Calculator Tag: Plumbing & HVAC

On this page:

• Calculator
• How to Calculate the Window AC Unit Size You Need
  • Factors to Consider When Sizing an Air Conditioner
  • Room Size
  • Sunlight
  • Kitchens and Cooking Appliances
  • Number of People
  • Window Size
  • Electrical Power Available
  • Operating Cost
• References

How to Calculate the Window AC Unit Size You Need

There are many window air conditioner models and sizes available on the market to help cool your home, but choosing one can be quite confusing if you don’t know what size you need to cool your space.

Air conditioners are sized in a few different ways. Large air conditioners are sized by the ton, while smaller air conditioners may be sized by the number of square feet they are designed to cool or by the BTU.

BTUs stand for British Thermal Units and are the measure of energy required to raise the temperature of one pound of water by one degree Fahrenheit. Choosing the right size air conditioner for your room is very important.

If the air conditioner is too small, then the unit will run all day and will not be able to comfortably cool the room on warm days.

If the air conditioner is too big, then the unit will move too much air, causing it to cycle on and off too frequently. This may cause discomfort due to frequent blasts of cool air, then warm air. Frequent on/off cycles will also shorten the lifespan of the unit

In either case, an incorrectly sized unit will also use more energy. This will mean that your energy bills are higher than they could be with a correctly sized unit while you stay more comfortable at the same time.

Factors to Consider When Sizing an Air Conditioner

There are many factors to consider when sizing a window air conditioner. The main factor is the size of the room.

Room Size

The size of the room is the main factor in choosing a window air conditioner size. A larger room will naturally require more energy to cool and more airflow in CFM than a smaller room.

Window AC Size Chart

The chart below from Energy Star shows how many BTUs are needed for a given room size.[1]

These air conditioner sizes assume a ceiling height of 8ft; if your ceiling is higher than 8ft, consider increasing the size in BTUs by 10% to account for the added volume of the room.

To find the size of the room you are planning on cooling, measure the length and width of the room in feet and multiply those two numbers together. Ideally, you want to be at the low to mid range of the unit’s capacity.

For example, to cool 1,100 sq ft, you would want a unit with 21,000 BTUs.

Sunlight

If the room gets a lot of sunlight, it will require additional energy to comfortably cool the room. Consider increasing the size of the air conditioner by 10% if the room gets a lot of sun.

To do this, take the size of the room in square feet, and add 10%, then find the recommended BTU range.

Kitchens and Cooking Appliances

Kitchens require much more energy to cool and require a larger air conditioner. Appliances add a lot of extra heat to a kitchen, making them tougher to cool. Consider the added heat of the oven or stove on a warm day.

When cooling a kitchen, consider adding an additional 4,000 BTUs to the size of the air conditioner.

Number of People

Body heat can warm a room significantly. The estimates above assume two occupants in the room. If your room has more than two people on a consistent basis, consider increasing the window air conditioner size by 10% – 20%.

Window Size

Window size can dictate how large of an air conditioner can be added to a room. If your room has windows that do not allow for the right size air conditioner, consider using two smaller units if your room has two windows, or consider an in-room air conditioner instead of a window unit.

Window units are available in different configurations as well as sizes. They should fit snugly, however; if the window is larger than the unit, you may need to add insulation or another material around the unit to help hold it in place and to stop thermal transfer around the unit

Electrical Power Available

Larger window air conditioners require more power. Some units require a dedicated circuit, while others require more current to the outlet. If the unit requires more power than you are able to provide, it may continually trip your circuit breaker. For example, if it needs its own circuit, but you plug in a vacuum on the same circuit, it will trip the breaker and stop both appliances from working.

Consider the amount and type of power available to the window and choose an air conditioner that will work with the existing wiring. If you can’t afford a dedicated circuit, take care not to use other appliances at the same time you use the air conditioner to avoid overloads.

If you’re unsure what your electrical requirements are, consider a consultation with an electrician.

Operating Cost

It’s important to also consider the operating cost of an air conditioner. Larger appliances consume more electricity and thus will cost more to operate. Use our electricity cost calculator to estimate how much it will cost to run the appliance.

