Slope ratings explained: percent, degrees and what your mower can really climb

Percent and degrees are different scales

Percent grade measures how much the ground rises for every 100 units you travel horizontally. A 30% slope rises 30 cm over 1 m of level distance. Degrees measure the angle between the slope and level ground. The two are linked by trigonometry (percent equals the tangent of the angle, times 100), and the relationship is not linear, so doubling the degrees more than doubles the percent.

This is why spec sheets can look confusing. A mower rated for 45% sounds close to one rated for 45°, but 45° is 100%, more than twice as steep.

How to measure the steepest part of your lawn

You do not need surveying tools. The simplest reliable method uses a straight board or spirit level and a tape measure.

• Find the steepest section the mower will actually drive on, including the boundary edge.
• Lay a straight board of known length (for example 1 m) on the slope, pointing straight downhill.
• Lift the low end until the board is level, checking with a spirit level or a level app.
• Measure the gap between the low end of the board and the ground. That is the rise.
• Divide the rise by the board length and multiply by 100 for percent. A 25 cm rise over 1 m is 25%.

Smartphone level apps can read the angle directly: lay the phone on the board along the slope. Take several readings and use the steepest.

Reading a manufacturer’s slope rating

Treat a slope rating as the maximum the mower can manage in good conditions, not a figure to plan right up to. Grip depends on grass density, moisture, soil and wheel design. Many manufacturers also list a separate, lower limit for slopes at the boundary, because turning on a slope near an edge is where a mower is most likely to slide.

Manufacturers sometimes round conversions differently, so a model listed as 42% and 22° is not a contradiction worth worrying about (22° is 40.4%). What matters is that your measured slope sits comfortably inside the figure.

Matching the slope to the right machine

Most two-wheel-drive robot mowers are designed for gentle to moderate lawns, often up to around 18° to 25°. All-wheel-drive robot mowers push that further. Beyond that, the safest option is often a remote-control slope mower that you operate from a distance, because steep banks are exactly where an unattended machine needs the most margin.

If a single steep bank is the only problem, consider mowing that area separately rather than buying a more expensive robot for the whole lawn.