CFM Calculator

CFM is cubic feet of air per minute. A 12 × 15 ft bedroom with an 8 ft ceiling holds 1,440 cubic feet, so six air changes an hour is 1,440 × 6 ÷ 60 = 144 CFM. A 3 ton central air conditioner moves about 1,200 CFM, and an 8 × 10 ft bathroom needs an 80 CFM exhaust fan. Pick the tab that matches what you are sizing.

How CFM is calculated

Four formulas cover almost every question:

Room: CFM = length × width × height (cu ft) × air changes per hour ÷ 60

System: CFM = tons × 400

Duct: CFM = cross-section area (sq ft) × velocity (ft/min)

Bathroom fan: CFM = floor area (sq ft) × 1, never under 50

In metric the calculator shows litres per second (L/s) and cubic metres an hour; 1 CFM is 0.472 L/s.

CFM from air changes per hour

Air changes per hour (ACH) says how many times the room's air is replaced in an hour. Trane gives these ranges, and picking a room type fills in the upper value:

For a 20 × 15 ft living room with 9 ft ceilings at 8 ACH: 2,700 cu ft × 8 ÷ 60 = 360 CFM. Room size in square feet comes from the room square footage calculator if the room is not a simple rectangle.

CFM per ton for HVAC

Trane puts a typical central air conditioner or heat pump at about 400 CFM per ton. A 2.5 ton system therefore needs ducts for 1,000 CFM, a 3 ton system for 1,200 CFM. The per-ton value is an input because equipment differs: the blower table in the installation manual gives the real airflow at each fan speed. If you don't know the tonnage, the AC size calculator estimates it from floor area.

CFM calculator for a duct

Airflow through a duct is its inside area times the air velocity. An 8 in round duct has an area of π × (8 ÷ 12)² ÷ 4 = 0.349 sq ft; at 700 feet per minute it carries 244 CFM. A 12 × 8 in rectangular duct (0.667 sq ft) at the same speed carries 467 CFM. Measure velocity with a vane anemometer at the register or duct, or use it backwards: to keep a supply duct under the 900 fpm that ACCA Manual D gives as the supply maximum (quoted by Energy Vanguard), size the duct in the duct size calculator.

Bathroom exhaust fan CFM

The Home Ventilating Institute sizes bath fans by floor area: 1 CFM per square foot for bathrooms up to 100 sq ft, with 50 CFM as the minimum. An 8 × 10 ft bathroom needs 80 CFM, a 5 × 8 ft one still needs 50. Above 100 sq ft, HVI adds the fixtures instead: 50 CFM each for a toilet, shower and bathtub, 100 for a jetted tub. A 120 sq ft primary bath with a toilet, a shower and a tub needs 150 CFM.

ASHRAE 62.2, the residential ventilation standard, sets the same 50 CFM for an on-demand bath fan, or 20 CFM if the fan runs continuously (Tables 5-1 and 5-2, as published by Energy Code Ace). Fan ratings are measured at a set pressure; long, crushed or kinked ducts cut the real airflow, so buy a little above the number when the duct run is long.

Range hood CFM

HVI recommends 100 CFM per foot of hood width for a wall-mounted hood and 150 CFM per foot for an island hood, which has no wall to guide the smoke. Its minimums are 40 and 50 CFM per foot. ASHRAE 62.2 asks at least 100 CFM from any vented range hood.

HVI also notes that high-BTU gas cooking equipment should always be ducted outside.

From CFM to ducts and equipment

• Size one duct for a known CFM: duct size calculator.
• Split the blower's CFM between rooms and size the trunk and branches: ductwork calculator.
• Turn room size into cooling BTU first: BTU calculator.