Work out the true size of your lawn in square feet and acres, subtract the parts you do not mow, and see how long it actually takes to cut. Add as many sections as your garden has. Nothing is stored and nothing is sent anywhere.
1. Lawn sections
Add a row for each separate piece of grass, such as a front lawn and a back lawn.
2. Areas to subtract
Anything inside your measurements that is not grass: the house footprint, patio, driveway, beds, pond, shed.
3. How you cut it now
How to measure your lawn
The single biggest mistake is using your plot size. A quarter acre plot is not a quarter acre of grass once the house, driveway, patio, beds and paths are taken out. On a typical suburban lot the difference is often 40 to 60 percent, which is the difference between buying the right mower and buying too much of one.
Measure the grass itself. Break an awkward garden into simple shapes rather than trying to find one formula that fits it: a rectangle for the main lawn, a triangle for a corner, a circle for a rounded end. Add each as its own row above, then subtract the things sitting inside those measurements.
Pacing is accurate enough for this. A normal adult stride is close to 2.5 feet, so counting paces along each edge and multiplying by 2.5 will get you within a few percent, which is well inside the tolerance that matters when choosing a mower.
The formulas
Standard geometry, nothing proprietary, so you can check any result by hand.
- Rectangle: length × width
- Circle: π × (diameter ÷ 2)²
- Triangle: ½ × base × height
- Acres: square feet ÷ 43,560
- Square metres: square feet × 0.0929
Mowing time is calculated as area divided by an effective cutting rate, where the rate is deck width × travel speed × 80 percent. The 80 percent accounts for overlap between passes and time spent turning. The three presets are a 21 inch push mower and a 30 inch walk-behind at a 2.5 mph walking pace, and a 42 inch riding mower at 4 mph. That works out at roughly 2 hours 20 minutes per acre with a push mower, which lines up with the figure most people quote from experience.
Why lawn area decides which robotic mower you need
Robotic mowers are rated by the area they can maintain, not by how fast they cut. That is a genuinely different way of thinking about capacity, and it catches people out.
A robot does not need to finish your lawn in one session. It cuts a little every day, returns to its dock to charge, and goes out again. What the coverage rating really tells you is whether the machine can keep on top of your lawn's growth over a week, not whether it can cut it all in one go.
So the number above is the one to match against a mower's rating. Then check two things the rating does not tell you:
- Your steepest slope. A mower rated for your area will still fail on a bank steeper than its grade rating. Find the worst part of your garden and buy for that.
- How many separate zones you have. A front and back lawn separated by a driveway is a different problem to one continuous area, and not every mower handles it the same way.
Frequently asked questions
How many square feet are in an acre?
43,560 square feet. A quarter acre is 10,890 square feet, and a half acre is 21,780.
How much of my property is actually lawn?
Less than you think. On a typical suburban lot, the house, driveway, paths, patio and beds commonly account for 40 to 60 percent of the plot. That is exactly why this calculator lets you subtract them.
How long does it take to mow an acre?
Roughly 2 hours 20 minutes with a 21 inch push mower, about 1 hour 40 minutes with a 30 inch walk-behind, and around 45 minutes on a 42 inch riding mower. Trimming and edging are extra.
What size robotic mower do I need for a quarter acre?
A quarter acre is 10,890 square feet, which sits inside the range most residential robotic mowers are built for. Coverage is unlikely to be your constraint, so choose on navigation type and slope rating instead.
Do I measure the whole garden or just the grass?
Just the grass. Robotic mower coverage ratings refer to the area actually being cut, so including the house and driveway will push you toward a larger machine than you need.
How accurate does my measurement need to be?
Within about ten percent is fine. Coverage ratings are broad bands rather than precise thresholds, so pacing out your lawn is accurate enough to choose correctly.
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