How to Measure Your Lawn Size and Slope for a Robot Mower

Updated 2026-09-24

Robot mowers are sold by lawn area and rated for a maximum slope. Guessing either one is the most common way to buy the wrong mower. Both take about ten minutes to measure.

Measure the area

Measure only the grass, not your whole lot. Driveways, the house, patios and flower beds don't count.

Option 1: Google Maps (desktop).

1. Find your house in satellite view and zoom in close.
2. Right-click the edge of your lawn and choose Measure distance.
3. Click around the lawn's outline. When you click back on the first point, Maps shows the total area.
4. Repeat for each separate lawn section (front, back, side strip) and add them up.

Option 2: Your property records. Many county websites list lot size. Subtract roughly the house footprint, driveway and landscaping. This is less accurate — use it as a sanity check.

Converting: 1 acre = 43,560 sq ft. A quarter acre is 10,890 sq ft.

Buy some headroom. Manufacturers rate area under good conditions. If your lawn uses more than about 85% of a mower's rated area, it may struggle to keep up in peak growing season.

Measure the slope

Slope is given as a percentage (rise ÷ run × 100) or in degrees. They are not the same: a 45% slope is 24°, and a 100% slope is 45°.

The string method:

1. Tie a string to a stake at the top of your steepest section.
2. Run it 10 feet out horizontally toward the downhill side, using a line level (a few dollars at a hardware store) to keep it level.
3. Measure straight down from the string to the ground at the 10-foot mark.
4. Slope % = drop ÷ 10 feet × 100. A 3-foot drop over 10 feet is a 30% slope.

The phone method: lay a phone on a straight board on the slope and use a level or inclinometer app to read the angle in degrees.

Measure the steepest part of the lawn, especially near edges — mowers find slopes hardest when they have to turn on them.

Got your numbers? Put them in the yard matcher.