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Slope calculator

Mower makers state slope limits in percent, and some in degrees. Measure the rise over a known run, and the calculator gives both and draws the slope to scale.

Updated

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Start from
Measured in

Use one unit for both numbers. The slope is their ratio, so any unit works.

centimeters

How far the ground climbs: the gap under the lifted end of a level board.

centimeters

The level distance over which it climbs: the length of the board.

Result

Enter the rise and the run.

How to measure a slope with a board and a level

  1. Find the steepest part of the lawn the mower has to climb. Measure there, not the average across the garden.
  2. Lay a straight board, about 2 m or 6 ft long, on the slope pointing straight downhill, with its upper end on the ground.
  3. Lift the lower end until a spirit level on the board reads level.
  4. Measure from the ground up to the underside of the board at the lifted end. That is the rise. The length of the board is the run.
  5. Enter both. A 30 cm rise over a 2 m board is 15%. An 18 in rise over a 6 ft board is 25%.

Percent and degrees

Percent is the rise divided by the run, times 100. Degrees measure the angle of the ground. The two do not grow in step: 100% is 45°, because the rise equals the run. 30% is 16.7° and 45% is 24.2°.

The mower sizer passes a mower on slope when its rating is at least 5 percentage points above your slope, and marks it at its limit when the margin is smaller. The methodology states the rule.

Questions about slope

Can a phone measure the slope?
Many phones have a level or inclinometer app that reads the angle in degrees. Lay the phone on a straight board along the slope rather than on the grass, then enter the degrees in the converter.
Which is steeper, 30% or 20°?
20° is steeper. It equals 36.4%.