CFM per Ton Converter
The 400 CFM per ton rule of thumb in HVAC derives directly from the psychrometric heat balance of standard air across a cooling coil. The refrigeration ton is defined as 12,000...
Formula
Source: ASHRAE Handbook | Last reviewed: June 7, 2026
Examples
400 CFM
= 1 ton
400 CFM ≈ 1 ton
1200 CFM
= 3 ton
3-ton system needs ≈ 1200 CFM
4000 CFM
= 10 ton
10-ton rooftop unit
Quick Reference Table
| CFM | Tons |
|---|---|
| 400 | 1 |
| 600 | 1.5 |
| 800 | 2 |
| 1200 | 3 |
| 2000 | 5 |
| 4000 | 10 |
Where is this used?
Used by engineers to quickly estimate the cooling capacity corresponding to a given airflow, or to determine required airflow for a specified tonnage.
In commercial HVAC retrofit projects, the CFM/ton rule is the first-pass sizing check — if an existing air handler is delivering 4,000 CFM and the calculated building load is 12 tons (144,000 BTU/hr), the actual CFM/ton is 333, suggesting either a coil that is performing better than standard assumptions (likely a high-performance chilled water coil with closer approach) or an over-sized blower (wasteful fan power).
In residential HVAC replacement, the CFM/ton rule is used to verify that a new system has been correctly sized: a 3-ton (36,000 BTU/hr) residential AC should deliver about 1,200 CFM, and a field measurement within ±10% of this value is considered good commissioning practice per ACCA Manual D.
Data center cooling design uses CFM/ton to back-calculate required CRAH (Computer Room Air Handler) capacity from the IT load — a 100 kW rack at 1,200 CFM/ton requires 24,000 BTU/hr of cooling per ton, so the rack needs 100 kW × 3,412 BTU/hr per kW ÷ 12,000 BTU/hr/ton = 28.4 tons of cooling and 11,360 CFM of supply air.
Cleanroom design for semiconductor, pharmaceutical, and biotech facilities uses the CFM/ton rule in conjunction with the air change rate (ACH) — for a 1,000 ft² cleanroom with 10 ft ceiling (10,000 ft³) at 600 ACH, the supply is 100,000 CFM, and the corresponding cooling at 400 CFM/ton is 250 tons — a value that often exceeds the sensible cooling load, requiring reheat coils or variable-volume supply to maintain temperature and humidity during low-load periods.
Industrial process cooling applications (plastic injection molding, die casting, printing press cooling, pharmaceutical reactor cooling) often use the CFM/ton rule for chilled water air-handling unit sizing where the cooling coil is downstream of a process exhaust stream — the CFM/ton calculation interacts with the process exhaust rate and the make-up air heating/cooling load.
Hotel, hospital, and laboratory HVAC systems that require once-through outside air (operating rooms, isolation rooms, BSL-3 labs) have very high CFM/ton values because the outside air load dominates over the recirculated air load — a 100% OSA lab in a hot-humid climate might need 800–1,000 CFM/ton to condition the massive outside air stream.
Frequently Asked Questions
Is 400 CFM/ton always accurate?
No, it's a rule of thumb. Actual design values range from 350–450 CFM/ton depending on climate, humidity, and system design. High-humidity climates may use 350 CFM/ton for better dehumidification.
How does altitude affect this?
At higher altitudes, air is less dense, so more CFM is needed per ton. At 5000 ft, use approximately 480 CFM/ton as the rule of thumb.
What's the equivalent in metric units?
400 CFM/ton ≈ 54 L/s per kW ≈ 0.193 m³/s per kW. The same concept applies but in different units.
Reviewed for accuracy
· Last reviewed: June 7, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.