Aviation Rules of Thumb

#1 - To convert climb gradient to climb rate, multiply the gradient by the airspeed in knots.

Climb rate (fpm) = Climb gradient (%) x Airspeed (kts)

Assumes:
-- 1% climb gradient over a mile = 60 ft (1% of 6000ft=1nm)
-- No wind; groundspeed = airspeed

example: Climb gradient = 5.5 %

Airspeed = 220 knots

Climb rate = 5.5 x 220 = 1210 feet per minute

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#2 - To convert the climb gradient to the climb rate in hundreds of feet, divide your current ground speed by 60 and multiply by climb gradient.

example: If you want to gain 200 ft per nm and have a 150 kts ground speed, your rate of climb in hundreds of feet is 500. (150/60 * 200 = 500)