CFM & Tonnage Calculator
Free HVAC airflow calculator. Find required CFM from sensible BTU/h and Delta T, convert tons to CFM with the 400 CFM/ton rule, and convert between tons, BTU/h, and kW.
Required airflow from sensible heat
Tonnage ↔ BTU/h ↔ kW & airflow
How the airflow calculations work
Required CFM from sensible heat
The amount of air a system must move to deliver its sensible capacity at a given temperature split is:
CFM = Sensible Heat (BTU/h) ÷ (1.08 × Delta T)
The 1.08 is the sensible heat factor for standard air — it’s 60 min/h × 0.075 lb/ft³ × 0.24 BTU/lb·°F. Rearranged, the same relationship gives you sensible heat (Qs = 1.08 × CFM × ΔT) if you already know airflow.
Worked example: A system with 24,000 BTU/h sensible capacity across a 20°F split needs:
24,000 ÷ (1.08 × 20) = 24,000 ÷ 21.6 = 1,111 CFM
Tonnage, BTU/h, kW, and the 400 CFM/ton rule
- 1 ton = 12,000 BTU/h = 3.517 kW
- Tons = BTU/h ÷ 12,000
- Nominal airflow ≈ 400 CFM per ton (practical range 350–450: lean toward 350 in humid climates for more dehumidification, up to 450 in dry climates).
A 3-ton system is therefore 36,000 BTU/h and wants roughly 1,200 CFM at the 400 CFM/ton rule.
How FieldPad helps in the field
Size the airflow here, then keep the numbers where the work lives. FieldPad logs measured airflow and capacity against the equipment, ties them to the job, and turns your findings into a client-ready estimate or invoice — offline, on your iPhone, no re-keying.
Stop Re-Running These Numbers by Hand
FieldPad is the all-in-one iPhone CRM for solo HVAC techs — clients, scheduling, estimates, invoices with signatures, inventory, and equipment history. Keep your readings on the job, the client, and the equipment, all in one offline-ready app. EPA Section 608 and Subpart C compliance included.
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