CFM Calculator
Size the airflow for a room. Enter the room dimensions and a target air-changes-per-hour (pick a room type or set your own), and get the required CFM, the room volume, how often the air turns over, the CFM per outlet and the round duct size, plus by-room-type and duct-by-velocity tables and a downloadable PDF.
| Room type | ACH | Required CFM | Change time |
|---|---|---|---|
| Enter a room to compare | |||
| Velocity | Duct area | Exact dia. | Round duct |
|---|---|---|---|
| Enter a room to compare | |||
How to use the CFM calculator
Four quick steps turn a room and a ventilation target into the airflow it needs — the required CFM, the room volume, the air-change time, the CFM per outlet and a sized round duct, with by-room-type and duct tables and a downloadable report.
Enter the room
The length, width and ceiling height, in feet.
Set the ventilation need
Pick a room-type preset or set your own air changes per hour (ACH).
Add outlets & velocity
The number of supply registers and the target duct velocity.
Read results & save a PDF
Get the CFM, CFM per outlet and the duct size — then download a report.
The formulas behind the airflow
Airflow comes from the room volume and the air-change target; the duct size then follows from the airflow per outlet and the velocity you choose.
▶ Worked example
A 15 × 12 ft room with an 8 ft ceiling, living-room target of 6 ACH, one outlet at 700 FPM.
Volume is 15 × 12 × 8 = 1,440 ft³, so CFM = 1,440 × 6 ÷ 60.
= 144 CFM · air changed every 10 min · 7″ round duct
One outlet carrying 144 CFM at 700 FPM needs a 6.1 in duct, which rounds up to the next standard size, 7".
CFM quick reference
Typical air changes by room type, CFM by floor area, and the round duct size for a given airflow.
| Room type | Typical ACH |
|---|---|
| Bedroom | 5 |
| Living room | 6 |
| Garage | 6 |
| Bathroom | 8 |
| Laundry | 8 |
| Kitchen | 15 |
Rules of thumb. The right value depends on use, occupancy and local code.
| Floor area | Volume (8 ft) | Required CFM |
|---|---|---|
| 100 ft² | 800 ft³ | 80 |
| 150 ft² | 1,200 ft³ | 120 |
| 200 ft² | 1,600 ft³ | 160 |
| 300 ft² | 2,400 ft³ | 240 |
| 400 ft² | 3,200 ft³ | 320 |
| 500 ft² | 4,000 ft³ | 400 |
At an 8 ft ceiling and 6 ACH this works out to about 0.8 CFM per square foot. Higher ceilings or ACH raise it.
| Airflow (CFM) | Exact diameter | Standard duct |
|---|---|---|
| 50 | 3.6 in | 4″ |
| 100 | 5.1 in | 6″ |
| 150 | 6.3 in | 7″ |
| 200 | 7.2 in | 8″ |
| 300 | 8.9 in | 9″ |
| 400 | 10.2 in | 12″ |
| 600 | 12.5 in | 14″ |
Diameter = √(4 × area ÷ π), rounded up to the next standard duct. A lower velocity needs a larger duct.
Everything the calculator gives you
One room produces a full ventilation profile, not just a single number.
CFM reference numbers
Built for anyone sizing ventilation
Homeowners, HVAC installers, and remodelers all need the airflow a room requires.
Sizing a bathroom or kitchen fan, or a vent for a workshop or garage, and needing the CFM for the room before buying.
- Get the CFM a room needs from its size
- Match a fan’s rating to the airflow
- Use a room-type preset for a quick target
Sizing supply runs and registers — the CFM per outlet and the round duct for the target velocity, as a quick check before a full design.
- Split the CFM across the registers
- Size the round duct for the velocity
- Hand a client a clean PDF of the figures
Planning ventilation for a new or remodeled space, and getting a sensible airflow target before the mechanical layout is set.
- Work out the CFM for each room
- Plan duct sizes from the airflow
- Set a baseline before an HVAC pro signs off
Get the airflow right
Small habits make a CFM estimate far more reliable.
Work from volume
CFM is the room volume times the ACH over 60 — measure the ceiling height, not just the floor.
Match ACH to the room
Kitchens and baths need high air changes; bedrooms and living areas much less.
Split across outlets
Divide the total CFM across the supply registers to size each duct run.
Keep supply velocity sensible
About 600–900 FPM for supply runs; 700 is a good, quiet default.
Round up the duct
Pick the next standard duct size up from the exact diameter, never down.
Appliances have own rules
Range hoods, bath fans, dryers and combustion appliances follow their own sizing, not just ACH.
Confirm with the code
For a real design, check ASHRAE 62.2, the mechanical code and an HVAC professional.
Frequently asked questions
Common questions about CFM, air changes per hour, room volume, and duct sizing.
