CFM Calculator (Air Changes, ASHRAE 62.2, Load and Tonnage)

Background.

"CFM calculator" is four questions wearing one name, and the four answers for the same room routinely differ by a factor of ten. That is not a flaw in any of them. A bathroom fan clearing shower moisture, a whole-house ventilation system meeting a code minimum, a supply register delivering a cooling load, and an air handler matched to a three-ton condenser are genuinely different problems that happen to share a unit. This page asks which one you are solving before it gives you a number, and then tells you, beside the answer, what that particular method does not cover.

The air-change method is the oldest and the loosest. Take the room's volume, decide how many times an hour you want to replace it, divide by sixty. A 300 square foot room with an 8 foot ceiling holds 2,400 cubic feet, and eight air changes an hour is 320 cfm. It is the right method when you know the rate that clears the nuisance you care about — moisture, odour, solvent vapour, waste heat — and it is worth saying plainly that air-change targets by room type are trade conventions rather than code. There is no standards body that publishes "a bathroom needs eight air changes per hour". That is why this calculator leaves the rate as a field you fill in rather than picking one for you from a hidden table.

The ASHRAE 62.2 method is the one with actual standing. Standard 62.2, Ventilation and Acceptable Indoor Air Quality in Residential Buildings, sets a minimum outdoor-air rate for a whole dwelling unit, and several energy codes and above-code programmes reference it. Section 4.1.1's Equation 4.1a is Qtot = 0.03 × Afloor + 7.5 × (Nbr + 1): three hundredths of a cfm for each square foot of floor area, plus 7.5 cfm for each bedroom plus one. The area term stands in for what the building itself emits — materials, finishes, furnishings — and the bedroom term stands in for the people, on the assumption of two occupants in a studio or one-bedroom unit and one more per additional bedroom. A 2,400 square foot three-bedroom house comes out at 102 cfm, which works out to about a third of an air change per hour and strikes most people as far too small until they realise it is outdoor air, continuously, on top of whatever the house leaks.

The sensible-load method is the one an installer uses. The rate at which an airstream carries heat is Qs = 1.08 × cfm × ΔT, so the airflow needed to deliver a known load at a known supply temperature difference is that equation rearranged. The 1.08 is not a fudge factor: it is the density of standard air, 0.075 pounds per cubic foot, times the specific heat of air, 0.24 BTU per pound per degree Fahrenheit, times sixty minutes in an hour. Two consequences follow immediately and both matter. Airflow is inversely proportional to the temperature difference, so designing around a 10 °F drop instead of a 20 °F drop doubles the ductwork you need. And it handles sensible heat only — nothing in that equation removes moisture, which is why a system sized purely on sensible capacity can hold temperature perfectly and still leave a house clammy.

The tonnage method is the shortcut, and it is the one most often quoted without its caveat. Multiply nominal tons by an airflow per ton, usually 400. That figure comes from equipment rating conditions rather than from design: ENERGY STAR's central air conditioner and heat pump specification defines several ducted system classes by their static pressure when operated at a full-load air volume rate "not exceeding 400 cfm per rated ton of cooling" — while rating small-duct high-velocity systems at 220 scfm per ton instead. So 400 is a convention attached to particular equipment classes, not a law of airflow, and the number for a specific machine comes out of its manufacturer's expanded performance data.

One thing this page deliberately will not do: calculate cfm from a duct diameter and an air velocity. That is the continuity equation, Q = A × v, and it is already answered by the flow rate calculator, which takes exactly those two inputs and returns CFM among its outputs. Duplicating it here would split one question across two pages for no benefit to anyone.

And one limit worth stating before you read any number: in ASHRAE 62.2 mode this returns Qtot, the total required ventilation rate, not Qfan, the required mechanical ventilation rate. Section 4.1.2 lets you subtract an infiltration credit from Qtot — but only where a blower door test has actually been performed, and that credit is capped at two-thirds of Qtot. No web page can do that step for you, so this one does not pretend to.

What is cfm calculator?

CFM is cubic feet per minute, the customary US unit of volumetric airflow. Its SI counterpart is litres per second, and the conversion is exact rather than approximate because the foot is defined exactly: one cubic foot per minute is 0.3048³ ÷ 60 cubic metres per second, or 0.471947 litres per second. It measures volume of air per unit time, not mass and not heat, which is why every method on this page needs a second quantity — a rate, a floor area, a temperature difference, a capacity — to turn a requirement into an airflow.

Air changes per hour, ACH, expresses the same airflow relative to the size of the space: how many times in an hour the airflow equals the room's volume. Converting between them is arithmetic — cfm = volume × ACH ÷ 60 — and the useful consequence is that the same cfm means completely different things in different rooms. Fifty cfm is about six air changes an hour in a small bathroom and about a tenth of one in a great room.

