Short answer: Measure only the grass the robot will mow, not your whole lot. Break the lawn into simple shapes, find each area, add them up, and subtract nothing twice. A roughly right figure is good enough, because robot mowers are sold in broad size ranges and the claimed areas are maximums.
Why do I need the lawn size at all?
Makers state the largest lawn each robot can handle. That number sets the size band you shop in, so you need a rough idea of your own. If you do not have it yet, this is the first step of how to choose a robot lawn mower.
What counts as lawn?
Count the grass you want the robot to cut. That usually means the lawn itself, including narrow strips you want mowed and any separate patches the robot will drive to.
Leave out:
- the house and any garage or shed
- patios, decks, driveways and paths
- flower beds and vegetable gardens
- a pool and its surround
- ponds, play surfaces and any area with fencing the robot will not enter
- areas under dense shrubs where grass does not grow
If your property record says the lot is 10,000 sq ft, the lawn may be well under that once the house and paving are subtracted.
Free ways to measure
1. Walk it and count steps
This needs no tools. First learn your own step length. Mark 10 steps in a straight line on flat ground, measure the distance with a tape, and divide by 10. Many adults take steps of roughly 2.5 feet (about 0.75 m), but yours may differ, so test it.
Then walk the length and the width of each section of lawn, count the steps, and convert to feet or meters.
2. Use a tape or a measuring wheel
A tape measure works for small areas. For long runs, a measuring wheel or a long tape saves time. Measure the length and width of each rectangle and write them down.
3. Draw it on a map or satellite view
Many free map apps and websites let you draw a shape on a satellite image and show the area. Trace around the grass, skipping the house, beds and paving. Pick a clear, recent image. Check the result against one measured distance, such as the length of a fence, to be sure the scale is right.
4. Look up property records
County or city property sites often list the lot size. That is the whole lot, not the lawn. Use it as an upper limit and subtract the house footprint, paved areas and beds. If you are not sure of the building footprint, the map method above is simpler.
How do I handle odd shapes?
Cut the lawn into pieces you can calculate.
- Rectangle: length times width. A lawn 20 m by 30 m is 600 m2. In feet, 60 ft by 100 ft is 6,000 sq ft.
- Triangle: half the base times the height.
- Circle: 3.14 times the radius times the radius.
- Curved or irregular edges: draw a box around the shape, find its area, and subtract the corner bits you can see are not grass. Or approximate with a few rectangles that slightly over- and under-fill the edge, so errors cancel out.
Add the pieces together. If the yard has a front lawn and a back lawn, measure each one and add them, and note whether a robot could actually drive between them.
How do I subtract the house, beds and pool?
Measure each non-lawn feature the same way and subtract. For a large flat area such as a house, a quick option is to measure the full rectangle of the yard first, then subtract the house, patio and beds as separate rectangles.
If a bed sits inside a lawn (an island), subtract its area, but remember that islands make the robot's job harder than the number suggests. See how to choose a robot lawn mower for how clutter affects the choice.
Conversion table
Use 1 m2 = 10.764 sq ft, and 1 acre = 43,560 sq ft = about 4,047 m2. To go from m2 to sq ft, multiply by 10.764. To go from sq ft to m2, divide by 10.764. To get acres, divide square feet by 43,560 or square meters by 4,047.
| Square feet | Square meters | Acres |
|---|---|---|
| 1,000 | 93 | 0.02 |
| 2,500 | 232 | 0.06 |
| 5,000 | 465 | 0.11 |
| 10,000 | 929 | 0.23 |
| 21,780 | 2,023 | 0.5 |
| 43,560 | 4,047 | 1 |
Quick check: 500 m2 is about 5,382 sq ft, and 1,000 m2 is about 10,764 sq ft.
Does exactness matter?
Not much. Robot mowers are listed by the largest area their makers say they can cover, and these are maximums. A lawn that is 480 m2 or 520 m2 makes little difference to which models suit you. What matters more is that you do not undercount.
So round up. If your pieces add up to 680 m2, think of it as about 700 m2. Then, as a common-sense rule, look for a model that claims more than that, not exactly that, because narrow gaps, slopes and clutter reduce what a robot can handle in practice.
What this means for you
You do not need a surveyor. A satellite-map trace plus one tape check is enough for most yards. Write down the total in both units, note the steepest slope and any separate patches, and bring those numbers to your shopping.
For the details-minded
Map apps can be off if the image is old or the tool measures on a flat projection. The tape check catches most errors. Count a lawn split into separate zones as separate areas, since some robots handle only one connected zone at a time and some need extra setup to cross a path or gap. Check the manual for how a model treats multiple zones. For related guides, see robot mowers for small yards and robot mowers for large lawns and acreage.
Next step
Once you have your number, take the quiz at /help-me-choose/ to narrow down models that suit your lawn.
Quick answers
Do I measure my whole lot or only the lawn? Only the lawn the robot will mow. The lot includes the house, driveway and beds.
What if I only know square feet and a maker lists square meters? Divide your square feet by 10.764. For example, 5,000 sq ft is about 465 m2.
How close does my measurement need to be? Roughly right is fine. Round up, and choose a robot that claims more than your lawn.
Does a sloped lawn count as a bigger area? Map and tape measures give the flat area, which is what most people use. Check the maker's slope limit separately, and read the manual for how it counts area.