CFM Calculator

CFM Calculator: Find the Right Airflow for Any Room, Duct or Fan

CFM tells you how much air moves through a space every minute. If you pick an exhaust fan that is too small, the bathroom stays damp and the mirror stays foggy. If you pick one that is too big, you pay more, hear more noise and waste energy. A good CFM number saves you from both problems.

This CFM calculator works in four ways. It sizes airflow for a room using air changes per hour, the ASHRAE 62.2 home rule or the ASHRAE 62.1 occupancy rule. It checks duct size, velocity and friction loss. It turns a heating or cooling load into airflow. And it converts between flow units, corrects for altitude and temperature, and applies the fan laws when you change speed.

It is made for homeowners, HVAC installers, electricians, contractors, facility managers and students. You do not need to remember any formula. Type your numbers, read the answer, and see the working beside it.

What Is CFM?

CFM stands for cubic feet per minute. One cubic foot is a box of air that is one foot long, one foot wide and one foot tall. If a fan is rated at 100 CFM, it can move one hundred of those boxes of air in a minute.

People use CFM for bathroom fans, range hoods, window fans, air conditioners, furnaces, dust collectors, grow tents, paint booths and shop vacuums. Outside the United States you will often see the same thing written in cubic metres per hour (m³/h) or litres per second (L/s). One CFM is about 1.7 m³/h or 0.47 L/s, and the calculator shows all of them together.

CFM only tells you the volume of air. It does not tell you how hard the fan can push against resistance. That second number is static pressure, and it matters as soon as ducts, bends and filters get involved. A fan can be rated at 200 CFM in free air and deliver far less once it is connected to a long duct.

The CFM Formulas Used in This Calculator

There is no single CFM formula. The right one depends on what you know and what you want to find. These are the ones built into this tool.

• Room method: CFM = Room volume (ft³) x ACH / 60. ACH means air changes per hour, and 60 turns hours into minutes.
• Duct method: CFM = Velocity (FPM) x Duct area (ft²). FPM means feet per minute.
• HVAC method: CFM = BTU/h / (1.08 x ΔT). ΔT is the temperature difference between supply and return air in degrees Fahrenheit.
• ASHRAE 62.2 home rule: CFM = 0.03 x Floor area (ft²) + 7.5 x (Bedrooms + 1).
• ASHRAE 62.1 occupancy rule: CFM = (People x Rp) + (Floor area x Ra), where Rp and Ra depend on the type of space.
• Fan laws: CFM2 = CFM1 x (RPM2 / RPM1). Pressure changes with the square of the speed and power with the cube.

How to Calculate CFM Step by Step

Start with a simple room. Take a bathroom or small kitchen that is 15 ft long, 12 ft wide and has an 8 ft ceiling. The volume is 15 x 12 x 8, which is 1,440 cubic feet. If you want 8 air changes per hour, multiply 1,440 by 8 to get 11,520, and divide by 60. The answer is 192 CFM.

Now a duct. Say you have an 8 inch round duct and you want to keep the air at 800 feet per minute. The area is pi x (8 / 12)² / 4, which is about 0.349 square feet. Multiply by 800 and you get roughly 279 CFM.

Now a cooling system. A house has a sensible cooling load of 27,000 BTU/h and you want a 20 degree supply-to-return temperature difference. Multiply 1.08 by 20 to get 21.6. Divide 27,000 by 21.6 and you get 1,250 CFM. That is close to the old rule of 400 CFM per ton for a 3 ton system, because 3 tons x 400 is 1,200.

The calculator does these steps for you and shows the working underneath the result, so you can check every number.

Air Changes Per Hour (ACH): Typical Values by Room

ACH counts how many times the full volume of air in a room is replaced in one hour. A higher number means a stronger exchange. The values below are common rules of thumb. Your local code or the equipment maker may ask for something different, so treat them as a starting point.

• Bedroom or living room: about 4 ACH for general comfort.
• Office: about 6 ACH.
• Garage: about 6 ACH to clear fumes.
• Bathroom: about 8 ACH to remove moisture.
• Laundry room, workshop, gym and restaurant dining area: about 10 ACH.
• Home kitchen: about 15 ACH.
• Commercial kitchen: 20 to 30 ACH because of heat and grease.
• Warehouse: about 3 ACH since few people work in a very large space.

How to Use This CFM Calculator

Pick the tab that matches your job. Everything updates as you type, so there is no calculate button to press.

• Room tab: enter length, width and height, then choose a room type or type your own ACH. You can switch to the ASHRAE 62.1 occupancy method or the ASHRAE 62.2 home method if your project follows those rules.
• Add a safety margin. Real ducts, elbows and grilles cut airflow, so most people add 10 to 20 percent. The tool then tells you the next standard fan size.
• Duct tab: pick round or rectangular, enter the size and the velocity, and the tool gives the CFM. You can also enter CFM and get the velocity, or ask for the duct size that fits a target velocity.
• HVAC tab: enter the load in BTU/h, tons or kW, set the temperature difference, and add your altitude if you live in the hills.
• Convert tab: change CFM into m³/h, L/s and others, correct for altitude and temperature, or see what happens to airflow, pressure and power when a fan changes speed.
• Use the Metric or Imperial switch at the top for feet and inches or metres and millimetres.
• Save up to five results to compare options. Then copy the summary, download a CSV file or print the page.

Which Room Method Should You Use?

The three room methods give different answers because they measure different things. Choose the one that matches your goal.

Use ACH when you care about how fast the air in a room is refreshed, such as a bathroom, workshop or kitchen. It is simple and widely understood, though it ignores how many people use the room.

