Fan Airflow / CFM Calculator

Works out the airflow — in CFM — needed to carry a given heat load away while keeping the temperature rise within a chosen limit. The same underlying formula whether you're sizing fans for a server rack, a network cabinet, or a small equipment enclosure that's been quietly cooking itself in a cupboard somewhere.

The formula, and why it's simpler than it looks

CFM = BTU/hr ÷ (1.08 × ΔT°F). The 1.08 constant comes from the specific heat capacity and density of air at roughly sea-level, room-temperature conditions — it's a physical property of air itself, not something you tune. Feed it total heat output (converted from watts to BTU/hr — multiply by 3.412) and the maximum temperature rise you're willing to accept between intake and exhaust air, and it tells you the airflow needed to hold that line.

Why the temperature rise number is a real design choice, not a fixed constant

A smaller allowed temperature rise means more airflow required for the same heat load — you're asking more air to carry away the same amount of heat, each cubic foot picking up less heat per pass. A larger allowed rise needs less airflow but means hotter exhaust air, which matters if that exhaust is being recirculated anywhere nearby (a real risk in a cramped equipment cupboard with no dedicated exhaust path) or if downstream equipment has a strict intake temperature spec it needs to stay under. 8–15°C rise is a fairly typical starting range for IT equipment cooling — tighter for equipment with strict thermal tolerances, looser where you've got the airflow headroom to spare.

Where the number actually gets used

Once you've got a target CFM, that's the number you compare against a fan's actual rated airflow — and rated CFM on a fan's spec sheet is measured at zero static pressure, which practically never applies once air has to move through a filter, a perforated door, or any real ducting. Real-world delivered airflow is always lower than the spec sheet number once there's any resistance in the path, which is exactly why real installations build in headroom rather than sizing a fan to the calculated minimum and hoping.

This isn't a substitute for actually checking

Manufacturer-rated equipment cooling requirements, when available, should always take priority over a generic CFM formula — this calculator is for the far more common case where you're sizing supplemental cooling for a cabinet, cupboard or small server room and there's no vendor spec to fall back on.