What the slope number on the box means
A slope rating tells you how much the ground can rise as the mower drives forward. The brand prints it as a percent. A 30% slope rises 30 ft30 m for every 100 ft100 m forward. A 100% slope rises 100 ft100 m for every 100 ft100 m forward, so it climbs at 45 degrees.
A phone level shows the angle in degrees, and some brands print both. Segway Navimow prints the i210 AWD’s slope limit as “45% (24°)”. Those two numbers describe the same hill. The table below turns one into the other.
| Rise for every 10 ft3 m forward | Slope in percent | About this many degrees | A mower rated for this |
|---|---|---|---|
| 1 ft0.3 m | 10% | 6° | Segway Navimow i105, at the lawn edge |
| 1.7 ft0.5 m | 17% | 10° | ECOVACS GOAT O1000 RTK, at the lawn edge |
| 3 ft0.9 m | 30% | 17° | Segway Navimow i105, inside the lawn |
| 4.5 ft1.4 m | 45% | 24° | Segway Navimow i210 AWD, inside the lawn |
| 5 ft1.5 m | 50% | 27° | ECOVACS GOAT A2500 RTK, inside the lawn |
| 8 ft2.4 m | 80% | 39° | Mammotion LUBA 3 AWD 3000, inside the lawn |
How to measure your slope
You need two numbers. One is how far forward you measured, called the run. The other is how much the ground drops over that distance, called the rise. Divide the rise by the run, then multiply by 100. That gives you the slope in percent.
With two stakes, string and a line level
This is the method in a slope lesson from New York City’s Department of Environmental Protection. A line level is a small bubble level that hooks onto a string.
- Push one stake into the ground at the top of the steepest part. Tie the string to it at ground level.
- Walk straight downhill and push in the second stake.
- Hook the line level on the string. Slide the string up or down the lower stake until the bubble sits in the middle.
- Measure along the string from stake to stake. That is the run.
- Measure up the lower stake from the ground to the string. That is the rise.
Say the string runs 5 ft1.5 m and the lower stake shows a rise of 18 in46 cm. Use the same unit for both: 5 ft1.5 m is 60 in152 cm. Then 18 in46 cm divided by 60 in152 cm is 0.3. Times 100 gives a slope of 30%.
On a steep bank, keep the string short. A run of 3 ft0.9 m is fine. The math works the same way, and the lower stake does not need to be tall.
With the level on your phone
Open the level on your phone. Lay a straight board, at least 3 ft0.9 m long, on the grass, pointing straight down the hill. Rest the phone on the board, along its length. Read the angle in degrees, then look it up in the table below. If your reading falls between two rows, use the steeper row.
A board is better than the bare phone. The phone alone measures only the few inches of grass under it, and one bump changes the reading.
Turn a phone reading into a percent
| Phone level reads | Slope in percent |
|---|---|
| 6° | 11% |
| 8° | 14% |
| 10° | 18% |
| 12° | 21% |
| 14° | 25% |
| 16° | 29% |
| 18° | 32% |
| 20° | 36% |
| 21° | 38% |
| 22° | 40% |
| 24° | 45% |
| 26° | 49% |
| 28° | 53% |
| 30° | 58% |
| 32° | 62% |
| 34° | 67% |
| 36° | 73% |
| 38° | 78% |
The percents in this table are rounded to the nearest whole number, so they can differ by one from the table above.
- 45%
- Slope of 45 ft45 m rise per 100 ft100 m, about 24°
- 38%
- The same rating after our 15% wet-grass margin
- 17%
- ECOVACS's edge limit for the GOAT O1000 RTK
Where on the lawn to measure
Measure in two kinds of place.
The steepest strip inside the lawn. Walk the whole lawn and find the steepest part. Measure straight down that part. The mower must cross every part of the lawn, so the steepest part tells you how high a slope rating you need.
Any stretch of edge that goes up or down a slope. Some brands set a second slope limit for the boundary. You draw the boundary around the lawn in the app. ECOVACS lists the GOAT O1000 RTK at 45% for the lawn itself and 17% for the boundary. Segway Navimow rates the i105 for a slope of 30% inside the lawn and 10% at the boundary. So walk each edge, and measure wherever it crosses a slope.
If a steep edge sits right on the line you planned, you could draw the boundary a few feet inside it. Then trim that strip by hand.
Comparing your number with a mower’s rating
Segway Navimow says it measured the i210 AWD’s slope rating on dry grass about 2 in5 cm long. Your grass is often wet with dew or rain. So for every mower, we plan for 15% less than the brand’s slope rating. Our ranking method uses the same margin. A rating of 45% becomes about 38%. A rating of 80% becomes about 68%. To be safe, take the same 15% off an edge limit too. ECOVACS’s edge limit of 17% then becomes about 14%.
Take the Segway Navimow i210 AWD. It’s rated for a 45% slope, which climbs 45 ft45 m over 100 ft100 m forward. After our margin, it suits a lawn whose steepest strip measures about 38% or less. On a phone level, that is about 21 degrees. It uses satellite signals and a camera to tell where it is on the lawn. Segway Navimow says it keeps a steady signal under tall trees. Satellite mowers still work best where most of the lawn has open sky. Look at how much open sky your lawn has. If your steepest strip has a slope of more than about 38%, pick a mower with a higher slope rating.
The LUBA 3 AWD 3000 is rated for an 80% slope, about 39 degrees. After our margin, it suits a lawn whose steepest strip has a slope of about 68% or less. That is about 34 degrees on a phone level. All-wheel drive means every wheel can pull, so the mower has more grip on a hill. It uses three systems to tell where it is on the lawn. Satellites give its position, and data sent over the internet makes that more exact. It also has two cameras and a spinning laser called lidar. Mammotion says the three together tell the mower where it is, even under trees.
Our shortlist for hilly yards compares the i210 AWD, the LUBA 3 AWD 3000 and the LUBA mini 2 AWD 1500H. Once you know your steepest slope, compare it with the ratings on that shortlist.
You can also get a first estimate without leaving the house. On our yard finder, you type your address and trace your lawn on a satellite photo. The finder works out the lawn’s size from your outline. It reads the slope from government maps of ground height. It also makes a rough guess at tree cover from the photo, and you can correct it with a slider. Then it checks each mower against the brand’s limits. If a mower is rated for less slope or less lawn than yours, the finder leaves it off the list. A mower close to its limit stays on the list, but it ranks lower and shows a warning. Close means your slope is over 85% of the mower’s slope rating. For a mower rated for 45%, that is any slope above about 38%. Close also means your lawn is over 80% of the mower’s rated lawn size. Nobody drove these mowers on your lawn, so treat the list as a place to start. The government maps show ground height in fine detail across most of the US, and in less detail elsewhere. So check the steepest spot by hand as well.
When the slope ends at water, a road or a drop
Some lawns slope down to a pond, a street, a retaining wall or a ditch. A slope rating says nothing about what happens if the mower slides at the bottom. Read the safety section of the mower’s manual before you buy. Then walk that slope yourself. Decide whether to block it off in the app or leave it out of the lawn.