CFM Calculator: Room Airflow and HVAC Duct Size

How to calculate CFM

CFM is cubic feet of air per minute. For a room, the exact sum is volume × air changes per hour ÷ 60: length × width × ceiling height gives the cubic feet, the air change rate says how many times an hour that volume is replaced, and dividing by 60 turns hours into minutes. The calculator does not assume an air change rate, because it depends on what the room is for and on the code that applies; type the one you have been given.

Worked example: a 20 × 15 ft workshop with a 10 ft ceiling is 3,000 cubic feet. At 4 air changes an hour, 3,000 × 4 ÷ 60 = 200 CFM. Turned round, a 200 CFM fan in that room gives 4 air changes an hour.

CFM from tons of cooling

A ton of cooling is 12,000 BTU per hour, and the rule of thumb is about 400 CFM of air across the coil per ton. A 2.5-ton system therefore moves about 1,000 CFM and a 4-ton system about 1,600. ACCA's own blog calls the figure typical rather than fixed: humidity, temperature, air density and elevation all move it, and many designs run 350 or 450 CFM per ton on purpose.

Use this method to check a system or rough out a quote, not to design one. Room-by-room airflow comes from an ACCA Manual J load calculation, and the blower setting from the equipment's airflow tables.

Bathroom fans and whole-house ventilation

For a bathroom fan, HVI's consumer guide gives a 50 CFM minimum for 50 square feet or less, then 1 CFM per square foot of floor up to 100 square feet. Bigger bathrooms are sized by fixture: 50 CFM each for a toilet, a shower and a bathtub, 100 CFM for a jetted tub. HVI adds that ceilings over 8 ft may need more and that an enclosed toilet needs its own fan.

For a whole house, ASHRAE Standard 62.2 sets a continuous ventilation rate: 0.03 CFM per square foot of floor plus 7.5 CFM per person, with people counted as bedrooms + 1. ACCA's example: a 1,525 square foot, two-bedroom house needs 0.03 × 1,525 + 7.5 × 3 = 68.25 CFM, before any credit for air leakage measured with a blower door.

Duct size from CFM and velocity

The duct tab divides the airflow by the air speed to get the cross-section, then finds the round diameter: area = CFM ÷ fpm, diameter = √(4 × area ÷ π) × 12 in. It rounds up to the next common size and shows how fast the air moves in that size. Faster air means a smaller duct but more noise and more pressure drop.

The velocity presets come from ACCA Manual D, Table 3-1, as printed in ACCA's Manual D checklist. Recommended / maximum: supply trunks 700 / 900 fpm in rigid duct and 600 / 700 in flex; supply branches 600 / 900 rigid and 600 / 700 flex; return trunks 600 / 700; return branches 400 / 700. Example: a 1,200 CFM supply trunk at 700 fpm needs 17.73 in, so an 18 in duct.

Round to rectangular ducts

A rectangular duct carries the same airflow at the same friction as a round duct when its equivalent diameter matches: De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25, the Huebscher formula. An 8 × 12 in duct works out at 10.7 in, as in Engineering ToolBox's table. The calculator lists, for each duct height from 4 to 12 in, the narrowest width in 2 in steps that reaches the round size needed.

Equal friction is not equal velocity: a rectangle has a little more area than its round equivalent, so the air moves a little slower. Flat, wide ducts also have more surface for the same air, so the narrower options are usually the better ones.

What this calculator does not do

It does not replace ACCA Manual J (the load of each room) or Manual D (duct design), which residential codes call for. Manual D sizes each run from the blower's available static pressure, the effective length of the run and its fittings, through a friction rate; none of that is modelled here. Use these numbers to check a design, size a single run or explain a quote.

For HVAC sales and service techs

The numbers in an HVAC quote come from the walk-through: room sizes, ceiling heights, the rooms that never cool, where the return is, how old the ductwork is and what the homeowner has already been told by someone else. Miss one and the quote, or the install, comes back.

Sources, each read on 2026-10-24. All results are estimates; Manual J, Manual D and the equipment data decide:

• ACCA: 400 CFM Per Ton, Or Is It? (Matt Akins, 2021; 350 humid, 450 dry)
• Home Ventilating Institute: Bathroom Exhaust Fans, A Consumer Guide (50 CFM minimum, 1 CFM per sq ft, fixtures)
• ACCA: 2016 Update for ASHRAE 62.2 (Don Prather; whole-house equation and example)
• ACCA: Verifying ACCA Manual D Procedures checklist (Manual D Table 3-1 velocities)
• Engineering ToolBox: Equivalent Diameter of rectangular ducts (Huebscher formula)

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