Percent and degrees
Percent grade is rise divided by run, times 100. Wikipedia gives the formula as percent grade equals 100 times the tangent of the angle. To go back, take the arctangent of the percent divided by 100. At 45 degrees, rise equals run, so the grade is 100 percent.
This is why percent numbers look large. A lawn that rises 1 ft over 2 ft of run is a 50 percent slope, and that is only 26.6 degrees. A mower maker's table lists these pairs (Robomow):
| Percent | Degrees |
|---|---|
| 10 | 5.7 |
| 15 | 8.5 |
| 20 | 11.3 |
| 25 | 14.0 |
| 30 | 16.7 |
| 35 | 19.3 |
| 40 | 21.8 |
| 45 | 24.2 |
| 50 | 26.6 |
| 60 | 31.0 |
| 100 | 45.0 |
Check which unit a spec sheet uses before you compare two models.
Measure your own slope
Robomow describes a simple method. Lay a 1 m stick on the slope, raise the downhill end until it is level, and measure the gap to the ground. A 35 cm gap equals a 35 percent slope. Measure the steepest spot, not the average. Also measure near the boundary and along the route to the dock.
What maker ratings mean
A maximum slope is not a promise for every part of your lawn. Read the fine print.
Husqvarna's manual shows the pattern. It specifies 45 percent (1:2.2, about 24 degrees) and says access on steeper slopes cannot be guaranteed. It adds that slopes above 45 percent are normally reachable only in very favorable conditions. The values are approximate and assume the tilt sensor is correctly calibrated. The same manual says that on an incline over 67 percent (about 34 degrees) the mower stops, reverses and looks for a less steep route. After two failed attempts, it stops.
Some makers give two numbers. One RTK mower's manual lists a maximum climbing ability of 80 percent (38.6 degrees) but a maximum slope at the edge of 45 percent (24 degrees). The edge number matters if your steep ground borders a drop or a street. Use the lower figure for those spots.
Ratings also assume dry grass. Robomow warns that moisture can make wheels spin or the mower slip, especially on steep slopes. Fallen leaves reduce grip too. One review site advises that slope ratings assume dry conditions, so leave headroom.
AWD vs 2WD
Drive type changes how a mower climbs and turns. Robomow says rear-wheel drive is most common. It says four-wheel drive handles steep inclines well but can struggle with turns on steep spots. Front-wheel drive is more maneuverable.
Here is a rough starting point from one review site. Under about 30 percent is generally fine with rear-wheel-drive models, 30 to 45 percent calls for AWD, and 45 to 80 percent needs AWD or 4WD with strong navigation. Treat those as guidelines, not guarantees.
Husqvarna's own range shows the gap. Its page says all Automower models manage at least 25 percent, while the 435X AWD handles 70 percent (35 degrees).
Do not pay for AWD you do not need. One buying guide says to skip AWD on a flat, firm yard.
Wire and layout on slopes
If you use boundary or guide wire, direction matters. Husqvarna says to run a guide wire straight from the bottom of the slope to the top. If you cannot, run it diagonally. Do not run it parallel to the slope, because that increases grass wear.
Robomow also notes that many mowers need a run-out zone with a maximum slope of 15 percent to turn easily. Slopes reduce area output, so plan a capacity buffer.
Safety limits
Respect the limits.
- Pick a rating comfortably above your steepest measurement.
- Keep people and pets off the lawn while the mower runs. Robotic mowers fall under a safety standard, ANSI/OPEI 60335-2-107, but no standard makes a steep slope safe to stand on.
- Skip wet-lawn runs. One RTK mower's manual says not to operate in rain or on a wet lawn.
- Mark ditches, holes and drop-offs as no-go zones, and keep 10 to 15 cm from walls and ditches, per that manual.
For the full buying checklist, see how to choose. To filter models by slope rating, use the comparison table.
