CFM Calculator

Calculate required airflow in cubic feet per minute from room volume and air changes per hour, or from a heating/cooling load and temperature difference.

Includes the ASHRAE 62.2 whole-house ventilation rate and the 400 CFM per ton rule of thumb.

Last updatedHow we build & check our tools
ft
ft
ft
BTU/h
°F
ft²
Results will appear here once you enter your values.

The two CFM formulas and when to use each

Room volume method: CFM = length × width × height × ACH ÷ 60. Air changes per hour (ACH) is how many times the full volume of the room is replaced each hour; dividing by 60 converts an hourly volume to a per-minute flow. Use it to size an exhaust fan, a supply register for a single room, a garage or workshop fan, or an air purifier target.

Load method (sensible heat equation): CFM = BTU/h ÷ (1.08 × ΔT). The 1.08 constant is 60 minutes per hour × 0.075 lb/ft³ (density of standard air) × 0.24 Btu/lb·°F (specific heat of air). Use it to check how much air a furnace, air handler or duct run must move to deliver a known load at a given supply-air temperature difference. In metric, L/s = W ÷ (1.206 × ΔK), from 1.2 kg/m³ × 1.005 kJ/kg·K.

Worked examples with real numbers

  • Home office: 12 × 12 × 8 ft = 1,152 ft³. At 6 ACH: 1,152 × 6 ÷ 60 = 115.2 CFM (about 54 L/s, 196 m³/h).
  • Furnace airflow: a 40,000 BTU/h sensible load with a 20 °F supply-to-room difference needs 40,000 ÷ (1.08 × 20) = 1,851.9 CFM. The same furnace at a 50 °F temperature rise needs only 741 CFM, which is why heating airflow is much lower than cooling airflow.
  • Cooling rule of thumb: a 3-ton (36,000 BTU/h) air conditioner is typically set for 3 × 400 = 1,200 CFM; in humid climates installers often drop to 350 CFM/ton (1,050 CFM) to improve dehumidification.
  • Whole-house ventilation: a 2,000 ft² home with 3 bedrooms needs 0.03 × 2,000 + 7.5 × (3 + 1) = 60 + 30 = 90 CFM of total continuous ventilation under ASHRAE 62.2-2013 and later (before any infiltration credit).

ASHRAE 62.2 governs whole-house ventilation, not room ACH

Room ACH presets are design rules of thumb for moving air through one space.

Fresh-air ventilation for a dwelling is set by ASHRAE Standard 62.2, which the International Residential Code (IRC M1505) and many energy codes reference.

The formula used here, from the 2013 and later editions, is Q = 0.03 × Afloor + 7.5 × (Nbr + 1).

That is the total ventilation rate; the standard lets you subtract an infiltration credit calculated from a blower-door test, so a leaky house may need a smaller fan.

The older 2010 edition used 0.01 × Afloor, so check which edition your jurisdiction adopted.

Local exhaust also has separate minimums (for example 50 CFM intermittent or 20 CFM continuous for a bathroom, 100 CFM intermittent for a kitchen range hood).

Common mistakes

  • Using total load in the sensible equation. 1.08 × CFM × ΔT covers sensible heat only. Latent load (moisture) is removed at the coil and is not included, so plugging a total cooling capacity in gives an airflow that is too high.
  • Mixing up ΔT. For cooling use room temperature minus supply-air temperature (typically 18–22 °F). For a furnace use the temperature rise printed on the nameplate range (often 35–70 °F).
  • Ignoring altitude. The 0.075 lb/ft³ density is sea-level standard air. At 5,000 ft air is about 17% less dense, so the constant falls to roughly 0.9 and more CFM is needed for the same load.
  • Assuming fan rating equals delivered CFM. Fan ratings are at a stated static pressure; long or undersized ducts (see ACCA Manual D) can cut delivered airflow by a third or more.

Frequently Asked Questions

Common questions about the CFM Calculator

Multiply length × width × ceiling height in feet to get cubic feet, multiply by the air changes per hour you want, then divide by 60. A 12 × 12 × 8 ft room at 6 ACH needs 1,152 × 6 ÷ 60 = 115.2 CFM.

Sources & References

ANSI/ASHRAE Standard 62.2: Ventilation and Acceptable Indoor Air Quality in Residential Buildings

Source of the whole-house mechanical ventilation rate Q = 0.03A + 7.5(Nbr + 1) and local exhaust minimums.
→Source: ASHRAE

ASHRAE Handbook: Fundamentals (psychrometrics and air properties)

Standard air density (0.075 lb/ft³) and specific heat (0.24 Btu/lb·°F) behind the 1.08 sensible heat constant.
→Source: ASHRAE

Whole-House Ventilation

DOE guidance on exhaust, supply, balanced and energy-recovery ventilation systems for tight homes.
→Source: U.S. Department of Energy, Energy Saver

Manual D: Residential Duct Systems

ANSI-recognized method for sizing duct systems to deliver the design airflow at available static pressure.
→Source: ACCA

Explore More Tools

Continue your financial journey with these related calculators

energy

Attic Insulation Calculator

Work out how much R-value, depth and how many bags of blown-in cellulose, blown fiberglass or batts you need to bring your attic up to ENERGY STAR recommendations for your climate zone.

Try it now
energy

Battery Runtime Calculator

Estimate how long a battery, portable power station or UPS will run a load, using capacity, usable depth of discharge, inverter efficiency and an optional Peukert correction for lead-acid batteries.

Try it now
energy

Box Fill Calculator

Size electrical boxes to NEC 314.16 by counting conductors, devices, clamps, fittings and grounds. Compare the required cubic inches with a standard metal box or a marked volume and see how much room is left.

Try it now
energy

Conduit Fill Calculator

Check NEC conduit fill for EMT, RMC, IMC, PVC and flexible metal conduit using Chapter 9 conductor and raceway areas. Mix wire sizes and insulation types, see pass or fail, and find the smallest trade size that works.

Try it now
energy

PC Power Supply Calculator

Add up your CPU, graphics card, drives, fans and peripherals to estimate peak system draw, the PSU wattage you should buy with headroom for GPU transient spikes, and what the PC costs to run each year.

Try it now
energy

Refrigerator Wattage Calculator

Turn the kWh per year on a refrigerator or freezer EnergyGuide label into average watts, compressor running and starting watts, daily to yearly energy cost, and the generator or inverter size it needs.

Try it now