How much air does your heat load need? Enter the power to remove and the air temperature rise you can accept, and get the required volumetric flow - with the altitude derate most quick calculators skip.
The diagram is labeled with the same symbols as the input fields below.
The airflow equation is the workhorse of electronics cooling: at sea level and room temperature it collapses to the familiar rule of thumb CFM ≈ 1.8 × Q / ΔT. This calculator keeps the full form because the shortcuts fail exactly where designs get into trouble - at altitude. Air at 2,000 m is about 18% less dense, so the same CFM carries 18% less heat; at 4,000 m the penalty is a third. Telecom and defense specs call this out; consumer designs discover it in the field.
Choose the allowed rise deliberately. A 10 C rise is a comfortable default for rack equipment; pushing to 20 C halves the required flow but raises every downstream component's inlet temperature and cuts the margin the hottest device sees. And remember the margin input: real chassis leak, recirculate, and bypass air around the heat sinks that need it - 25% is a sensible allowance, tight ducting earns less, open enclosures need more.
What this estimate cannot tell you is the temperature of any specific component - that depends on local velocity, heat-sink design, and the flow path. When the question shifts from "how much air" to "how hot does THIS part get", a conjugate network or CFD-class simulation of the actual layout is the correct escalation.
At sea level with a 10 C air rise, roughly 0.18 CFM per watt - i.e. CFM is approximately 1.8 x Q(W) / dT(C). This calculator applies the exact form with density corrected for your altitude.
Yes: heat capacity per unit VOLUME falls with density. At 2,000 m you need about 22% more CFM for the same load and rise; at 4,000 m about 50% more. High-altitude derating is a standard requirement in telecom and aerospace specifications.
Fan ratings are free-delivery (zero back-pressure). Mounted in a real chassis with filters, grilles, and heat sinks, the operating point slides down the P-Q curve - 50-70% of free delivery is typical. Size from the fan curve at your system impedance.