Use ASHRAE 62.2 for the whole-home ventilation rate in houses and low-rise apartments. For a 2,000 square foot house with 3 bedrooms, the answer is 0.03 x 2,000 plus 7.5 x 4, which is 60 plus 30, or 90 CFM. This is a continuous background rate, not a bathroom fan size.

Use ASHRAE 62.1 for offices, classrooms, shops and restaurants. It adds a per-person amount to a per-area amount. An office with 20 people and 2,000 square feet needs (20 x 5) + (2,000 x 0.06), which is 100 plus 120, or 220 CFM of outdoor air.

Duct Size, Velocity and Friction Loss

A duct that is too small makes the air rush through. That brings noise, whistling at the grilles and a fan that works too hard. A duct that is too large wastes sheet metal and space. The calculator shows the velocity and warns you when it is outside a good range.

As a guide, 400 to 700 feet per minute is quiet and good for bedroom branches. Between 700 and 1,000 feet per minute is normal for home trunk lines. Offices and shops often run between 1,000 and 1,500. Above 2,000 feet per minute you should expect a lot of noise and pressure loss.

The tool also estimates friction loss per 100 feet of straight duct. It uses the common ASHRAE equation for round duct and works out an equivalent round diameter for rectangular duct. Elbows, filters and dampers add more loss on top, so treat the result as a base figure and keep a margin.

Bathroom Fans and Range Hoods

For a bathroom up to 100 square feet, the usual rule is 1 CFM per square foot of floor, with a minimum of 50 CFM. A 60 square foot bathroom needs a 60 CFM fan. Larger bathrooms are sized by fixtures instead, and each toilet, shower or bathtub adds about 50 CFM.

For a kitchen hood there are two common rules. One is 100 CFM for each foot of cooktop width on a wall, or 150 CFM for an island. A 30 inch range on a wall is 2.5 feet wide, so that gives 250 CFM. The other rule is 1 CFM for every 100 BTU/h of gas burner power. A range with 60,000 BTU/h of burners points to 600 CFM. The calculator shows both numbers and uses the higher one.

Very strong hoods pull a lot of air out of the house. Many building codes ask for a makeup air supply once the exhaust goes above about 400 CFM, so check with your local office before you buy a big hood.

Heating and Cooling: CFM per Ton and Temperature Difference

Air conditioners are often sized at about 400 CFM per ton of cooling. One ton is 12,000 BTU/h. The number is a rule of thumb that fits a normal humid climate. Dry climates can use 450 or more, and very humid ones can drop to 350.

The more exact way is to start from the load. A larger temperature difference means each cubic foot of air carries more heat, so you need less airflow. A 20 degree difference is common for cooling. Gas and electric heating often runs 40 to 70 degrees. Use the HVAC tab and test different values to see how the airflow changes.

Keep one thing in mind. The 1.08 factor is for sensible heat only, and it is made for air at sea level. The calculator adjusts it for altitude when you enter your height above sea level.

SCFM vs ACFM: Altitude and Temperature

Air gets thinner as you go up and as it gets warmer. A fan moves the same volume of air, but that air has less weight and carries less oxygen, heat and dust. This is why engineers use two terms. ACFM is the actual volume at your real conditions. SCFM is the flow converted back to standard air at 70 degrees Fahrenheit and sea level.

At 5,000 feet, air is about 83 percent as dense as at sea level. A fan that moves 1,000 ACFM there is delivering only about 830 SCFM of standard air. If your process needs a certain mass of air, such as combustion or cooling, you must size the fan for the higher ACFM number.

The Convert tab does this correction in both directions. Enter your altitude and air temperature, and you get the density ratio and both CFM values.

Fan Laws: What Happens When You Change Fan Speed

If you have a variable speed fan, the fan laws give a quick estimate of the new numbers. Airflow changes in direct proportion to speed. Pressure changes with the square of the speed. Power changes with the cube.

Here is what that means in practice. If you raise the speed by 10 percent, airflow goes up by 10 percent, pressure goes up by 21 percent and power goes up by about 33 percent. If you slow a fan down by 20 percent, airflow drops by 20 percent but power falls by almost half. That is why a small speed cut on a big fan can save a lot on the electricity bill.

These laws are exact for the same fan in the same system and with the same air density. They are a good guide for most real work, but a fan curve from the maker is more accurate near the edges of the fan's range.

Common Mistakes When Sizing for CFM

Most bad installs come from a few easy mistakes. Here are the ones to watch for.

• Reading the CFM on the box as if it were real. Most ratings are taken with no duct at all, so the real number is lower.
• Forgetting about static pressure. A long run or many bends can cut a fan's output by a third or more.
• Using floor area when you should use volume. A tall ceiling holds more air, so it needs more airflow.
• Choosing a fan only by CFM and ignoring noise. Check the sone rating, and pick 1.0 sone or less for bedrooms and bathrooms.
• Ignoring makeup air. A tight house cannot exhaust air that has nowhere to come back in.
• Mixing up units. 100 m³/h is only about 59 CFM, not 100.
• Skipping the safety margin. Add 10 to 20 percent so the system still works well when filters get dirty.

Important Notes Before You Buy or Install

This calculator is a planning and learning tool. It uses well-known published formulas and rules of thumb, and it gives you an estimate. It does not replace a proper design by a licensed engineer, and it does not replace your local building code.

For projects with fire safety, health care, kitchens with heavy grease, hazardous fumes or any life-safety need, ask a qualified HVAC designer to check the final numbers. Always compare the fan maker's published curve against your real duct system before you order.