Altitude Derate Calculator
Altitude Derate – Input Notes
- Equipment Type – Pick what you’re sizing. Choosing A/C or Heat Pump switches capacity to a tonnage dropdown and hides the efficiency box (not used for cooling).
- Altitude (ft) – Site elevation. If unsure, use city elevation. Higher altitude = thinner air = lower capacity.
- Cooling Capacity (tons) (AC only) – Select nameplate tons (1 ton = 12,000 BTU/h). Use Custom… to type tons or BTU/h.
- Custom Capacity & Units (AC only) – Enter a value, then pick tons or BTU/h. Example: 3 tons = 36,000 BTU/h. If you typed “36000” but units say “tons,” switch units to BTU/h.
- Rated Capacity or Input (BTU/h) (non-AC) – From the nameplate. For gas heat, enter the input unless your manual specifies otherwise.
- Optional Efficiency (%) (non-AC) – AFUE/thermal efficiency used to estimate output BTU/h after derate. Not shown for AC.
- Derate Rate (% per 1000 ft) – Rule-of-thumb default:
- Gas/Tankless: ~4% per 1000 ft above 2000 ft
- Cooling: ~3% per 1000 ft from sea level
- Derate Starts At (ft) – Elevation where derating begins. Defaults to 2000 ft (gas) and 0 ft (cooling). Adjust if your manual says otherwise.
How the math works: Derated = Rated × (1 − (%/1000 ft × thousands of feet above start)).
Output: Shows total derate %, adjusted BTU/h, and for AC also the equivalent tons after derate.
Output: Shows total derate %, adjusted BTU/h, and for AC also the equivalent tons after derate.