Keep in mind that air conditioners are available with different energy efficiency levels. These are known as SEER levels or Seasonal Energy Efficiency Ratings. The higher the SEER, the less your unit will cost to run.

However, higher SEER units are also more costly. If you live in a climate where you need the air conditioner to run all day, it may be worthwhile to invest in a high SEER unit. However, if you only plan on using it periodically, a lower SEER unit may be your best choice.

If you’re sizing a heating system, check out our heating system BTU calculator.

On this page:

• Calculator
• How to Calculate Water Velocity
  • Water Velocity Formula
  • Alternative Formula
• References

How to Calculate Water Velocity

Water velocity is a measure of the rate of water flowing through a closed pipe system.[1] Knowing the water velocity of a pipe can help determine things like pressure. Having the right velocity is important if you want to extend the life of your pipes; water velocity that is too high for your system can result in the pipes corroding faster and needing to be replaced sooner.

Water Velocity Formula

Water velocity for a pipe of any size or length can be found using a simple formula:

Thus, the velocity v is equal to the flow rate Q divided by the cross-sectional area A.

When using this formula, all units should be in the same form of measurement. If the flow rate is measured in cubic meters per second, then the area should be in square meters, and if the flow rate is measured in cubic feet per second, then the area should be in square feet.

The cross-sectional area of a pipe can be found using the formula:

The area A is equal to π times the radius r squared.
For example, let’s find the water velocity given a flow rate of 35 cubic feet per second and a 1″ diameter pipe.

Start by finding the cross-sectional area of the pipe. You will need to convert the diameter from inches to feet, then divide this in half to find the radius.

diameter in feet = 1″ ÷ 12 = 0.0833′
radius = 0.0833′ ÷ 2 = 0.04167′
area = π × 0.04167² = 0.0054542 sq ft

Next, calculate the velocity.

Alternative Formula

The formula above works great when the flow rate is measured in the cubic form of a standard length unit. But when the flow rate is measured in gallons per minute or liters per minute, a different method is needed.

In this case, start by converting the flow rate to cubic feet per second or cubic meters per second.

Alternatively, if the flow rate is measured in gallons per minute, the following formula can be used to calculate water velocity.

Thus, the velocity v in feet per second is equal to 0.408 times the flow rate Q in gallons per minute (GPM) divided by the pipe diameter D in inches squared.[2]

For example, let’s find the water velocity given a flow rate of 15 GPM and a 0.75″ diameter pipe.

Start by finding the cross-sectional area of the pipe.

So, the velocity of water inside a 0.75″ pipe with a flow rate of 15 GPM is equal to 10.88 ft/s.

On this page:

• Calculator
• How to Calculate How Much Charge is Needed For a Split-System Air Conditioners and Heat Pumps
  • Estimating Charge Needed for Refrigerant Lines
• Charge Needed per Foot of Line

How to Calculate How Much Charge is Needed For a Split-System Air Conditioners and Heat Pumps

When charging a system there are a few components that need to be accounted for when estimating the amount of refrigerant needed. The manufacturer of the equipment being charged will provide specs for the amount of refrigerant to add to the system and the manual should be consulted for that information.

However, this leaves the liquid and suction lines between the units unaccounted for. The length of those lines will need to be accounted for to avoid under or over-charging the system.

To more accurately approximate the amount of charge needed, add the amount recommended by the manufacturer with the amount needed for the refrigerant lines.

Estimating Charge Needed for Refrigerant Lines

To calculate the refrigerant needed for the lines, start by noting the size of the liquid and suction lines.

Then, consult the table below to find the charge weight needed per foot of lines. Note that the amount of charge will be very different for the liquid and suction lines, and they should be added together to find the total weight per foot.

Finally, multiply the length of the lines in feet by the weight per foot to find the total charge needed. Note that you might need to convert from ounces to pounds at this point.

Charge Needed per Foot of Line

The tables below show the amount of charge needed per foot of line, for various line sizes. Always double-check your charge rates with those recommended or required by the manufacturer of the charge, equipment, and piping in use.

R-410A Refrigerant

Amount of R-410A refrigerant needed per foot of liquid line. Assumes a 40° suction temp and 105° liquid temp.

