Top of Descent Calculator
Estimate top-of-descent distance using the common 3-to-1 rule.
What this tool calculates
Estimate top-of-descent distance using the common 3-to-1 rule. The main inputs on this page are Current altitude (ft), Target altitude (ft). The result is intended to be readable immediately, with the explanation below available when you need to verify the method.
Method and calculation basis
A common planning approximation is altitude to lose ÷ descent gradient. For a nominal 3° path, the distance is roughly altitude to lose in thousands of feet × 3 nautical miles, but the exact planning method should match the aircraft and operation.
Worked examples
Training cross-check
Compare a known example with the POH/AFM or training material before using new inputs.
Units
Knots with nautical miles gives time in hours; mixing statute miles and knots produces the wrong result.
Understanding the result
The output should be read in the units shown beside the result. If you are comparing alternatives, keep all unchanged assumptions identical and alter one input at a time. That makes it easier to see which value is responsible for the difference.
Assumptions, edge cases and accuracy
Planning/training aid only. Do not use a generic web result as the sole basis for flight operations. Use the aircraft POH/AFM, approved performance data, current weather, charts, NOTAMs and applicable operational rules.
Sources and methodology
- UK Civil Aviation Authority (CAA) — Flight planning, safety and operational guidance
- Aircraft POH/AFM — Aircraft-specific limitations and approved performance data
Method reviewed for Instant Solvers V10.0 · October 2026. If you find a calculation or explanation that needs correcting, use the site Contact page and include the tool URL and the values you entered.