Climb gradient conversions
A climb gradient is a slope: how many feet you climb (or descend) for each nautical mile of ground traveled. The four common ways of writing it, at a groundspeed of GS knots, are
ft/NM = fpm × 60 / GS
fpm = (ft/NM) × GS / 60
percent = (ft/NM) / 6,076.12 × 100
degrees = arctan((ft/NM) / 6,076.12)
The same identities work for a descent. Groundspeed, not indicated airspeed, is the speed that belongs in the formula, because the gradient is defined on the ground track. A headwind (lower GS) helps you make a required slope; a tailwind hurts.
The standard 200 ft/NM departure climb
U.S. instrument departure procedures are designed, unless a larger number is published, to a climb gradient of 200 feet per nautical mile — about 3.3% or 1.9°. At 90 kt that is 300 fpm; at 120 kt, 400 fpm; at 150 kt, 500 fpm. A SID or obstacle departure procedure may publish a higher close-in or all-engine gradient. Crossing restrictions on a SID (“climb to 5,000 in 10 NM”) are also a gradient: 500 ft/NM in that example, before you even look at obstacles.
Worked example
A light airplane departing at 90 kt groundspeed, required 200 ft/NM:
- Required climb rate = 200 × 90 / 60 = 300 fpm.
- 200 ft/NM is 3.29% and 1.89°.
- If the same airplane later has to make a 400 ft/NM SID at 140 kt, the required rate is 400 × 140 / 60 ≈ 933 fpm — a very different problem, and one to check against the POH before you accept the clearance.
For engine-out glide angles, see the glide distance calculator.
Frequently asked questions
How do I convert feet per nautical mile to feet per minute?
Multiply ft/NM by groundspeed in knots and divide by 60. Two hundred ft/NM at 90 kt is 300 fpm.
What is the standard departure procedure climb gradient?
200 ft/NM in the U.S., unless a higher figure is published on the SID or ODP. Always check the procedure and the POH.
How do I convert a climb gradient to a percent or an angle?
Percent is (ft/NM) / 60.7612; the angle is arctan((ft/NM) / 6,076.12). Two hundred ft/NM is about 3.29% and 1.89°.
Why does groundspeed matter for a required climb gradient?
A faster airplane covers each mile in less time, so it needs a higher climb rate in fpm to hold the same slope.