R-22 Refrigerant

Amount of R-22 refrigerant needed per foot of liquid line. Assumes a 40° suction temp and 105° liquid temp.

You might also be interested in our BTU heating calculator.

How to Find the Volume of a Pipe

You can find the volume of fluid in a pipe using the inner diameter of the pipe and the length. To estimate pipe volume, use the following formula:

Thus, the volume of a pipe is equal to pi times the pipe diameter d squared over 4, times the length of the pipe h.

This formula is derived from the cylinder volume formula, which can also be used if you know the radius of the pipe.

You can use either formula to get the same results.

To use the formulas, start by getting the diameter and length of the pipe in inches or millimeters. Use our feet and inches calculator to calculate a length in inches or millimeters.

If you’re unsure what the inner diameter of a pipe is but you know the outer diameter, refer to the common pipe dimensions tables to find the most likely inner diameter of your pipe.

Enter the length and diameter values into the formula above to calculate the volume of the pipe.
Example: calculate the volume of a 2″ diameter pipe that is 50′ long

Chart showing the reference angle for various angles in degrees

Volume and Weight of Water for Common Pipe Sizes

Volume and weight of water per foot for common pipe sizes

Metric Pipe Sizes

Volume and weight of water per meter for common metric pipe sizes

You can also use our water weight calculator to estimate the weight of the water in a pipe.

On this page:

• Calculator
• What Are BTUs
• How to Calculate Furnace Size in BTUs
  • BTUs per Square Foot Based on Climate
  • Home Insulation Affects BTU Requirements
  • Furnace Efficiency Affects Heat Output
  • Use a Smart Thermostat
  • Consider a Detailed Analysis
• References

What Are BTUs

A British Thermal Unit, or BTU, is a unit of measurement of energy. 1 BTU is the energy needed to heat 1 pound of water by 1° Fahrenheit. We often refer to the British Thermal Unit as a BTU, but the rating used for heat measurement or furnace size is actually the BTUs per hour, or BTU/h. 1 BTU is the equivalent of 1,055 joules or 0.293 watts.

BTUs are the measurement given for furnace sizes. The more BTUs a furnace produces, the larger the space that it is capable of heating. BTUs can be influenced by the size of a home, its climate, and how well-insulated the building is.

How to Calculate Furnace Size in BTUs

It is very important to size your heating and cooling system correctly for your home. If a furnace or air conditioner is too small, it will run constantly and not be able to comfortably heat or cool the space.

On the coldest or hottest days, the system will not be able to keep up. Alternatively, if the system is sized too large, it will heat or cool the space too quickly, which will cause it to cycle on and off more frequently, causing unnecessary wear and shortening the lifespan of the system.

According to the Star Tribune, the constant cycling of an oversized furnace is less comfortable as well since certain areas of the home heat up too quickly and become too hot before the system turns off.[1] Both furnaces that are too small and too large will also cost you more to run, while still leaving your home uncomfortable. They’ll also wear out faster, costing you more in replacement costs.

There are several factors to consider when trying to figure out how many BTUs your furnace should output.

BTUs per Square Foot Based on Climate

Experts suggest between 30 and 60 BTUs of heat per square foot of living space, which is a pretty wide range.

The location of your home is a major factor in how many BTUs you’ll need. A home in the northern region of the US will require more BTUs to heat than a home in the southern region because it gets significantly colder in the north.

The table below shows how many BTUs per square foot you’ll need based on your climate zone.

Table showing suggested BTU/sq. ft. of heat for various climate zones.

Based on these figures, a 2,000 sq. ft. home in a moderate climate would need a furnace with an output between 80,000 and 90,000 BTUs. When making your calculations, you may need to round up or down to the nearest furnace size. Keep things like insulation, sunlight, and the height of your ceilings in mind when choosing whether to go with a smaller or larger furnace.

Home Insulation Affects BTU Requirements

The ranges in the heating climate zone chart above assume a home with average insulation. Homes that are poorly insulated may require 10% more BTUs due to heat loss through thermal transfer.

Very well-insulated homes and homes with a tight building envelope will require 10% fewer BTUs to heat since very little heat is lost. It may be worth adding insulation to attic spaces and walls to reduce the amount of energy needed to heat the home.

