What is the standard 3:1 descent rule?

The 3:1 rule states you need 3 nautical miles of distance for every 1,000 ft of altitude to lose, at a typical jet approach speed. It corresponds to roughly a 300 ft/nm gradient, or about a 3-degree glide path. This calculator lets you use your actual descent rate and groundspeed instead of the rule of thumb.

What descent rate should I use?

Light GA piston aircraft typically use 300–500 ft/min. Jets often use 1,000–2,000 ft/min in cruise descent. Check your aircraft POH or SOPs for the recommended value. The default here is 500 ft/min, suitable for most light aircraft.

Why does groundspeed matter?

Groundspeed converts descent time into distance. A faster aircraft covers more ground during the same descent time, so its TOD point is farther from the destination. Always use actual groundspeed, not indicated airspeed.

What does descent gradient mean?

Descent gradient (ft/nm) is how many feet of altitude you lose per nautical mile traveled. A standard 3-degree ILS glide slope is approximately 318 ft/nm. Comparing your calculated gradient to this reference helps verify a stabilized approach.

Can I use this for IFR approach planning?

Yes, but always cross-check with your instrument approach plate, published crossing altitudes, and ATC instructions. This tool is a planning aid only — it does not replace certified avionics or official procedures.

Top of Descent Calculator

Name: Top of Descent Calculator
Availability: InStock
Author: Nham Vu

Enter current altitude, target altitude, groundspeed, and descent rate to find your top of descent distance, time to descend, and descent gradient.

Flight Parameters

Current Altitude (ft)

Your current pressure altitude in feet

Target Altitude (ft)

Pattern altitude or crossing altitude at destination

Groundspeed (knots)

Actual groundspeed from GPS or E6B

Desired Descent Rate (ft/min)

Typical GA: 300–500 ft/min; jets: 1,000–2,000 ft/min

Top of Descent Results

Enter your flight parameters and click Calculate

3:1 Rule Comparison

Results will appear after calculation

Descent Profile

Summary

Enter current altitude, target altitude, groundspeed, and descent rate to find your top of descent distance, time to descend, and descent gradient.

How it works

Enter your current altitude (ft) and target altitude (ft) at the destination.
Input your groundspeed in knots.
Set your desired descent rate in feet per minute (default 500 ft/min).
The calculator computes the altitude to lose and the time required at that descent rate.
TOD distance is calculated from groundspeed and descent time.
The descent gradient (ft/nm) is also shown as a cross-check against the 300 ft/nm rule of thumb.

Use cases

Plan descent from cruise altitude to pattern altitude before landing.
Calculate TOD for instrument approaches in general aviation.
Cross-check ATC descent clearances against your own calculations.
Train student pilots on energy management and descent planning.
Verify TOD for mountain flying where precise altitude management is critical.
Use alongside an FMS when cross-checking computed descent points.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu