Take-off Distance Calculator
Correct your POH take-off figures for density altitude, weight, wind, surface and slope, then check them against the runway you have.
Enter the book figures from your POH (sea level, ISA, max weight, paved and level), then the conditions on the day. Elevation is always in feet, weights in any unit as long as both match.
- Take-off distance required (over 50 ft)
- โ
- Ground roll
- โ
- Total correction
- โ
- Pressure altitude
- โ
- Density altitude
- โ
- Headwind / tailwind
- โ
- Crosswind
- โ
Where the correction comes from
A planning aid built on published rules of thumb, not a certified performance calculation. Always fly the numbers in your aircraft's current POH or AFM.
How it works
Start with the two numbers your flight manual gives for a sea-level, standard-day, maximum-weight
take-off from a dry level runway: the ground roll and the total distance to clear a 50 ft obstacle.
Everything else is a correction applied on top. Pressure altitude comes from the field elevation and
the altimeter setting (PA = elevation + (29.92 โ inHg) ร 1000, or 27 ft per hPa below
1013.25). Density altitude follows from the temperature against the ISA value at that pressure
altitude, DA = PA + 118.8 ร (OAT โ ISA), where ISA cools by 1.98 ยฐC per 1,000 ft from
15 ยฐC at sea level.
The corrections are the standard rules of thumb: 10% more distance per 1,000 ft of density altitude,
compounded; distance scaling with the square of the weight ratio; 10% less per 9 knots of headwind
and 10% more per 2 knots of tailwind; a surface factor of 1.2 for short dry grass, 1.3 for wet grass
and 1.25 for soft ground or snow (UK CAA Safety Sense figures); and 5% for each 1% of uphill slope.
Headwind and crosswind components come from the angle between the wind and the runway,
headwind = speed ร cos ฮธ and crosswind = speed ร |sin ฮธ|, with any gust
used for the crosswind and for a tailwind so the answer stays on the cautious side. The 1.33 safety
factor at the end is the margin the CAA recommends private pilots apply.
Deliberately, no credit is taken for a density altitude below sea level or for a downhill slope, and runway designators and reported surface winds are both magnetic, so the two work together without a variation correction. Everything is arithmetic done in your browser: nothing about your aircraft, your weights or your destination is uploaded, and there is no sign-up and no tracking.
Frequently asked questions
How do I calculate take-off distance?
Start with the ground roll and the distance over a 50 ft obstacle from your aircraft's flight manual at sea level, ISA and maximum weight, then apply a correction for each condition that differs. This tool works out pressure altitude and density altitude from your elevation, altimeter setting and temperature, adds roughly 10% per 1,000 ft of density altitude, scales by the square of the weight ratio, applies the 10%-per-9-knots-headwind and 10%-per-2-knots-tailwind rules, multiplies by a surface factor for grass, snow or soft ground, adds 5% per 1% of uphill slope, and finally applies a 1.33 safety factor.
Why does density altitude matter so much on take-off?
Thin air means less thrust from the propeller, less power from the engine and less lift at any given groundspeed, so the aeroplane needs a higher true airspeed and a longer run to reach it. The usual rule of thumb is about 10% more take-off distance for every 1,000 ft of density altitude, which compounds: a 4,000 ft density altitude is roughly 1.10โด = 1.46, so nearly half as much runway again. On a hot summer afternoon at a 5,000 ft field the density altitude can easily be 8,000 ft.
How much extra runway does grass need?
The factors used here come from the UK CAA Safety Sense guidance: short dry grass on firm ground multiplies the take-off distance by 1.2, wet grass by 1.3, and soft ground, snow or standing water by 1.25 or more. Slope adds another 5% for each 1% uphill. No credit is taken for a downhill slope, because the conservative assumption is the one that keeps you inside the fence.
Can I trust these numbers for a real flight?
No. This is a planning aid built on published rules of thumb, not a certified performance calculation, and it knows nothing about your specific aeroplane, its age, its engine or your technique. The figures your flight manual gives were flown by a test pilot on a new aircraft. Always work from the current POH or AFM for the aircraft you are flying, and treat any result here as a sanity check rather than an authority.