An energy audit can help you determine if this is necessary, or if you have other areas of thermal transfer that can be eliminated to help lower your heating bills.

Furnace Efficiency Affects Heat Output

Furnaces are rated on their BTUs of energy consumption, not their BTUs of energy output. A more efficient furnace will output more BTUs of heat than a less efficient furnace with the same BTU rating.

For example, if a furnace is rated at 100,000 BTUs and is 80% efficient, then the heat output will be 80,000 BTUs (100,000 × .8). However, if a furnace is rated at 100,000 BTUs and is 92% efficient, then the heat output will be 92,000 BTUs (100,000 × .92). It may be worth considering a more efficient furnace as less energy will be used to achieve the same heat output.

Keep in mind that electric furnaces will use nearly 100% of the energy they receive to produce heat. Natural gas furnaces can convert up to 98% of energy into heat, while oil furnaces are generally less efficient, with the highest efficiency reaching around 92%.

The higher the efficiency, the less energy you will need to heat your home and the lower your energy bills. Most high-efficiency furnaces are more expensive to purchase, so they may not be worth the savings in hot or warm climates, but they may be worth the investment in cold climates.

While electric furnaces do convert nearly 100% of energy into heat, the cost of electricity frequently makes them costly to run. Therefore, most electric furnaces don’t come in sizes large enough to heat the entirety of a home in colder climates.

Use a Smart Thermostat

Smart thermostats can also improve the efficiency of your heating and cooling system. They can work in different ways depending on the type. Some work by learning when you’re at home and when you’re away. These thermostats can turn down the heat while you’re typically away and turn it up prior to your normal arrival. By learning your behavior, these thermostats can save you money.

Other types will pair with your smartphone or another device. When you leave a designated area, such as outside of your neighborhood, this will indicate to the thermostat to lower the heat. When you reenter this area, the thermostat will raise the heat level again.

Most smart thermostats also allow you to control your heating and cooling from your smartphone. This can allow you to make changes even when you’re at work if you want to turn the heat on for a guest or if your schedule changes and the thermostat hasn’t had time to learn your new schedule.

A popularly used option is the but there are several other great smart thermostats on the market.

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Consider a Detailed Analysis

Professional heating and cooling installers can provide a much more detailed analysis of your home to select the correct furnace and air conditioning size, tailored to your needs. Consider getting a free heating and cooling estimate for your home.

Professional installers will consider your home’s insulation, door and window sizes and locations, ceiling heights, duct sizes, and other constraints that affect the choice of system. This is called the Manual J calculation. It takes into account all of these factors, as well as how many people live in the home.

The Manual J calculation can give a very accurate size for furnaces and BTUs, so if you think your current furnace may be the wrong size, consider having a professional size your home before making your next purchase. Learn more about the cost of the average furnace installation.

If you’re sizing a cooling system, check out our window air conditioner size calculator to help determine what size air conditioner you will need. If you’re charging a cooling system, check out our refrigerant line charge calculator.

On this page:

• Calculator
• How to Calculate Flow Rate
  • Flow Rate Formula
  • Alternate Flow Rate Formula
• Average Flow Rate Chart
• References

How to Calculate Flow Rate

If you’re installing a new filtration system, water softener, or other pipe-based system in your home or business, you will need to know your home’s flow rate to determine the right system size.

Volumetric flow rate, also referred to as volume flow rate, flow rate, or volume velocity, is defined as the volume of fluid passing a point in a given unit of time.[1] The SI unit of flow rate is cubic meters per second (m³/s).

Meaning that a system may be able to handle a given amount or volume of liquid at one time. Knowing what your plumbing flow rate is will enable you to choose a filter or other system type that will work with your plumbing.

Flow rate can be found by simply measuring the amount of fluid that fills a bucket in a given amount of time or using a formula.

Flow Rate Formula

The flow rate of a fluid can be calculated using the volumetric flow rate equation:

Thus, the volume flow rate Q is equal to the cross-sectional area of the pipe A times the velocity of the fluid

To use the flow rate formula, substitute the cross-sectional area of the pipe and the velocity of the liquid. For a round pipe, the cross-sectional area can be found using the formula for circle area A = πr².
For example, let’s find the flow rate for the fluid passing through a 1″ diameter pipe at a velocity of 3 feet per second.

