Engine Bay Airflow Calculator
Radiator airflow needed to reject engine heat.
Enter your values
Enter the engine power output in hp.
Enter the allowable air temp rise in F.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Estimated heat to radiator
- 229,050 BTU/hr
- Required radiator airflow
- 5,302 CFM
- Note
- Assumes ~30% of fuel energy rejected to coolant at rated power
Quick answer
Radiator airflow is sized from the engine's heat rejection rate and the allowable air-side temperature rise through the core.
CFM = (heat rejection BTU/hr) / (1.08 x dT)
At a glance
| What it does | Radiator airflow needed to reject engine heat. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Engine power output, Allowable air temp rise |
| Main output | Estimated heat to radiator |
| Formula | CFM = (heat rejection BTU/hr) / (1.08 x dT) |
| Cost | Free — no sign-up, no download |
How to use the Engine Bay Airflow Calculator
- 1Enter the engine's rated power output.
- 2Roughly 30% of an internal combustion engine's fuel energy goes to the coolant as heat.
- 3Divide the resulting BTU/hr by 1.08 times the allowable radiator air temperature rise.
Inputs explained
- Engine power output(hp)
- Enter the engine power output in hp.
- Allowable air temp rise(F)
- Enter the allowable air temp rise in F.
Worked example
Using the values the calculator loads with:
Inputs
- Engine power output300 hp
- Allowable air temp rise40 F
Results
- Estimated heat to radiator229,050 BTU/hr
- Required radiator airflow5,302 CFM
- NoteAssumes ~30% of fuel energy rejected to coolant at rated power
Frequently asked questions
Does this account for the fan alone?
It's the total airflow needed through the radiator core, whether from ram air, fan, or both combined.
Turbocharged engines?
Add intercooler airflow separately — this only covers the coolant radiator, not charge air cooling.
What do I need to enter into the Engine Bay Airflow Calculator?
Just 2 values: engine power output, allowable air temp rise. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Engine Bay Airflow Calculator work out the answer?
Radiator airflow is sized from the engine's heat rejection rate and the allowable air-side temperature rise through the core. It applies the formula CFM = (heat rejection BTU/hr) / (1.08 x dT) and shows the working so you can check each step by hand.
Is the Engine Bay Airflow Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.