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

Background.

Ask how much air a 2,400 square foot, three-bedroom house needs and you can defend four very different answers: 102 cfm, 800 cfm, 1,111 cfm or 2,560 cfm. None of them is an error. The first is the outdoor-air ventilation rate ASHRAE Standard 62.2 sets for the whole dwelling. The second is what the blower of a two-ton air conditioner moves at 400 cfm per ton. The third is the supply air needed to carry 24,000 BTU/h of sensible heat at a 20 °F temperature difference. The fourth is eight air changes an hour, a rate someone might pick for clearing steam from a bathroom, applied to an entire house. Cubic feet per minute is a unit; the requirement depends on what the air is for.

That is why the calculator begins by asking which question you have. Air changes per hour suits one room with one nuisance — shower moisture, cooking smells, solvent fumes in a workshop, heat from a server cupboard — where you pick the rate yourself, because no standards body publishes air-change targets by room type. The ASHRAE 62.2 mode gives the code-referenced minimum of outdoor air for a dwelling from its floor area and bedroom count. The sensible-load mode gives the airflow that delivers a known heating or cooling load to a space. The tonnage mode multiplies an air conditioner's nominal capacity by an airflow per ton.

Every answer is also expressed three other ways: in litres per second, as the air changes per hour it produces in the space you described, and as cfm per square foot. Those side figures are where a mismatched method gives itself away. The 102 cfm whole-house rate works out to about a third of an air change per hour, which looks feeble until you remember that it is fresh outdoor air arriving around the clock, on top of whatever the building already leaks.

If what you actually have is a duct diameter and an air speed, the answer is Q = A × v, and the flow rate calculator handles it directly.

What is cfm calculator?

CFM, cubic feet per minute, measures a volume of air moved per unit of time. Its metric counterpart is litres per second, and the conversion is exact because the foot is defined exactly: 1 cfm = 0.3048³ ÷ 60 m³/s, or 0.4719474432 L/s. Air changes per hour restate the same flow relative to a room, cfm × 60 ÷ volume, which is why 50 cfm is a brisk rate in a small bathroom and almost nothing in an open-plan living area. Ventilation is narrower than airflow: it means outdoor air brought in to dilute indoor contaminants, so a furnace blower recirculating 1,200 cfm through a house supplies no ventilation at all unless some of that air comes from outside. Sensible heat is the part of a heating or cooling load that changes air temperature; latent heat is the part that changes its moisture content. A ton of refrigeration is 12,000 BTU per hour by definition, a figure inherited from the heat needed to melt a short ton of ice over a day.

How to use this calculator.

1. 01Choose the method that matches your question: air changes for a room with a specific nuisance, ASHRAE 62.2 for whole-dwelling ventilation, sensible load when a load calculation gives you BTU/h, tonnage when you only know the equipment size.
2. 02Enter floor area and ceiling height in every mode, since they produce the volume, air-change and per-square-foot figures. In ASHRAE 62.2 mode the area is the whole dwelling unit, not one room.
3. 03For air changes, choose a rate you can justify for what you are clearing. For ASHRAE 62.2, enter the bedroom count as a whole number; a studio is 0, which the standard counts as 1.
4. 04For the sensible-load method, take the sensible figure from your load calculation, not the total, and enter the supply-to-room temperature difference. Cooling designs commonly sit around an 18–22 °F drop; for heating, use the temperature rise printed on the furnace nameplate, often somewhere in the 35–65 °F range.
5. 05For tonnage, enter nominal tons and the airflow per ton from the equipment's own performance data if you have it. 400 cfm per ton suits most conventional ducted systems; small-duct high-velocity systems are rated nearer 220.
6. 06Read the method note under the result, which shows the arithmetic with your numbers, and have a licensed mechanical contractor or designer confirm anything that will be installed or submitted for a permit.

The formula.

Each method is a single equation. Air changes: cfm = V × ACH ÷ 60, where V is floor area times ceiling height in cubic feet and 60 turns an hourly rate into a per-minute one. ASHRAE 62.2: Qtot = 0.03 × A + 7.5 × (N + 1), Equation 4.1a of Section 4.1.1, with A the dwelling's floor area in square feet and N the bedroom count, not less than 1. The area term stands in for emissions from the building and its contents; the bedroom term stands in for occupants, assuming two people in a studio or one-bedroom home and one more per extra bedroom. The SI version, Equation 4.1b, is 0.15 L/s per square metre plus 3.5 L/s per (N + 1); its rounded coefficients give 47.45 L/s for a house the I-P equation puts at 48.14 L/s.

Sensible load: cfm = Qs ÷ (1.08 × ΔT). The constant is derived rather than fitted: standard air weighs 0.075 lb per cubic foot and has a specific heat of 0.24 BTU per pound per °F, and there are 60 minutes in an hour, so 0.075 × 0.24 × 60 = 1.08 BTU/h per cfm per °F. Because ΔT sits in the denominator, halving it doubles the airflow. Thinner air carries less heat per cubic foot: at 5,000 feet air is roughly 14% less dense than at sea level, so the constant falls to about 0.93 and the required cfm rises accordingly. The calculator keeps the sea-level 1.08.

Tonnage: cfm = tons × cfm per ton. The 400 default is the full-load airflow ENERGY STAR's Version 6.2 specification uses when it defines low-static, mid-static and mobile-home blower coil systems; the same document defines small-duct high-velocity systems at no less than 220 scfm per ton.

In every mode the result is converted to litres per second at 0.4719474432 L/s per cfm, re-expressed as air changes per hour (cfm × 60 ÷ V) and divided by floor area. The 62.2 result is Qtot: no infiltration credit is subtracted and the separate kitchen and bathroom exhaust requirements are not included.

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 follow from the equation.
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.