What is the 3:1 descent rule?

It is a cockpit shortcut for a roughly 3-degree descent path. You lose 300 feet of altitude for every nautical mile travelled, so dividing the altitude to lose by 300 gives the distance at which to begin the descent.

Why is the required vertical speed groundspeed times five?

On a 3-degree path you descend about 5 feet per minute for each knot of groundspeed, so multiplying groundspeed by 5 gives the target vertical speed. The exact trigonometric value is also shown and is within a few percent of the rule of thumb.

Does this account for wind?

It uses groundspeed, so if you enter a wind-corrected groundspeed the vertical speed will be correct for that condition. A tailwind raises groundspeed and therefore the vertical speed needed to stay on the same 3-degree path.

Should I add a margin to the descent point?

Many crews add a few miles to allow for deceleration and configuration. The 3:1 figure assumes a clean idle descent at constant speed, so if you must slow down before the target, start a little earlier than the bare calculation.

Is a 3-degree path always appropriate?

Three degrees is the standard for jet descents and most instrument approaches because it is comfortable and fuel-efficient. Steeper paths are sometimes needed for terrain or close-in restrictions, which require a higher vertical speed than this tool shows.

What is the 3:1 Descent Gradient Calculator?

Apply the 3:1 descent rule to find your top-of-descent distance and the required vertical speed for a standard 3-degree glide path from cruise altitude. IFR and airline crews use it to plan efficient descents without the FMS. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

3:1 Descent Gradient Calculator

Name: 3:1 Descent Gradient Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

A clean, efficient descent that arrives level at a crossing restriction or pattern altitude is mostly about starting down at the right point. This calculator applies the classic 3:1 rule to give you the top-of-descent distance and the vertical speed to hold a 3-degree path.

How it works

The 3:1 rule loses 300 feet per nautical mile, and the vertical speed follows from groundspeed:

altitude to lose        = cruise altitude − target altitude
top of descent (nm)     = altitude to lose / 300
required vertical speed = groundspeed × 5   (feet per minute)

The groundspeed-times-five rule is a near-exact match for a 3-degree path; the tool also shows the precise trigonometric vertical speed for comparison. The exact value is groundspeed × tan(3°) × 101.27 (converting knots to feet per minute), which evaluates to groundspeed × 5.24 — close enough to 5 that the shorthand holds for practical cockpit use.

Why 3 degrees and why 300 feet per mile

Three degrees is the standard instrument approach slope for a reason: it provides a comfortable, stable descent that avoids excessive nose-down attitude, minimises engine spooling from idle to go-around thrust if needed, and lands the aircraft close to the touchdown zone. Steeper approaches (4.5° or 5.5°) are used at terrain-constrained airports but require much higher vertical speeds and faster configuration.

The 300 feet-per-mile figure comes directly from the geometry:

tan(3°) ≈ 0.0524
feet per nautical mile ≈ 6076
0.0524 × 6076 ≈ 319 feet per mile → rounded to 300 for mental arithmetic

The rounding introduces a small safety margin: the actual 3-degree path is slightly steeper than the rule suggests, so starting down at the 3:1 point puts you just ahead of the path rather than behind it.

Worked examples

Jet descent from cruise:

Descending from 35,000 ft to 3,000 ft means losing 32,000 ft:

Top of descent: 32,000 ÷ 300 ≈ 107 nm before the target
At 450 knots groundspeed: 450 × 5 = 2,250 ft/min required descent rate

Turboprop or regional jet:

Descending from 18,000 ft to 2,000 ft (losing 16,000 ft):

Top of descent: 16,000 ÷ 300 ≈ 53 nm
At 280 knots groundspeed: 280 × 5 = 1,400 ft/min

Light aircraft ILS approach:

Descending from 4,000 ft (pattern) to 500 ft (threshold crossing), losing 3,500 ft:

Top of descent: 3,500 ÷ 300 ≈ 12 nm
At 120 knots: 120 × 5 = 600 ft/min

Planning margins and common pitfalls

Deceleration and configuration. The bare rule assumes a constant-speed idle descent. If you need to slow from cruise speed to approach speed before the destination, add 2–5 nm to your top-of-descent point to allow room to configure. Extending flaps or gear on the way down adds drag and may require you to adjust the descent rate.

Tailwind versus headwind. The tool uses groundspeed, so enter your wind-corrected groundspeed. A strong headwind lowers groundspeed and therefore reduces the vertical speed needed — you need less feet-per-minute to hold the same 3-degree angle when you are covering fewer miles per minute. A tailwind does the opposite.

Altitude crossing restrictions. If ATC assigns a “cross [fix] at or above [altitude]”, use that altitude as your target, not the destination airport elevation. Run the calculation again from the restriction point if you need a second descent segment after it.