Ventilation rate, in the ASHRAE 62.2 sense, is specifically the rate of OUTDOOR air brought into a dwelling. That is a narrower idea than airflow: a furnace blower moving 1,200 cfm around a house is providing no ventilation at all if none of that air comes from outside. Standard 62.2 sets a whole-dwelling minimum for outdoor air, plus separate local-exhaust requirements for kitchens and bathrooms which this page does not compute.

Sensible heat is the part of a thermal load that changes air temperature, as distinct from latent heat, which changes its moisture content. The Qs = 1.08 × cfm × ΔT relation on this page governs sensible heat only. A cooling load calculation splits the total into both, and using the total where the equation wants the sensible part will overstate the airflow — a common and expensive error.

A ton of refrigeration is 12,000 BTU per hour by definition, a legacy of the rate at which a ton of ice melts over a day. Airflow per ton is a rating and design convention connecting a machine's capacity to the air it is expected to move, and it varies by equipment class rather than being universal.

How to use this calculator.

1. 01Pick the method that matches the question you actually have. If you are sizing an exhaust fan for a specific nuisance, use air changes. If you are meeting a residential ventilation code requirement, use ASHRAE 62.2. If you know a load in BTU/h, use the sensible-load method. If you have a tonnage and want a starting airflow, use tonnage.
2. 02Enter the floor area and ceiling height. These are read in every mode, because they are what turn any airflow into a volume and an air-change rate. In ASHRAE 62.2 mode the area must be the whole dwelling unit, not one room.
3. 03For the air-change method, set a rate you can justify for the nuisance you are clearing. The field is free because there is no code table of ACH by room type, whatever else you may have read.
4. 04For ASHRAE 62.2, enter the bedroom count. A studio is 0, and the standard counts it as 1.
5. 05For the sensible-load method, enter the SENSIBLE load only — not the total cooling load — and the supply-to-room temperature difference you are designing around.
6. 06For the tonnage method, enter nominal tons and the airflow per ton from the equipment's own data if you have it. 400 is a rating condition for most ducted classes, not a universal figure.
7. 07Read the method note under the result. It shows the arithmetic with your numbers in it and names what the method leaves out.
8. 08Check the resulting air-change rate as a sanity check. A whole-house ASHRAE 62.2 rate well under one air change per hour is normal and correct; a bathroom exhaust rate of eight or more is normal too. A number wildly outside what you expected usually means the wrong method, not a wrong answer.
9. 09Take any ventilation or airflow design to a licensed mechanical contractor or design professional before it is installed, and confirm which edition of ASHRAE 62.2 your jurisdiction references.

The formula.

The air-change method is a unit conversion with a decision inside it. Volume in cubic feet times air changes per hour gives cubic feet per hour; dividing by sixty gives cubic feet per minute. For a 300 square foot room with an 8 foot ceiling the volume is 2,400 cubic feet, and at 8 air changes per hour that is 2,400 × 8 ÷ 60 = 320 cfm. The decision is the rate, and the honest position is that nobody authoritative publishes one. Air-change targets circulate as trade rules of thumb, they vary between sources, and they exist because they are a convenient way to express "clear this nuisance at about this speed". Treat the rate as your judgement, not as a lookup.

The ASHRAE 62.2 method is Equation 4.1a of Section 4.1.1, quoted from the standard: Qtot = 0.03 × Afloor + 7.5 × (Nbr + 1), with Qtot in cfm, Afloor the dwelling-unit floor area in square feet, and Nbr the number of bedrooms, not to be less than one. For the worked example's 2,400 square foot three-bedroom house that is 0.03 × 2,400 = 72 cfm from the area term plus 7.5 × 4 = 30 cfm from the occupancy term, totalling 102 cfm. The SI form of the same rule, Equation 4.1b, is Qtot = 0.15 × Afloor + 3.5 × (Nbr + 1) in litres per second and square metres.

Two things about that coefficient deserve to be said out loud. First, 0.03 is edition-specific. Standard 62.2-2010 and earlier used 0.01, one third as much, and the change arrived with the 2013 edition — for this same house that is the difference between 54 cfm and 102 cfm, so citing "ASHRAE 62.2" without a year is meaningless. Second, Qtot is not the fan you buy. Section 4.1.2 allows an infiltration credit, Qfan = Qtot − (Qinf × Aext), where Qinf may be no greater than two-thirds of Qtot, but the credit is available only where a blower door test has been performed. This page returns Qtot and says so, because inventing an infiltration figure would be the difference between a code-compliant house and one that merely looks compliant on paper.

