Cooling Fan Airflow Requirement
CFM = P / (1.08 × ΔT)
Quick answer
This calculator solves CFM = P / (1.08 × ΔT) for Airflow. With Heat Dissipation 10 W, Temperature Rise 50 °C, the result is 0.1852. It solves in 3 directions, so you can enter the values you have and get the one you need.
Calculator
Formula
Description
The required airflow to cool an enclosure or heatsink depends on the total heat dissipation and the allowable air temperature rise. The constant 1.08 converts between watts and CFM·°F and accounts for the volumetric heat capacity of air at standard conditions (1.08 = ρ × cp in BTU/(min·ft³·°F) converted to W/(CFM·°C)). This is a first-order estimate for sizing cooling fans in electronics enclosures, server racks, and telecom cabinets. The actual required airflow may be higher due to altitude derating, recirculation, and non-uniform heat distribution.
Variables
- CFM — Required airflow in cubic feet per minute
- P — Total heat dissipation (W)
- ΔT — Allowable air temperature rise from inlet to exhaust (°C)
Practical Notes
A typical ΔT of 10-15°C is used for electronics cooling. At altitude, air density decreases (about 12% per 1000 m), requiring proportionally more CFM. For metric: 1 CFM ≈ 1.7 m³/h. Common fan sizes: 40 mm (5-10 CFM), 80 mm (20-40 CFM), 120 mm (40-100 CFM). Fan curves (pressure vs. flow) must be matched to system impedance (backpressure) for accurate airflow prediction.
Symbols and units
| Symbol | Meaning | Default unit | Accepted units |
|---|---|---|---|
| CFM | Airflow | — | — |
| P | Heat Dissipation | W | µW, mW, W, kW |
| dT | Temperature Rise | °C | °C |
Step by step
- 1. What you know: Heat Dissipation 10 W, Temperature Rise 50 °C
- 2. The relation: CFM = P / (1.08 × ΔT)
- 3. Substitute: P = 10 W, dT = 50 °C
- 4. Result: Airflow = 0.1852
Worked examples
Minimum range
Given: Heat Dissipation 10 mW, Temperature Rise 2.5 °C
Result: Airflow = 0.003704
Low range
Given: Heat Dissipation 100 mW, Temperature Rise 25 °C
Result: Airflow = 0.003704
Typical
Given: Heat Dissipation 10 W, Temperature Rise 50 °C
Result: Airflow = 0.1852
High range
Given: Heat Dissipation 1 kW, Temperature Rise 85 °C
Result: Airflow = 10.89
Maximum range
Given: Heat Dissipation 10 kW, Temperature Rise 850 °C
Result: Airflow = 10.89
Solving for each variable
Solve for Airflow
Heat Dissipation 10 mW, Temperature Rise 2.5 °C → 0.003704
Solve for Heat Dissipation
Airflow 0.025, Temperature Rise 2.5 °C → 67.5 mW
Solve for Temperature Rise
Airflow 0.025, Heat Dissipation 10 mW → 0.3704 °C
How Airflow changes with Heat Dissipation
| Heat Dissipation (input) | Airflow (result) |
|---|---|
| 1 W | 0.01852 |
| 2.5 W | 0.0463 |
| 5 W | 0.09259 |
| 7.5 W | 0.1389 |
| 10 W | 0.1852 |
| 15 W | 0.2778 |
| 20 W | 0.3704 |
| 40 W | 0.7407 |
| 70 W | 1.296 |
| 100 W | 1.852 |
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<p><a href="https://vbatpower.com/formulas/cooling-fan-cfm/">Cooling Fan Airflow Requirement</a> Calculator by vbatpower.com</p>Frequently asked questions
How do you calculate Cooling Fan Airflow Requirement?
Use CFM = P / (1.08 × ΔT). With Heat Dissipation 10 W, Temperature Rise 50 °C, Airflow works out to 0.1852.
What is the formula for Cooling Fan Airflow Requirement?
The formula is CFM = P / (1.08 × ΔT). The required airflow to cool an enclosure or heatsink depends on the total heat dissipation and the allowable air temperature rise.
How do you find Heat Dissipation when you know the other values?
Rearrange for Heat Dissipation: with Airflow 0.025, Temperature Rise 2.5 °C, Heat Dissipation is 67.5 mW.
How do you find Temperature Rise when you know the other values?
Rearrange for Temperature Rise: with Airflow 0.025, Heat Dissipation 10 mW, Temperature Rise is 0.3704 °C.
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