How to measure lawn slope for a robot mower
Updated
A straight board, a spirit level and a tape measure are all you need, and the measurement takes a few minutes.
Percent and degrees
Percent is the rise divided by the run, times 100. A slope that climbs 15 cm over 1 m of ground is 15%. Degrees measure the angle between the ground and level. The two scales do not grow in step: 100% is 45°, where the rise equals the run.
| Degrees | Percent |
|---|---|
| 10° | 17.6% |
| 15° | 26.8% |
| 20° | 36.4% |
| 24° | 44.5% |
| 30° | 57.7% |
| 35° | 70.0% |
| 40° | 83.9% |
| 45° | 100% |
Mixing up the two scales is an easy mistake. A mower rated for 45% climbs 24.2°, about half of a 45° slope. Read the unit on the spec sheet before you compare.
Find the steepest stretch
- Walk the lawn and look for the steepest part the mower has to drive up or across: a bank, the side of a raised bed, the ramp down to a lower lawn.
- Measure straight down the slope, along the line water would run. Across the slope the reading is lower and does not show the climb.
- A short, steep bump counts when the mower has to cross it to reach the rest of the lawn. A bank you plan to leave out of the mowing area does not.
Measure with a board and a level
- Lay a straight board, about 2 m or 6 ft long, on the slope, pointing straight downhill.
- Keep the uphill end on the ground and lift the downhill end until a spirit level on the board reads level.
- Measure from the underside of the board at the lifted end straight down to the ground. That is the rise.
- Divide the rise by the length of the board and multiply by 100. A 30 cm rise over a 2 m board is 15%. An 18 in rise over a 6 ft board is 25%.
- Repeat at two or three points along the bank and keep the highest reading.
Other ways to measure
- A phone level. The Measure app on an iPhone shows how many degrees a surface varies from level (Apple Support), and many Android phones have similar apps. Lay the phone on the board, not on the grass, so tufts and dips do not tilt it, then convert the degrees to percent.
- A string line. For a long bank, tie a string to a stake at the top at ground level, pull it level to a stake at the bottom with a line level, and measure the height of the string above the ground at the lower stake. Divide by the length of the string.
Slope limits in the dataset
65 of the 65 robot mowers in the Botsize dataset publish a slope limit. 46 of them are rated for 45% (24.2°) or more, and 19 for 70% (35.0°) or more. The lowest published limit is 25% and the highest 84%.
Some makers also publish a lower limit for the edge of the lawn, where the mower turns along the border. 28 records carry one, and each model page lists it as the steepest slope at the edge. The sizer compares your slope with the main limit, so read the edge figure when the steep part of your lawn is at its border.
How the sizer uses your slope
- Pass when the model's published limit is at least 5 percentage points above your slope.
- At its limit when the limit is at or above your slope but less than 5 points above it. The model moves to tight.
- Fail when the limit is below your slope. The model does not fit.
- Unknown when the maker publishes no slope limit. The model goes to needs a check and is never passed or failed on a guess.
On a 30% lawn a model rated for 35% passes, one rated for 32% is at its limit and one rated for 27% fails. The margin leaves room for measuring error and for wet grass. The methodology states the rule.
Convert your reading
The slope calculator below turns your rise and run into percent and degrees, or converts one into the other, and draws the slope to scale.
With area and slope in hand, the mower sizer checks every model at once. For a steep lawn, start from robot mowers for steep slopes.