Robot mowers on slopes: what the percentages mean

The short answer

Slope ratings are percentages, not degrees. A slope's percentage is how far it rises divided by how far it runs horizontally, times 100. So a 25% slope rises 25 cm over a metre (about 14 degrees), a 45% slope is about 24 degrees, and a 100% slope is 45 degrees, not a vertical wall.

Most two-wheel-drive robot mowers are rated somewhere between 25% and 50%. All-wheel-drive and four-wheel-drive models go much higher. Whatever the rating, treat it as a best case and leave a margin for wet UK grass.

Percentages and degrees

The conversion isn't linear, which is why percentages can mislead. Here are the ratings you'll see most often:

• 25% is about 14°
• 30% is about 17°
• 35% is about 19°
• 40% is about 22°
• 45% is about 24°
• 50% is about 27°
• 60% is about 31°
• 70% is about 35°
• 80% is about 39°
• 100% is 45°

Going the other way: a 10° slope is about 18%, 15° is about 27%, 20° is about 36% and 30° is about 58%.

If you want to work it out yourself, degrees are the arctangent of the percentage divided by 100. On a phone calculator, that's the "tan⁻¹" button: type 0.45, press tan⁻¹ and you get about 24.2.

How to measure your lawn's slope

You don't need special kit. The easiest method uses a straight board and a spirit level:

1. Find the steepest part of the lawn by eye, and the steepest bit near its edges.
2. Lay a straight board or batten exactly 1 m long on the slope, pointing straight uphill.
3. Keep the uphill end on the ground and lift the downhill end until the spirit level on top reads level.
4. Measure the gap between the downhill end and the ground, in centimetres. That number is the slope in percent. A 30 cm gap is a 30% slope.

For long slopes, repeat at a few points: banks are often steeper at the top or bottom than in the middle. Phone inclinometer apps also work; lay the phone on a straight board rather than directly on bumpy grass, and check whether the app shows degrees or percent.

Measure across the slope too. If the mower has to travel sideways along a bank, the cross-slope matters as much as the uphill one.

What current mowers are rated for

The ratings manufacturers publish vary widely:

• Small wired and budget mowers: often 25% to 35%. The Flymo EasiLife GO is rated for 25%.
• Mainstream two-wheel-drive wire-free mowers: commonly 30% to 50%.
• All-wheel drive and four-wheel drive: from about 60% upwards. The Mammotion LUBA 3 AWD is rated for 80%, the Husqvarna Automower 435X AWD NERA for 70%, and the Navimow X4 and Kress EyePilot 4×4 for 84%.
• The top of the range: the Lymow One Plus, a tracked mower for large grounds, is rated at 100%.

Why ratings are a best case

A manufacturer's slope figure generally describes what the mower can manage in good conditions. Real gardens are harder:

Wet grass. Wet grass reduces grip on any mower, and in the UK the lawn is damp for much of spring and autumn. A mower that climbs a bank on a dry July afternoon may slip on the same bank on a dewy October morning.

Turning and side slopes. Climbing straight up is easier than turning on a slope or driving across it. Mowers that mow in straight lines turn at the end of every pass, and some will cross a slope sideways.

Edges. Some manufacturers publish a lower limit for slopes along the boundary than inside the lawn. Husqvarna, for example, distinguishes the two. Near the edge the mower has to turn and stop precisely, and a slide there can take it off the lawn.

Surface. Bumpy, mossy or thin turf gives less grip than a dense, healthy lawn. Soft ground after rain can also rut.

How much margin?

There's no official figure, but as a rule of thumb, choose a mower rated at least 10 percentage points above your steepest slope, and more if that slope is at the edge of the lawn, often wet, or has to be crossed sideways. If your steepest bank is 35%, a 45% mower is a sensible minimum; a mower rated exactly 35% will probably struggle in wet weather.

Drive systems and grip

• Two-wheel drive (rear wheels driven, a castor or free wheel at the front) is the norm. Fine on flat and gently sloping lawns.
• All-wheel or four-wheel drive drives every wheel, keeping grip when weight shifts on a slope. These mowers tend to be heavier and more expensive, and some are louder.
• Tracks spread the load further; they're rare and mostly found on large-grounds machines.

Wheels matter too. Some brands sell terrain kits, spiked wheels or wheel brushes to improve grip and keep treads clean. Check what's available for the model you're considering in our accessories section.

Slopes, drops and safety

Slope ratings are about climbing, not about what's beyond the edge. Take extra care with:

• Drops and retaining walls. Keep the boundary well back from any drop so a slipping mower can't go over. A physical barrier, such as a low edge or fence, is safer than relying on the boundary alone.
• Ponds and water. Exclude them with a generous margin, and consider a physical barrier.
• Steps and paths at the bottom of a bank. A mower that slides may end up on them.

If part of a lawn is simply too steep, exclude it and cut it by hand or with a strimmer. That's often cheaper than buying a much more expensive mower just for one bank.

Looking after a sloping lawn

• Pause in heavy rain using the rain sensor or weather setting, so the mower isn't spinning its wheels on wet turf.
• Vary the mowing direction if the app allows, to avoid wheel ruts on the same line.
• Keep the wheels clean, as caked mud reduces grip.
• Raise the cutting height slightly on steep areas if the mower scalps the crest of a bank.

Next steps

Measure your steepest slope, add your margin, and then filter by it with our mower finder. Our best robot mowers for slopes page picks the strongest options at different budgets. If your garden has slopes and several separate areas, read our guide to complex gardens as well.