How to calculate a top of descent
For a constant flight-path angle, the distance to lose a given altitude is a matter of geometry:
distance (nm) = altitude to lose (ft) / (tan(angle) × 6,076.12 ft/nm)
descent rate (fpm) = groundspeed (kt) × tan(angle) × 6,076.12 / 60
A 3° path is about 318 feet per nautical mile. Two well-known rules of thumb sit close to that geometry:
- 3-to-1 rule: start down 3 nm before the fix for every 1,000 feet you have to lose. That is the same as 333 ft/nm, a slightly steeper stand-in for 3° (10,000 ft → 30 nm by the rule, about 31.4 nm exactly).
- Groundspeed × 5: a 3° path is actually closer to GS × 5.3 (120 kt → about 637 fpm). “Times five” (600 fpm) is an easy, slightly shallow target.
This page shows the exact path and the rules of thumb side by side so you can brief both. Use groundspeed in the descent, not indicated airspeed — convert with the true airspeed calculator and the wind triangle if you need to, and check the time with the time–speed–distance tool.
Worked example
Cruise 7,000 ft, target 1,000 ft (6,000 ft to lose), 120 kt, 3° path:
- Exact TOD ≈ 18.8 nm; 3-to-1 rule = 18 nm.
- Exact rate ≈ 637 fpm; GS × 5 = 600 fpm.
- Time to descend ≈ 9.4 minutes.
How to use it
- Enter cruise and target altitudes and the groundspeed you will use in the descent.
- Leave the angle at 3° or set a shallower or steeper path. Or switch to “Set rate” and type a target feet-per-minute.
- Optionally add a distance buffer if you want to start down early.
- Start the descent at the displayed distance from the point at which you want to be at the target altitude, and follow ATC and published restrictions.
Frequently asked questions
How do I calculate top of descent?
Divide the altitude you need to lose by the descent gradient. On a 3° path the exact gradient is about 318 feet per nautical mile, so TOD in nautical miles is altitude to lose ÷ 318. The mental 3-to-1 rule — 3 nautical miles per 1,000 feet — is a close, slightly short approximation (30 nm to lose 10,000 ft, versus about 31.4 nm exactly).
What is the groundspeed × 5 descent-rate rule?
For a 3° path, descent rate in feet per minute is about 5.3 times the groundspeed in knots. The “GS × 5” rule (120 kt → 600 fpm) is a convenient underestimate of the exact 637 fpm or so. This page shows both.
When should I start down?
Start down at the TOD distance before the point at which you want to be at the target altitude — the airport, a pattern altitude, or a crossing restriction. You can add a buffer so you arrive a little early and have room to shallow the path. Always follow ATC, published procedures, and terrain.
Can I plan from a chosen descent rate instead of an angle?
Yes. Switch to “Set rate,” enter a feet-per-minute value, and the calculator returns the distance, time, and the implied path angle at your groundspeed.