HVAC · Air side

CFM Calculator

The airflow a system needs, from nominal tonnage (400 CFM per ton) or from the sensible load (CFM = sensible BTU / (1.08 × ΔT)). The result feeds the duct size calculator.

Field brief

2 / 3 tonat 400 CFM per ton, a 2-ton system needs 800 CFM and a 3-ton needs 1200 CFM. The 350-450 band moves it with climate.800/1200 CFM

From loadCFM = sensible BTU / (1.08 × ΔT). 24,000 BTU sensible at 20°F is 1111 CFM.1111 CFM

1.08= 60 × 0.075 × 0.24 (min/h × std-air density × specific heat). Standard air, 70°F, sea level.std air

Nominal tons

CFM per ton

400 CFM/ton is the nominal comfort rule; 350 for hot, humid climates (more dehumidification), 450 for dry climates (more sensible cooling). Confirm against OEM blower data (ACCA).

Required airflow

1,200 CFM

Airflow1,200 CFM

Basis3 ton × 400 CFM/ton

Show the math

CFM = tons × CFM/ton = 3 × 400 = 1,200

Where this number comes fromIntry Verified

Calculated from

Sensible-heat equation (1.08 x CFM x dT) or 400 CFM per ton (ACCA). Computed from the standard-air 1.08 sensible-heat constant (60 x 0.075 x 0.24; a secondary, HVAC School, states its basis) or the ACCA 400 CFM-per-ton rule, at standard air. See the full receipt ↓

Run with

Basis3 ton x 400 CFM/ton
Required airflow1,200 CFM

Checked

Every figure an Intry calculator computes comes from a locked source module, not hand-typed. 31163 automated checks, one suite for the whole site rather than a per-tool count, re-derive the numbers before each deploy from the tables this site transcribed from the ASHRAE and ACCA airflow standards it was computed from. This is our own deterministic gate, not a third-party audit.

Final say

The OEM blower table for the actual coil, or a full ACCA Manual J/D design, has final say.

What Intry Verified means

Intry Verified · Build 2731D30 · 2026-10-15

Standard air: the 1.08 constant assumes standard air (0.075 lb/ft³, 70°F, sea level). At altitude or high supply temperatures the air is thinner, so the real CFM is higher for the same load. The 400 CFM/ton rule is a nominal starting point; the OEM blower table is the authority for a given coil.

3 ton1,200 CFM

The airflow formulas

From tonnage: CFM = tons × CFM-per-ton, with 400 CFM per ton nominal (ACCA; band 350 to 450). From sensible load: CFM = sensible BTU / (1.08 × ΔT). The 1.08 = 60 min/h × 0.075 lb/ft³ × 0.24 BTU/lb°F is the standard-air sensible-heat constant, so both assume air at 70°F and sea level. Latent load uses a separate constant and is not included here.

CFM Calculator FAQs

How many CFM for a 3 ton unit?

A 3-ton system needs about 1200 CFM at the nominal 400 CFM per ton (3 x 400). Hot, humid climates run closer to 350 CFM/ton (1050 CFM) for more dehumidification; dry climates run up to 450 CFM/ton (1350 CFM) for more sensible cooling. Confirm against the OEM blower table for the actual coil.

How many CFM for a 2 ton unit?

A 2-ton system needs about 800 CFM at 400 CFM per ton. The 350-450 CFM/ton band puts it between 700 and 900 CFM depending on climate and how the system is set up for latent versus sensible load.

How do I calculate CFM from BTU?

For the SENSIBLE portion of the load, CFM = sensible BTU / (1.08 x deltaT), where deltaT is the supply-to-room temperature difference. For 24,000 BTU sensible at a 20F deltaT that is 1111 CFM. The 1.08 constant is 60 min/h x 0.075 lb/ft3 standard-air density x 0.24 BTU/lb-F specific heat, so it assumes standard air at 70F and sea level.

What is the 1.08 constant in the CFM formula?

1.08 is 60 x 0.075 x 0.24: 60 minutes per hour, times the density of standard air (0.075 lb/ft3 at 70F, sea level), times the specific heat of air (0.24 BTU/lb-F). It converts a sensible BTU/hr load and a temperature difference into the airflow in CFM. At altitude or high supply temperatures the air is thinner, so the real constant is lower and the required CFM is higher.

Is 400 CFM per ton always right?

No. 400 CFM per ton is a nominal comfort starting point, not a law. ACCA gives a 350 to 450 CFM per ton band: 350 for hot, humid climates that need more moisture removal, 450 for dry climates that want more sensible cooling. The OEM blower performance table for the specific air handler is the authority; this calculator sizes the airflow, then feed it into the duct size calculator.

Next in the air-side job

Duct Size Calculator

Turn this CFM into a round or rectangular duct size by the equal-friction method.

Static Pressure & Friction Rate

Get the friction rate the duct sizer needs from your available static pressure and TEL.

BTU / Load Calculator

Size the cooling load first, then the airflow, then the duct.

01Source
receipt

Where these airflow numbers come from

The required airflow on this page is computed by the locked duct-airflow engine either from nominal tonnage at your CFM-per-ton, or from a sensible load and supply-to-room temperature difference, re-derived from the sourced constants below on every build. The 1.08 sensible-heat constant is never hand-typed: it is multiplied out from its three factors, so a drift in any one fails the build.

Standard-air sensible-heat basis (1.08): The airflow-from-load rule CFM = sensible BTU / (1.08 x deltaT). The 1.08 constant is derived, not typed: 60 min/h x 0.075 lb/ft^3 (standard-air density) x 0.24 BTU/lb-F (specific heat of air). The three factors are ASHRAE standard-air psychrometric constants and the 1.08 is multiplied from them at run time, never typed. A secondary, HVAC School, states the 70F / sea-level (14.7 psia) basis.
ACCA 400 CFM per ton airflow rule: The tonnage path uses 400 CFM/ton nominal, with a 350 (hot/humid, latent priority) to 450 (warm/dry, sensible priority) band, and 1 ton = 12,000 BTU/h by definition. Fetched from the ACCA HVAC Blog '400 CFM Per Ton, Or Is It?' (hvac-blog.acca.org). The OEM blower table for the actual coil is the authority; this is the nominal starting point.

These are looked-up field constants and an industry rule of thumb, applied by the engine to the tonnage or load you enter, not a measurement of your system. Everything here assumes standard air (0.075 lb/ft^3, about 70F, sea level); at altitude or high supply temperatures the air is thinner and the real airflow is higher for the same load. The OEM blower data for the coil, or a full ACCA Manual J/D design, has the final say.