This airflow calculator works out how much air your servers or racks need to carry their heat away, from the heat load and the temperature rise between the cold inlet and the hot exhaust. It runs entirely in your browser: nothing you type is sent to our server.
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Table of Contents
The formula
For standard air: heat (BTU/h) = 1.08 × CFM × ΔT (°F), so CFM = BTU/h ÷ (1.08 × ΔT). With watts and °F, that is roughly CFM = 3.16 × W ÷ ΔT. 1 CFM = 1.699 m³/h = 0.472 L/s.
What temperature rise to use
Servers typically heat the air by 10-15 °C (18-27 °F). The smaller the rise, the more air is needed: halving ΔT doubles the airflow. If the cooling units supply less air than the servers pull, hot exhaust air is drawn back into the front of the racks.
Practical checks
- Compare the result with the rated airflow of your cooling units and perforated floor tiles.
- Fit blanking panels in empty rack units so hot air cannot recirculate.
- Thin air at altitude carries less heat: add about 3% airflow per 300 m above sea level.
How we checked it
We checked the outputs on 11-12 October 2026: 5 kW (17,061 BTU/h) at an 11 °C rise needs 798 CFM, matching 17,061 ÷ (1.08 × 19.8).
Frequently asked questions
How much airflow does 1 kW need?
About 158 CFM at a 20 °F (11 °C) temperature rise, or about 268 m³/h.
What is ΔT in the CFM formula?
The temperature difference between the air entering the equipment and the air leaving it, in °F for the 1.08 constant.
Is the formula valid at any altitude?
The 1.08 constant assumes air near sea level and room temperature. At high altitude the air is thinner, so more airflow is needed for the same heat.