Start by finding the cross-sectional area of the pipe.

Next, it will be easier to solve later if the velocity and the area are in the same units. Convert the area in square inches to square feet by dividing by 144.

Finally, substitute the area and velocity in the flow rate equation and solve.

Thus, the flow rate is 0.01635 cubic feet per second.
Because most systems work in gallons per minute, or GPM, you will need to convert your cubic feet per second to gallons per minute.

Start by multiplying the cubic feet per second by 7.481, then by 60, which will give you cubic feet per gallon and per minute.

Alternate Flow Rate Formula

An alternative formula to solve flow rate is:

The volume flow rate Q is equal to the volume of liquid passed V divided by the time passed t.

To use this formula, you need to be able to measure the volume of liquid that passes in a given time.

For example, let’s find the flow rate for a garden hose that can fill a 5-gallon bucket in 3 minutes.

Average Flow Rate Chart

While water pressure and other factors can influence your total flow volume, most pipes will have general averages for flow rates. The following assumes minimum and maximum pressures on the pipe:

Table showing the average flow rate in gallons per minute for common pipe sizes.

Try our unit rate calculator to find the rate of other units.

On this page:

• CFM Calculator
• How to Calculate CFM
  • Step One: Measure the Space
  • Step Two: Calculate the Space’s Volume
  • Step Three: Determine the Air Changes per Hour Needed
  • Step Four: Calculate the CFM Needed
• Frequently Asked Questions
• References

How to Calculate CFM

Calculating the necessary airflow is a critical step when designing and maintaining an HVAC system or other air-moving application. The minimum airflow is the minimum amount of ventilation required to maintain a comfortable and healthy indoor environment with good air quality.

Airflow is often measured in CFM, or cubic feet per minute. CFM is the volume of air that is moved by or flows through a ventilation system.

You can calculate how much airflow you need in CFM by following a few easy steps.

Step One: Measure the Space

The first step is to measure the volume of space that needs to be ventilated. Larger spaces require more airflow and ventilation than smaller ones.

Start by using a tape measure to measure the length, width, and ceiling height of the space. It will be easiest if all of your measurements are in feet. You can round to the nearest whole foot if you are between full foot measurements.

You can use a length converter to convert your measurements to feet if needed.

Step Two: Calculate the Space’s Volume

The next step is to use the measurements from above to calculate the volume of the space. In order to calculate the amount of airflow in CFM, you’ll need to calculate the volume in cubic feet.

To do this, you can use the volume formula:

Thus, the volume in cubic feet is equal to the length times the width times the height, all in feet.

Step Three: Determine the Air Changes per Hour Needed

An important consideration when figuring the minimum airflow in CFM is how many air changes per hour (ACH) are needed in the space. A kitchen in a restaurant will need many more air changes per hour than a closet in a residence will need.

The air changes needed will multiply how many cubic feet per minute of airflow are required to adequately ventilate the space. Changing the air in a space more frequently improves the air quality.

While the number of ACH can vary, below are some of the recommended numbers for homes based on the room in question:

Table showing the suggested air changes per hour for various rooms in a home.

The larger the space, the higher the ACH may need to be in the range provided. Likewise, if the space is enclosed, it needs more ACH than a space that is open, and if the air is very humid or may have particles you want to filter out, a higher ACH is recommended.

If you are trying to filter out allergens, aim for at least 5 ACH in every room.

Step Four: Calculate the CFM Needed

To calculate the CFM needed to ventilate a space with a single air change per hour, you can use the following formula:

So, the cubic feet per minute of airflow needed to ventilate a space with a single air change per hour is equal to the volume of the space in cubic feet divided by 60.

If you need more than one air change per hour, multiply the room’s area by the number of air changes per hour that you need before dividing by 60. Round up to the nearest whole number.

For example, to calculate the CFM of a 100 sq. ft. bathroom with 8′ high ceilings, your formula would be:
CFM = (100 × 8 × 8) ÷ 60
CFM = (800 × 8) ÷ 60
CFM = 6,400 ÷ 60
CFM = 107