Top Of Descent Point Calculator

Work out top of descent point instantly with clear inputs, formula shown and shareable results.

About Top Of Descent Point Calculator

Top of descent is found by dividing the altitude to lose by the gradient in feet per nautical mile. A 3 degree path is about 318 ft/nm, which is where the pilot rule of thumb 'altitude to lose in thousands of feet times three' comes from.

Formula

Top of descent

TOD distance = altitude to lose / (tan(angle) x 6076.12 ft per nm)

Worked example

With the default values below, Top Of Descent Point Calculator returns Distance before the fix to start down of 100.49. Change any input above and every figure here is recalculated in your browser.

Inputs used

Cruise altitude: 35,000 ft
Target altitude at the fix: 3,000 ft
Planned descent gradient: 3 °
Ground speed in descent: 280 kt

Results

Distance before the fix to start down: 100.49 nm
Altitude to lose: 32,000 ft
Rate of descent: 1,486 ft/min
Descent duration: 21.53 min

Step by step

1Values used
2Top of descent
3Distance before the fix to start down= 100.49 nm
4Altitude to lose= 32,000 ft
5Rate of descent= 1,486 ft/min
6Descent duration= 21.53 min

Assumptions and limits

The result is derived only from the 4 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 2 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

Why is the rule of thumb 'times three'?

Three nautical miles per 1,000 ft is 333 ft/nm, very close to the 318 ft/nm of a true 3 degree path, so it is an easy and slightly conservative approximation.

Should I add distance for deceleration?

Yes. Most operators add a few miles to slow from cruise speed to approach speed, plus an allowance for a tailwind.