How do you calculate top of descent?

The 3-degree rule: distance in NM = altitude to lose in thousands of feet × 3, and required rate = groundspeed × 5. Losing 6,000 feet means starting down about 18 NM out; at 120 knots groundspeed that takes roughly 600 feet per minute. The exact math is time = altitude ÷ rate, distance = groundspeed × time.

What descent rate should I plan in a light airplane?

Around 500 feet per minute keeps passengers' ears comfortable and stays well within cruise-power descent capability for most GA airplanes. Use the calculator's required-rate mode: if the distance remaining demands much more than 700–1,000 fpm, you started down late; add track miles or slow down rather than diving.

How far can an airplane glide from altitude?

Glide distance = glide ratio × height. A typical trainer's 9:1 ratio from 5,000 feet AGL gives 45,000 feet, about 7.4 NM in still air. Best-glide speed and the ratio come from your POH; headwind, a windmilling propeller, and turns all shorten it, so treat the still-air circle as an outer bound.

Descent & Glide Calculator: Top of Descent, Rate & Glide Range

Solve for:

Altitude to lose (ft)

Cruise altitude minus pattern altitude (or the crossing restriction)

Groundspeed in the descent (kt)

Descents usually run faster than cruise; check the GPS on the way down

Descent rate (fpm)

500 fpm is the comfortable GA standard

Distance available (NM)

Track miles to the airport or the fix

Enter the descent to see the profile.

Start down –

Time in descent – minutes

Gradient – ft per NM

The 3° rule: start down at 3 NM per 1,000 ft to lose, and fly groundspeed × 5 as your rate. Losing 6,000 ft means a top of descent 18 NM out; at 120 kt that is 600 fpm. It is the same geometry an ILS glideslope uses, and it is the difference between a descent you planned and a dive you apologized for.

How descent planning works

Time = altitude to lose ÷ rate. 6,000 ft at 500 fpm is 12 minutes.
Distance = groundspeed × time. 12 minutes at 120 kt is 24 NM: that is your top of descent.
Required rate (when the distance is fixed, like a crossing restriction) = altitude ÷ time available. The moment that number gets big, you are negotiating with physics.
Gradient = altitude ÷ distance, in ft/NM; ÷ 100 gives roughly the descent angle in degrees (a 318 ft/NM path is the standard 3°).

Two cockpit shortcuts worth memorizing: 3 × altitude-in-thousands = distance, and groundspeed × 5 = the 3° rate. This calculator is those rules with the rounding removed.

Glide range

Same triangle, engine out. Distance = glide ratio × height. Enter your POH numbers:

Best glide ratio (X : 1)

From the POH; ~9:1 for a typical trainer

Height above terrain (ft AGL)

Still-air glide distance

–

Time to the ground (at ~70 kt)

–

Still air, wings level, best-glide speed, propeller as tested. Headwind, turns, and a windmilling prop all shorten it; treat this as the outer edge of the circle, not the middle.

Typical POH best-glide figures (verify in your POH; weight and configuration matter): Cessna 152 and 172 around 9:1 at 60–68 KIAS, Piper Archer about 10:1, Cirrus SR22 roughly 9:1 at 88 KIAS, Diamond DA40 up to 11:1. From 5,000 ft AGL that is a 7 to 9 NM circle in still air.

How pilots use these numbers

Every arrival: compute the top of descent before engine start (altitude to lose × 3 NM). Starting down there at 500 fpm arrives at pattern altitude 2 to 3 miles out, no diving, no shock cooling, no ear complaints.
Crossing restrictions: "cross 10 west at 4,000" turns into a required rate in one calculation. If it comes out over about 1,000 fpm, ask for lower now or add track miles.
The glide circle habit: knowing your AGL height and ratio means always having a radius in mind. Route selection over hostile terrain (water, mountains, forest at night) is choosing altitudes that keep a landable spot inside that circle.
Engine-out drill: height ÷ 1,000 × ratio ÷ 6 ≈ NM of glide. Practicing that mental math is what makes the real one calm.

Common Questions

Why plan descents at 500 fpm instead of just diving down late?

A late, steep descent stacks problems: airspeed creeping toward the yellow arc in bumps, engine temperatures dropping fast, ears hurting, and arriving at the pattern high and fast, which is how overruns and go-arounds start. The 3° profile costs nothing but a decision made 20 miles earlier.

Does wind change the top of descent?

Yes, through groundspeed: a tailwind on the way down means more track miles for the same rate, so start earlier; a headwind compresses it. Use the groundspeed you expect in the descent (usually cruise GS plus a few knots), and re-check against the GPS once established.

Is the glide ratio the same at any weight?

The ratio barely changes with weight, but the best-glide speed does: heavier means a faster best-glide speed for the same angle. What really costs distance is flying the wrong speed; a trainer glided 10 knots off best-glide can lose a quarter of its range. Nail the speed, then the ratio takes care of itself.

Related tools and guides

Time-Speed-Distance Calculator

The groundspeed your top of descent depends on.

Holding Pattern Entry

The hold that sometimes comes before the descent.

Density Altitude Calculator

What the airport's thin air does to the other end of the flight.

Plan the whole arrival, not just the slope

FlightDecide checks weather, NOTAMs, fuel, performance, and W&B for your specific route and window, and scores the flight GO, CAUTION, or NO GO with the data behind every call.

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Educational tool for flight-planning practice. It is advisory only and not a substitute for your POH, current charts, or your own judgment as pilot in command (14 CFR 91.3). Sources: FAA Airplane Flying Handbook (FAA-H-8083-3C), Ch. 4 and 18; Instrument Procedures Handbook (FAA-H-8083-16B), Ch. 3. Last reviewed: July 17, 2026.