CFM — cubic feet per minute of airflow — comes from the room’s volume and how many times you want to change its air each hour. The formula is the room volume times the air changes per hour, divided by 60 to go from per-hour to per-minute.
The room volume is the length times the width times the ceiling height, in cubic feet. The air changes per hour (ACH) is the ventilation target for the space.
For example, a 15-by-12-foot room with an 8-foot ceiling is 1,440 cubic feet, and at 6 air changes per hour that needs 1,440 times 6 divided by 60, which is 144 CFM. This calculator works out the airflow, the room volume and the air-change time.
ACH, air changes per hour, is how many times the full volume of air in a room is replaced each hour. A higher ACH means more complete and more frequent air turnover, which you want for spaces that build up moisture, odors or fumes.
Typical targets are about 5 to 6 ACH for bedrooms, 6 to 8 for living areas, 8 for bathrooms and laundries, and 15 for kitchens. These are rules of thumb; the right value depends on the use, the occupancy and local code.
This calculator has presets for common room types and lets you set your own ACH. The airflow in CFM then follows from the ACH and the room volume.
The round duct diameter comes from the airflow and the air speed you want in the duct. First find the duct area — the CFM divided by the velocity in feet per minute — then convert that area to a diameter and round up to the next standard duct size.
Residential supply runs use about 600 to 900 feet per minute, with 700 a good default; lower is quieter, higher lets you use smaller ducts but is noisier. The total CFM is split across the number of outlets first.
For example, 144 CFM through one outlet at 700 FPM needs about a 6.1-inch duct, which rounds up to a 7-inch round. This calculator sizes the duct from the CFM per outlet and your target velocity.
ACH is a rate of air turnover — how many times per hour the room’s whole air volume is replaced — while CFM is the actual volume of airflow per minute that achieves it. One is a target, the other is the flow that meets it.
They are linked by the room volume: CFM equals the volume times the ACH divided by 60. So a bigger room needs more CFM for the same ACH, and a higher ACH needs more CFM for the same room.
This calculator takes the ACH target and the room size and converts them into the required CFM, so you can size a fan or a duct.
The room volume is the floor area times the ceiling height: length times width times height, in cubic feet. It is the amount of air the room holds, and it is the basis for the CFM calculation.
Measure the length and width of the floor and the floor-to-ceiling height. Most homes have 8-foot ceilings, with 9 or 10 feet in newer or vaulted spaces — and a vaulted or sloped ceiling uses the average height.
For example, a 15-by-12-foot room with an 8-foot ceiling holds 1,440 cubic feet. This calculator computes the volume from the dimensions you enter and uses it for the airflow.
There is no single CFM-per-square-foot figure, because it depends on the ceiling height and the air-change target, not just the floor area. A room with a high ceiling holds more air and needs more CFM for the same floor area.
A rough way to see it: at an 8-foot ceiling and 6 air changes per hour, the airflow works out to about 0.8 CFM per square foot. Raise the ceiling or the ACH and that figure goes up.
So it is better to work from the actual volume and ACH than a flat per-square-foot rule. This calculator does that, and you can read off the CFM for your specific room and target.
A bathroom’s exhaust needs enough CFM to change the air roughly eight times an hour, so it follows from the room volume. For a typical small bathroom this often lands around 50 to 110 CFM, but a larger or taller bath needs more.
A common shortcut for small baths is about 1 CFM per square foot of floor, with a minimum around 50 CFM, but the air-change method is more accurate for larger rooms. Bath fans also have their own code sizing and duct-run limits.
This calculator works out the airflow from the room volume and an 8-ACH target when you pick the bathroom preset. Confirm the fan and duct against the fan maker’s data and the code.
Residential supply ducts are usually sized for an air speed of about 600 to 900 feet per minute, with 700 FPM a sensible default. Within that range, a lower velocity is quieter and has less pressure loss but needs a bigger duct, while a higher velocity lets you use a smaller duct but is noisier.
Main trunk lines can run a little faster than branch runs, and return ducts are usually sized for a lower velocity than supplies. Going much above 900 FPM in a home tends to make the system noisy.
This calculator lets you set the target velocity and sizes the round duct from it, so you can trade duct size against noise and pressure.
Disclaimer: US units (feet, CFM, FPM). Required airflow (CFM) = room volume × air changes per hour ÷ 60; room volume = L × W × H. Round duct is sized from CFM per outlet and the target velocity (area = CFM ÷ velocity), then rounded up to a standard size. ACH targets are rules of thumb and vary by use, occupancy and local code; kitchen hoods, bath fans, dryers and combustion appliances follow their own sizing rules, and this tool is not a full Manual J/D load or static-pressure calculation. A planning aid only — confirm with ASHRAE 62.2, the mechanical code and an HVAC professional.