ISA Deviation Calculator
OAT and altitude in, ISA± out — the deviation that indexes every cruise chart, drives density altitude and prices your climb performance.
ISA± is aviation's temperature currency: cruise tables index by it, turbine flat ratings expire at it, and forecast temps aloft convert to it. One subtraction, used everywhere.
Formula
⚠️ For planning and education only. Weight & balance must be computed from YOUR aircraft's actual empty weight, arm and current equipment list, and verified against the POH/AFM envelope before flight.
OAT and altitude in, ISA± out — the deviation that indexes every cruise chart, drives density altitude and prices your climb performance.
About ISA Deviation Calculator
Raw OAT means little until it's compared with what the altitude is supposed to be: −5 °C is a warm day at 12,000 ft and an arctic one at sea level. ISA deviation — OAT minus the standard temperature at your pressure altitude — is the single number that performance charts index by, turbine ratings expire against and density-altitude math consumes. This calculator does the subtraction, names the regime, and prices the deviation in feet of density altitude.
How to use ISA Deviation Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula deviation = OAT − ISA(PA); each +1 °C ≈ +118.8 ft of density altitude substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use ISA Deviation Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula deviation = OAT − ISA(PA); each +1 °C ≈ +118.8 ft of density altitude with sources cited on the page
- ✓ISA± is aviation's temperature currency: cruise tables index by it, turbine flat ratings expire at it, and forecast temps aloft convert to it. One subtraction, used everywhere.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Why do POH cruise charts use ISA± columns instead of plain temperature?+
Because one deviation column serves all altitudes: 'ISA+20' describes the same relative density loss at 4,000 ft (where it means 27 °C OAT) and 12,000 ft (where it means 11 °C). Charting by raw OAT would need a different table per altitude. The deviation is the altitude-independent way to say how non-standard the air is.
How do I get the deviation from a winds-aloft forecast?+
FB forecasts list OAT directly at each level (e.g. '−07' at 9,000). Subtract ISA there (15 − 2×9 ≈ −3 °C): −07 minus −03 is ISA−4. Many EFBs print the ISA± alongside. Negative deviations mean denser air — your true 'better than book' days, worth recognizing as much as the hot ones.
What does ISA+20 actually do to my airplane?+
Adds about 2,400 ft of density altitude wherever you are: a 6,000-ft cruise behaves like 8,400, a normally-aspirated engine sheds ~8% of the power it had at standard, TAS for a given CAS rises ~4%, and climb sags accordingly. The deviation is compact precisely because its consequences are everywhere — our density-altitude tools unpack each.
Why do turbine pilots obsess over ISA deviations?+
Flat ratings: turbine engines hold rated power only up to a reference temperature (often ISA+15 at the runway, ISA+10 at cruise); beyond it, available thrust falls with every degree. Hot-and-high airline ops, max takeoff weights and 'temperature-limited' departures are all ISA-deviation arithmetic — the same subtraction this page does, with revenue on it.
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