Furnace Airflow Calculator
CFM and temperature rise for a furnace.
Enter your values
Enter the input rating in BTU/h.
Enter the efficiency in %.
Enter the measured temperature 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.
- Output capacity
- 64,000 BTU/h
- Airflow
- 1,185 CFM
- CFM per 10,000 BTU output
- 185
- Rise check
- Within typical 30-70°F range
- Airflow at 45°F rise
- 1,317 CFM
Quick answer
Furnace airflow is output BTU divided by 1.08 times the temperature rise, which must fall inside the nameplate range.
CFM = output BTU / (1.08 x rise)
At a glance
| What it does | CFM and temperature rise for a furnace. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Input rating, Efficiency, Measured temperature rise |
| Main output | Output capacity |
| Formula | CFM = output BTU / (1.08 x rise) |
| Cost | Free — no sign-up, no download |
How to use the Furnace Airflow Calculator
- 1Read input rating and efficiency from the furnace nameplate.
- 2Measure supply and return temperatures to get the actual rise.
- 3Adjust blower speed until the rise sits mid-range on the nameplate.
Inputs explained
- Input rating(BTU/h)
- Enter the input rating in BTU/h.
- Efficiency(%)
- Enter the efficiency in %.
- Measured temperature rise(°F)
- Enter the measured temperature rise in °F.
Worked example
Using the values the calculator loads with:
Inputs
- Input rating80000 BTU/h
- Efficiency80 %
- Measured temperature rise50 °F
Results
- Output capacity64,000 BTU/h
- Airflow1,185 CFM
- CFM per 10,000 BTU output185
- Rise checkWithin typical 30-70°F range
- Airflow at 45°F rise1,317 CFM
Frequently asked questions
Rise too high?
Airflow is too low — check the filter, closed registers and duct restriction before touching the blower tap.
Rise too low?
Airflow is excessive, which can cause cold drafts and, on some furnaces, condensation in the heat exchanger.
What do I need to enter into the Furnace Airflow Calculator?
Just 3 values: input rating, efficiency, measured temperature 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 Furnace Airflow Calculator work out the answer?
Furnace airflow is output BTU divided by 1.08 times the temperature rise, which must fall inside the nameplate range. It applies the formula CFM = output BTU / (1.08 x rise) and shows the working so you can check each step by hand.
Is the Furnace 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.