Descent Distance

Calculator

Where should the descent begin?

deg
Calculated descent distance A side view showing the descent path from the starting altitude to the target altitude. 6500 ft 2000 ft 14.1 NM 4500 ft 3.0° TOP OF DESCENT TARGET POINT
Altitude to lose 4500 ft
Path gradient 319 ft/NM
Descent distance 14.1 NM 1-in-60: about 15 NM (÷ 300)

Method

From angle to distance

01

Find the height

Only the altitude that must be lost belongs in the calculation.

6500 − 2000 = 4500 ft
02

Turn the angle into ft/NM

Tangent gives the vertical side of a right triangle when its angle and horizontal side are known.

tan(angle) = vertical ÷ horizontal tan 3.0° × 6080 = 318.6 ft/NM

One nautical mile is normally 6076 ft. Using 6080 ft is a practical rounded value.

03

Find the distance

Divide the altitude to lose by the number of feet lost in each nautical mile.

4500 ÷ 318.6 = 14.1 NM

1-in-60 mental calculation

4500 ÷ 300 = 15 NM
3.0° → ÷ 300 3.5° → ÷ 350 4.0° → ÷ 400

Explore the approximation

The 1-in-60 rule

Adjustable 1-in-60 triangle An adjustable flight-path angle compares the exact tangent with the 1-in-60 approximation. 1 NM ≈ 6000 ft for mental calculation 319 ft 3.0°
Move the angle to compare the geometric result with the rule of thumb.

For small angles, a change of about 1° produces approximately 1 unit of vertical change over 60 horizontal units.

Using tangent 319 ft/NM tan 3.0° × 6080
Using 1 in 60 300 ft/NM 3.0 × 6000 ÷ 60

Difference: 19 ft/NM (5.8%)

For mental calculation, round one nautical mile to 6000 ft. One degree is then approximately 100 ft/NM. Multiply the angle by 100 to get the divisor: 3.0° uses 300, 3.5° uses 350 and 4.0° uses 400.

Practice · Distance

Find the top of descent

You are at 7000 ft and need to reach 1500 ft. Use the 1-in-60 calculation for 3.0°.

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