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Engine Power At Altitude Calculator

Work out engine power at altitude instantly with clear inputs, formula shown and shareable results.

About Engine Power At Altitude Calculator

A normally aspirated piston engine loses power in proportion to air density. The Gagg and Ferrar relation, power = rated x (sigma - 0.117)/0.883, matches test data well and shows why such engines make only about 75% of rated power at 8,000 ft and cannot maintain cruise power much above 12,000 ft.

Formula

Power lapse

P = P0 x (sigma - 0.117) / 0.883, sigma = density ratio

Worked example

With the default values below, Engine Power At Altitude Calculator returns Power available of 136.47. Change any input above and every figure here is recalculated in your browser.

Inputs used

Rated sea-level power
180 hp
Pressure altitude
8,000 ft
Outside air temperature
-1 °C

Results

Power available
136.47 hp
Power lost
24.2% %
Density ratio (sigma)
0.7865
Power available
101.77 kW

Step by step

  1. 1Values used
  2. 2Power lapse
  3. 3Power available= 136.47 hp
  4. 4Power lost= 24.2 %
  5. 5Density ratio (sigma)= 0.7865
  6. 6Power available= 101.77 kW

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Aviation results are planning estimates only. Always verify take-off, landing, fuel, weight and balance figures against the certified Aircraft Flight Manual, current performance charts and applicable aviation regulations before flight.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Aviation results are planning estimates only. Always verify take-off, landing, fuel, weight and balance figures against the certified Aircraft Flight Manual, current performance charts and applicable aviation regulations before flight.

Frequently Asked Questions

Does this apply to turbocharged engines?

No. A turbocharger maintains manifold pressure up to its critical altitude, so power stays roughly constant until that height and only then lapses.

Why is the relation not simply proportional to sigma?

Friction losses inside the engine do not fall with altitude, so the usable power falls slightly faster than air density does.

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