Why is 400 CFM per ton the standard?

ASHRAE and most equipment manufacturers establish 400 CFM/ton as the baseline airflow needed to achieve the rated sensible and latent capacity of a DX cooling coil at standard conditions (75°F DB / 63°F WB entering air). Deviating more than 15% in either direction reduces efficiency and may void equipment warranties.

When should I use a lower CFM/ton ratio?

In humid climates (Gulf Coast, Southeast US, tropical regions), slowing airflow to 350–380 CFM/ton increases coil contact time, lowering the leaving dewpoint and removing more moisture. This trades some sensible capacity for improved dehumidification.

When should I use a higher CFM/ton ratio?

In dry or arid climates (Southwest US, high-altitude regions), where latent load is minimal, 420–450 CFM/ton maximizes sensible cooling and reduces fan energy relative to the cooling delivered.

What is a ton of refrigeration?

One ton of refrigeration equals 12,000 BTU/h of heat removal — the rate at which one short ton (2,000 lb) of ice melts over 24 hours. Residential systems are typically 1.5–5 tons; commercial systems may reach hundreds of tons.

Does this account for duct leakage or static pressure?

No. This tool calculates design airflow at the equipment. Actual delivered CFM depends on fan performance curves, duct system resistance, and leakage. Always add a leakage allowance (typically 5–10%) when sizing ductwork.

CFM from Tonnage Calculator

Enter cooling tonnage and CFM-per-ton ratio to instantly calculate the required airflow in cubic feet per minute.

System Parameters

Cooling Capacity (tons)

Residential systems: 1.5–5 tons. Commercial: 5–500+ tons.

CFM per Ton

Required Airflow

1,200

CFM

3 tons × 400 CFM/ton

Common Sizes at 400 CFM/ton

Tons	CFM	BTU/h

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Summary

This calculator converts HVAC system cooling capacity (measured in tons of refrigeration) to the required supply airflow in CFM (cubic feet per minute). The industry-standard ratio is 400 CFM per ton, suitable for most temperate climates. For humid climates where latent heat removal is critical, a lower ratio (350–380 CFM/ton) is often used; for dry or arid regions, a higher ratio (420–450 CFM/ton) may be appropriate.

How it works

Enter the cooling capacity of the HVAC system in tons.
Adjust the CFM-per-ton ratio if needed (default is 400, the ASHRAE standard).
The calculator multiplies tonnage by the CFM/ton ratio to compute total airflow.
Review the result in CFM and the breakdown table for common system sizes.
Use the copy button to capture the result for use in duct sizing or equipment selection.

Use cases

Size ductwork for a new residential or commercial HVAC installation.
Verify that an existing air handler can deliver enough airflow for the connected cooling coil.
Adjust airflow targets for high-humidity climates that require more aggressive dehumidification.
Compare airflow requirements across multiple system sizes during an HVAC design review.
Estimate total fresh-air volume for ventilation code compliance checks.
Cross-check equipment submittals against design airflow specifications.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu