Find the airflow in cubic feet per minute needed to carry a heating or cooling load.
For the electrical side of the same plant, use the main kVA to kW converter.
Every diagram and table on this page updates with the calculator above.
The 1.08 combines air density, specific heat and 60 minutes per hour at sea level.
Note: Standard air at sea level; correct for altitude and high temperatures.
| Property | Air | Water |
|---|---|---|
| Density | 1.2 kg/m³ | 1,000 kg/m³ |
| Specific heat | 1.005 kJ/kg·K | 4.18 kJ/kg·K |
| Heat per m³ per K | 1.2 kJ | 4,180 kJ |
| Relative capacity | 1 | about 3,500 |
A cubic metre of water carries roughly 3,500 times the heat of a cubic metre of air for the same temperature change. That is why air systems need ducts measured in hundreds of millimetres while the equivalent pipework fits in your hand.
Required airflow across a range of loads at your design temperature difference.
| kW | Airflow (CFM) | Airflow (m³/h) |
|---|---|---|
| 1 kW | 158 | 268 |
| 2.5 kW | 395 | 671 |
| 5 kW | 790 | 1,342 |
| 7.5 kW | 1,185 | 2,013 |
| 10 kW | 1,580 | 2,684 |
| 15 kW | 2,370 | 4,026 |
| 20 kW | 3,159 | 5,368 |
| 30 kW | 4,739 | 8,052 |
| 50 kW | 7,898 | 13,420 |
| 75 kW | 11,848 | 20,129 |
| 100 kW | 15,797 | 26,839 |
| 150 kW | 23,695 | 40,259 |
| 250 kW | 39,492 | 67,098 |
A wider supply temperature difference cuts the airflow needed — but too wide causes draughts and poor mixing, so comfort sets a practical floor on supply temperature.
| kW | 10 °F | 15 °F | 20 °F | 25 °F | 40 °F |
|---|---|---|---|---|---|
| 2 kW | 632 | 421 | 316 | 253 | 158 |
| 5 kW | 1,580 | 1,053 | 790 | 632 | 395 |
| 10 kW | 3,159 | 2,106 | 1,580 | 1,264 | 790 |
| 20 kW | 6,319 | 4,213 | 3,159 | 2,528 | 1,580 |
| 50 kW | 15,797 | 10,531 | 7,898 | 6,319 | 3,949 |
| 100 kW | 31,594 | 21,063 | 15,797 | 12,638 | 7,898 |
| 200 kW | 63,188 | 42,125 | 31,594 | 25,275 | 15,797 |
A common rule of thumb in comfort cooling is 400 CFM per ton of refrigeration, which corresponds to roughly a 20 °F supply temperature difference. Duct velocity then sets the cross-section.
| Location | Velocity limit | Reason |
|---|---|---|
| Main plant risers | 1,500 – 2,500 fpm | Space efficiency, noise acceptable |
| Branch ducts | 800 – 1,200 fpm | Balance between size and noise |
| Final runouts | 500 – 700 fpm | Noise in occupied spaces |
| Return air grilles | 400 – 600 fpm | Avoid whistling at the grille |
Use the figure from this calculator as the starting point.
Fresh air rates per person may exceed the thermal airflow in densely occupied spaces.
Some rooms have a minimum ACH requirement regardless of load.
The governing figure is whichever is largest — never the average.
The 1.08 constant assumes standard sea-level air. Thinner air carries less heat per cubic foot, so high-altitude installations need more airflow for the same duty.
| Altitude | Density ratio | Effective constant |
|---|---|---|
| Sea level | 1.00 | 1.08 |
| 1,000 m (3,300 ft) | 0.91 | 0.98 |
| 1,500 m (5,000 ft) | 0.86 | 0.93 |
| 2,500 m (8,200 ft) | 0.78 | 0.84 |
About 16% more airflow — and a correspondingly larger fan and duct.
29 calculators built on the same maths as the tool above. Each one has its own inputs, interactive diagrams and worked examples.
Common questions about converting kw to cfm.