The sensible-load method rearranges the airstream heat equation. The rate at which moving air carries sensible heat is mass flow times specific heat times temperature difference; converting cubic feet per minute into pounds per hour using standard air density and then into BTU per hour gives Qs = 0.075 × 0.24 × 60 × cfm × ΔT, and 0.075 × 0.24 × 60 is exactly 1.08. Solving for airflow, cfm = Qs ÷ (1.08 × ΔT). For 24,000 BTU/h at a 20 °F difference that is 24,000 ÷ 21.6 = 1,111 cfm. Because ΔT sits in the denominator, the relationship is inverse: design around 10 °F instead of 20 °F and the same load needs 2,222 cfm, twice the duct. The constant shifts with air density, so work well above sea level needs a corrected value.

The tonnage method is a multiplication whose interest is entirely in the second number. Two tons at 400 cfm per ton is 800 cfm. ENERGY STAR's Version 6.2 central air conditioner and heat pump specification defines low-static, mid-static and mobile-home blower coil systems by the static pressure they produce "when operated at the cooling full-load air volume rate not exceeding 400 cfm per rated ton of cooling", and defines small-duct high-velocity systems at "at least 220 scfm per rated ton" — nearly half. So 400 belongs to particular equipment classes as a rating condition, and using it as a universal design figure will be wrong for anything unusual.

Every mode also reports the same three derived figures, which is deliberate: the volume, the air-change rate the answer works out to, and the airflow per square foot. Those let you cross-check one method against another's intuition. All arithmetic is carried in exact decimal at forty significant digits and rounded once, at the end, to ten decimal places. The only step function anywhere on the page is ASHRAE's rule that Nbr is not to be less than 1, and it is applied to the value you entered, not to a rounded one.

A worked example.

Frequently asked questions.

References& sources.

1. [1]ANSI/ASHRAE Addendum m to ANSI/ASHRAE Standard 62.2-2013, "Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings". Read directly from the ASHRAE-published addendum PDF on 2026-07-29. Source of Section 4.1.1 "Total Ventilation Rate" and Equation 4.1a (I-P), Qtot = 0.03·Afloor + 7.5·(Nbr + 1), together with its variable definitions — including the clause that Nbr is "not to be less than 1" — the SI form Equation 4.1b, and Tables 4.1a and 4.1b, all fifty cells of which this calculator regenerates from the equation in its test suite.
2. [2]ANSI/ASHRAE Addendum h to ANSI/ASHRAE Standard 62.2-2019. Read directly from the ASHRAE-published addendum PDF on 2026-07-29. Source of the infiltration-credit relation this page deliberately does not apply: Equation 4-2, Qfan = Qtot − (Qinf × Aext), with Qfan the required mechanical ventilation rate, Qinf the effective annual average infiltration rate from a single-point blower-door test at 50 Pa per ASTM E1827 or ANSI/RESNET/ICC Standard 380, and Aext equal to 1 for detached dwelling units. This addendum revises Section 4.1.2 while continuing to refer to Section 4.1.1 for Qtot, which is the evidence that Equation 4.1a is unchanged in the 2019 edition.
3. [3]ASHRAE, ANSI/ASHRAE Standard 62.2, "Ventilation and Acceptable Indoor Air Quality in Residential Buildings" — the governing standard. Sold by ASHRAE; the full current edition was not opened. Sections 4.1.1 and 4.1.2 reach this page through the two ASHRAE-published addenda cited above, which are open documents. Section 5's separate local-exhaust requirements for kitchens and bathrooms are not implemented here.
4. [4]U.S. Environmental Protection Agency and U.S. Department of Energy, ENERGY STAR Program Requirements, Product Specification for Central Air Conditioner and Heat Pump Equipment, Version 6.2, Section 1 (Definitions). Read directly from the PDF on 2026-07-29. Source of the 400 cfm per rated ton figure and of the fact that it is a rating condition rather than a design mandate: low-static, mid-static and mobile-home blower coil systems are defined by their external static pressure "when operated at the cooling full-load air volume rate not exceeding 400 cfm per rated ton of cooling", while small-duct high-velocity systems are defined at "at least 220 scfm per rated ton of cooling".
5. [5]National Institute of Standards and Technology, "Guide for the Use of the International System of Units (SI)", NIST Special Publication 811, Appendix B.8. Retrieved 2026-07-29. Source of the exact airflow conversion used here: cubic foot per minute (ft³/min) → 4.719 474 × 10⁻⁴ m³/s. The calculator carries this to full precision as 0.3048³ ÷ 60 = 0.000 471 947 443 2 m³/s, or 0.471 947 443 2 L/s per cfm.
6. [6]ASHRAE Handbook — Fundamentals, psychrometrics and standard-air properties. Source of the standard-air density of 0.075 lb/ft³ and specific heat of 0.24 BTU/lb·°F from which the sensible-heat constant 1.08 = 0.075 × 0.24 × 60 is derived. ASHRAE Handbook access is gated behind purchase or membership; the constant is not taken on authority here but reconstructed from its published components, and that arithmetic is asserted in this calculator's test suite.