Question 1

What is CFM in HVAC?

Accepted Answer

CFM stands for cubic feet per minute — the volume of air a system moves. It's how HVAC airflow is measured for blowers, ducts, registers, and ventilation. Required CFM scales directly with cooling tonnage and the sensible heat ratio.

Question 2

How many CFM per ton of AC?

Accepted Answer

Standard residential systems target 400 CFM per ton at design conditions. High-efficiency variable-speed systems often run lower (350 CFM/ton) to improve dehumidification. Two-stage systems may run as low as 300 CFM/ton on first stage.

Question 3

How do I calculate CFM for a room?

Accepted Answer

Three approaches: (1) by tonnage — multiply tons × 400 CFM/ton; (2) by ACH — multiply room volume × ACH ÷ 60; (3) from sensible load — divide BTU/hr by (1.08 × supply ΔT). Use the load-based approach for accurate per-room balancing.

Question 4

What is ACH (air changes per hour)?

Accepted Answer

ACH is how many times the entire air volume of a space is replaced each hour. Living rooms typically need 6 ACH, bathrooms and kitchens 8 ACH, restaurants 10–12 ACH, hospital operating rooms 20+ ACH. Higher ACH = better ventilation but more energy use.

Question 5

What's the CFM formula?

Accepted Answer

From sensible load: CFM = sensible BTU/hr ÷ (1.08 × ΔT). From ACH: CFM = (room volume in cu ft × ACH) ÷ 60. From tonnage: CFM = tons × 400 (or 350 for high-efficiency). Different inputs, same airflow target.

Question 6

How many CFM for a 1,500 sq ft house?

Accepted Answer

Typically 800–1,200 CFM at the air handler — 2–3 tons × 400 CFM/ton. Per-room CFM should be balanced by load: a sunny living room may need 200 CFM while a shaded bedroom needs 80 CFM. The CFM total at the registers should equal the air handler output.

Question 7

What is supply air ΔT?

Accepted Answer

ΔT is the temperature drop across the cooling coil — typically 18–22°F in residential cooling. Lower ΔT = higher CFM needed for the same load. Higher ΔT = less airflow but risk of cold drafts and underdehumidification. Most balanced systems target 20°F.

Question 8

Should I use 350 or 400 CFM per ton?

Accepted Answer

Use 400 CFM/ton for standard single-stage AC and most heat pumps. Use 350 CFM/ton for variable-speed equipment that prioritizes dehumidification (humid climates), or for high-SEER systems where the manufacturer specifies it. Never go below 300 CFM/ton — coil freeze risk.

Space	Recommended ACH	Why
Bedroom	5–6	Quiet, low odor
Living room	6	General ventilation
Kitchen	7–8	Cooking + odor removal
Bathroom	8	Moisture removal
Office	6	Equipment heat + IAQ
Garage	4–6	Off-gas exhaust
Restaurant dining	8–10	High occupancy + odor
Restaurant kitchen	20+	Heavy cooking equipment
Hospital surgery	15–25	Infection control

CFM / Airflow Calculator

Inputs

Results

Tonnage-driven airflow at 400 CFM per ton

Tonnage to CFM (400 CFM/ton)

Three ways to calculate required CFM

Recommended ACH by space type

Common CFM sizing mistakes

CFM FAQ