How to measure your lawn for a robot mower
Updated
You need a tape, a sketch and twenty minutes. The mower sizer adds a margin of 20 to 50% for the shape of the lawn, so a small error in your measuring does not change the answer.
What counts as lawn
- Count every patch of grass the mower should cut, including strips along the side of the house and grass under trees it can drive beneath.
- Leave out the footprint of the house, garage and sheds, the drive and paths, patios and decks, flower and vegetable beds, ponds and gravel.
- Leave out grass the mower cannot reach: a strip behind a step, or a corner behind a gate it cannot pass. Either keep cutting those by hand or plan a route to them.
Split the lawn into shapes
Draw the lawn from above on paper. Split it into rectangles, triangles and circles, rounding curved edges off to the nearest shape. Measure each shape, work out its area, add them up and take away the beds and patios that sit inside them.
| Shape | Measure | Area |
|---|---|---|
| Rectangle | length and width | length × width |
| Triangle | base, and height at a right angle to it | base × height ÷ 2 |
| Circle | radius (half the width) | 3.14 × radius × radius |
| Half circle | radius | 3.14 × radius × radius ÷ 2 |
| Long curved strip | length along the middle, and average width | length × average width |
Tools that work
- A long tape of 30 m or 100 ft gives the most accurate lengths. A second person holding the end helps.
- A measuring wheel suits long edges and large lawns: roll it along each side and read the distance.
- Pacing gives a first estimate. Walk ten steps along a tape to find the length of one step, then count steps along each side.
- A map with an aerial view gives lengths without leaving the house. Google Maps measures the distance between points you click (Google Maps Help). Aerial images can be years old and hide edges under trees, so check one or two lengths with a tape.
Starting from the lot size
A deed, a plat or the county property record gives the size of the whole parcel. Take away everything that is not grass and you are close to the lawn area. Use the ground the house stands on, not its floor area: a two story house covers half its floor area.
| Item | sq ft | m² |
|---|---|---|
| Lot | 10,890 | 1,012 |
| House and garage footprint | minus 2,000 | minus 186 |
| Drive and paths | minus 800 | minus 74 |
| Patio | minus 300 | minus 28 |
| Beds | minus 600 | minus 56 |
| Lawn | 7,190 | 668 |
The figures are an example to show the working. Here the lawn is two thirds of the lot, so a sizer run on the lot size alone would ask for a mower sized for half as much lawn again.
Units and conversions
Makers in Europe rate mowers in square meters, and US pages often use acres or fractions of an acre. One acre is 43,560 sq ft, or 4,046.86 m². Since January 1, 2023 the acre is defined from the international foot of exactly 0.3048 m (NIST). One square meter is 10.76 sq ft.
| Acres | sq ft | m² |
|---|---|---|
| 1/8 | 5,445 | 506 |
| 1/4 | 10,890 | 1,012 |
| 1/2 | 21,780 | 2,023 |
| 3/4 | 32,670 | 3,035 |
| 1 | 43,560 | 4,047 |
Several lawns and narrow links
Add a front and a back lawn together when one mower will cut both. Then check how the mower gets between them. The sizer asks for the number of separate lawns and rules out a model whose maker says it handles one area only. Measure the narrowest gap on the way, such as a side gate or a path between a wall and a bed: some makers publish the narrowest passage their mower can work through, and the sizer compares your gap with it.
How exact it needs to be
A rating does not say how much beds, corners and short mowing hours cost, so the sizer adds a margin, a rule of thumb: it multiplies your lawn by 1.2 for one open shape, 1.35 for a typical lawn with some beds and trees, and 1.5 for a complex one. For a lawn of 800 m² (8,611 sq ft) of typical shape that margin adds 280 m². A measuring error of 5% on the same lawn is 40 m². Getting the shape right matters more than the last square meter.
Add up your shapes
The lawn area calculator below adds and takes away shapes in feet or meters, or starts from the lot size. It gives the area in m², sq ft and acres and opens the sizer with the number filled in.
Next, find the steepest stretch the mower has to climb: how to measure lawn slope.