What the result means
Air carries heat. If you know how much heat is being added or removed and how much the air temperature changes, you can back out the airflow. It also works the other way: measured airflow and temperature split give you the delivered sensible capacity.
On a furnace, use the output Btu/h and the temperature rise to check airflow against the rated rise range. On cooling, the 1.08 formula covers sensible heat only, so it underestimates total capacity when the coil is also removing moisture.
How it's calculated
- CFM = Btu/h ÷ (1.08 × ΔT)
- Btu/h = 1.08 × CFM × ΔT
- Rule of thumb: CFM ≈ tons × 400
1.08 = 0.075 lb/ft³ × 0.24 Btu/lb·°F × 60 min/h (standard air). Correct for altitude if needed.
Worked example
A 24,000 Btu/h sensible load with a 20 °F temperature split.
- CFM = 24,000 ÷ (1.08 × 20)
- CFM = 24,000 ÷ 21.6 = 1,111
About 1,111 CFM is needed to move that heat at a 20 °F split.
Reference table
The values below are computed from the same calculator functions used above. They let you compare common sizes and operating points without replacing the interactive calculation or its assumptions.
| Tons | 350 CFM/ton | 400 CFM/ton | 450 CFM/ton |
|---|---|---|---|
| 1 | 350 | 400 | 450 |
| 2 | 700 | 800 | 900 |
| 3 | 1050 | 1200 | 1350 |
| 4 | 1400 | 1600 | 1800 |
| 5 | 1750 | 2000 | 2250 |
| 6 | 2100 | 2400 | 2700 |
| 7 | 2450 | 2800 | 3150 |
| 8 | 2800 | 3200 | 3600 |
| 9 | 3150 | 3600 | 4050 |
| 10 | 3500 | 4000 | 4500 |
Rule of thumb only. Confirm airflow against the equipment manufacturer and design.
Sources and assumptions
1.08 = 0.075 lb/ft³ × 0.24 Btu/lb·°F × 60 min/h (standard air). Correct for altitude if needed. Calculator formulas are published with their units so a technician can check a result instead of treating it as a black box. Where a code table, manufacturer specification or field measurement is required, use the applicable local source.
Last reviewed . Spot an error? Email support@tiermenu.com.
FAQ
Why 1.08?
It combines standard air density, specific heat and 60 minutes per hour. At high altitude air is thinner, so the factor is lower.
Is 400 CFM per ton always right?
No. It's a rule of thumb. Humid climates often run 350 CFM per ton for better moisture removal, dry climates higher. Follow equipment data.
Can I use this to check furnace airflow?
Yes. Use the furnace output (not input) Btu/h and the measured temperature rise.
Does this include latent heat?
No. For total cooling capacity use enthalpy with the psychrometric calculator.
Using a reference result in the field
A calculator is most useful when the inputs match the job being checked. Confirm whether a value is nominal or measured, and whether readings were taken at design or actual operating conditions. The unit label beside the result is part of the result; do not carry a number into another calculation without checking its units.
Reference rows are provided for comparison, not as universal design limits. They use the values and conditions stated with each table. Elevation, air temperature, equipment and installation details can change a field result. Where the page calls out a code check, equipment rating or manufacturer table, that check remains part of the job.
Use the interactive inputs for the exact conditions at hand, then review the displayed formula and assumptions. If a value looks unexpected, change one input at a time and compare it with a known point in the reference table. Keep the measured readings and job-specific notes with the work record so another technician can understand how the result was obtained.
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