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.

3:1 Descent Gradient Calculator

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.

Example and notes

Descending from 35,000 ft to 3,000 ft means losing 32,000 ft, so the top of descent is 32000 / 300 ≈ 107 nm before the target. At 450 knots groundspeed the required vertical speed is about 450 × 5 = 2,250 ft per minute down. Add a few miles if you must decelerate or configure before reaching the target, because the bare rule assumes a constant-speed idle descent.