How to use the top of descent calculator
- Enter your current altitude, target altitude and ground speed.
- Choose a descent plan: three nautical miles per 1,000 feet, a fixed descent angle, or a fixed rate of descent.
- Add an extra lead distance in the advanced options if you want to be level before the point.
- Read how far before the point to start the descent, plus the angle, rate and time.
Formula
Altitude to lose = current - target (feet)
3:1 rule: distance (nm) = altitude to lose in thousands of feet x 3
Angle plan: distance = altitude to lose / (tan(angle) x 6076.115 ft per nm)
Rate plan: distance = ground speed x (altitude to lose / rate) / 60
Rate for an angle: ground speed x 6076.115 x tan(angle) / 60 feet per minute
One nautical mile is 6,076.115 feet. A path that loses 1,000 feet in 3 nautical miles is a descent angle of about 3.14 degrees.
Three ways to lose altitude
To go from 12,000 to 3,000 feet at 120 knots, the 3:1 rule gives 9 x 3 = 27.0 nm. That is an angle of 3.14 degrees, a rate of about 667 feet per minute, and 13 minutes 30 seconds.
With a fixed 3-degree angle the same drop takes 28.3 nm and 637 feet per minute. A drop from 18,000 feet to sea level at 250 knots and 1,500 feet per minute takes 12 minutes and covers 50.0 nm, an angle of 3.39 degrees.
The idea behind the rule
A steady descent follows a straight line from where you are down to where you want to be. The horizontal distance needed is the altitude to lose divided by the slope of that line. The familiar 3:1 rule is a handy shortcut: three nautical miles for every thousand feet works out to a slope close to 3 degrees, which is why it also gives a comfortable rate of descent for common ground speeds.
The rate of descent you must hold depends on ground speed. At 3 degrees you need about 5.3 feet per minute for every knot of ground speed, so 120 knots needs about 637 feet per minute and 250 knots needs about 1,327 feet per minute.
What the number does not include
Real descents are affected by wind, slowing down, level segments and changes of path. This page gives the plain geometry for a constant ground speed. It is a learning and checking aid; follow approved procedures, charts and the aircraft manual for real flight planning.
Related tools: the wind triangle calculator for ground speed and heading, the flight arrival time calculator for the trip timeline, and the true airspeed calculator to turn indicated speeds into true speeds. For the slope of a road or trail instead of an air path, the slope calculator gives the angle from rise and run.
Assumptions and limits
- Ground speed stays constant during the descent, and the path is a straight line.
- The 3:1 rule and the angle plan use nautical miles and feet; the rate plan uses feet per minute and knots.
- The extra lead distance is added before the point at which the target altitude is reached.
- Results are for planning and learning, not for navigation.
Frequently asked questions
Where does the 3:1 rule come from?
One nautical mile is about 6,076 feet, so losing 1,000 feet over 3 nautical miles is a slope of about 1 in 18, or 3.14 degrees. Pilots remember it as 3 miles per 1,000 feet to lose.
Does wind change the starting point?
Yes. A tailwind increases the ground speed, so a given rate of descent covers more distance. A headwind does the opposite. Use your expected ground speed during the descent for the best estimate.
Why is the rate of descent tied to ground speed?
To follow a fixed angle you must drop a fixed amount for every mile covered. The faster you cover miles, the faster you must lose altitude each minute, so the rate scales with ground speed.
Can I plan a descent in other units?
The page uses feet, knots and nautical miles, the units used in aviation. To plan with kilometers or meters, convert first, for example with the unit converter, and enter the converted values.