How big a robot mower do I need?
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You size it to the grass you mow. A rating assumes an open, simple lawn. If yours has heavy tree cover or is split into two zones, a sensible rule of thumb is to divide the rating by 1.25. On that basis a 1-acre mower is a comfortable fit for about 0.8 acres. That's our own rule of thumb, not a figure from the maker.
Swipe sideways to read every column.
| Rated area | Square feet | Plan on, with heavy trees and two zones |
|---|---|---|
| 0.25 acres | 10,890 sq ft | 0.2 acres (8,712 sq ft) |
| 0.5 acres | 21,780 sq ft | 0.4 acres (17,424 sq ft) |
| 0.62 acres | 27,007 sq ft | 0.5 acres (21,780 sq ft) |
| 0.75 acres | 32,670 sq ft | 0.6 acres (26,136 sq ft) |
| 1 acre | 43,560 sq ft | 0.8 acres (34,848 sq ft) |
| 1.5 acres | 65,340 sq ft | 1.2 acres (52,272 sq ft) |
One acre is 43,560 sq ft, which is the whole lot, not just your grass.
What an acre looks like on a lot
On a lot, a quarter acre is 10,890 sq ft. In the worked example, take out a 1,200 sq ft house, a 600 sq ft drive, and 400 sq ft of beds from that lot and about 8,690 sq ft of lawn is what's left. That's smaller than the 0.25 acre a maker might assume.
A lot 90 ft by 121 ft is a quarter acre, with a front lawn and a back lawn. The Navimow i210 AWD is the one we'd look at here, rated 0.25 acres. Comfortable for about 0.2 acres with trees or two zones. how we size i210 pick.
A lot 132 ft by 165 ft is a half acre, with a 3 ft side passage between the front and the back. Two zones still matter even when your grass fits a rating. The Dreame A3 AWD Pro 2500 is rated 0.62 acres. Its page also says each of two maps needs its own charging station. Dreame pick.
A lot 180 ft by 242 ft is one acre. The Navimow X430 and the Worx Landroid Vision Cloud WR340 are both rated 1 acre. Comfortable for about 0.8 acres with trees or two zones. how we size The slope and the trees decide which of those two is the honest fit for your yard. Navimow X4 review.
When the maker already discounts the rating
If the maker already gives a smaller figure for a complex lawn, we'd use that figure as printed. Dividing it by 1.25 again would have you buying a bigger mower than you need.
Husqvarna rates the Automower 420 iQ at 1 acre and 45% (degrees not published), and 15% at the boundary. The same page describes 0.5 acre for an irregular lawn and 1 acre for systematic mowing. We'd plan from those notes, and the estimator below doesn't apply them again.Automower iQ review.
Automower 420 iQ, Husqvarna US, checked Oct 10, 2026.
The Automower 410 iQ is rated 0.5 acres, with a ±20% note on that area, and 45%, and 15% at the boundary. We'd read that note before you stretch the rating.
Automower 410 iQ, Husqvarna US, checked Oct 10, 2026.
Makers usually publish one maximum slope and don't say whether it applies in the middle of your lawn or at the edge. When a model also gives a lower figure for the edge, that's the one to use there. If your bank runs into a bed or a fence, we'd leave some margin, because edges are usually harder.
The Navimow X4 page rates the X450 at 1.5 acres and lists a map maximum of 1.8 acres for that model. That 1.8 acre figure belongs to the X450. It's not a general Navimow multiplier, so we wouldn't borrow it for a smaller model.
Navimow X4 series, Segway Navimow US store, checked Oct 10, 2026.
Wirecutter, updated Aug 3, 2026, suggests a complex lawn closer to a quarter acre for the WR340, whose maker rates it at 1 acre. If your acre is broken up, we'd plan closer to that.
Wirecutter, The Best Robot Lawn Mowers (updated Aug 3, 2026), checked Oct 10, 2026.
Check a lawn against current ratings
You can match your lawn to current ratings with the same rule, JavaScript off or on. An open 0.5 acre in one zone needs a rating of 0.5 acre. The same 0.5 acre with heavy trees, or with more than one zone, needs 0.625 acre. A 0.4 acre lawn with two zones needs 0.5 acre. That's the grass we'd size, not the whole lot.
Trees shrink the useful area
Yes, trees shrink it. Honestly, canopy over about a third of your lawn is enough to weaken a satellite fix along the mower's path. The LUBA 3 AWD 3000 is the one we'd look at here, rated 0.75 acres. Comfortable for about 0.6 acres with trees or two zones. how we size LUBA 3 AWD review. How navigation fails under trees.
Ratings that do not include a slope
These ratings don't include a slope. The Dreame A3 AWD Pro 3500 is rated 0.87 acres, and the 5000 is rated 1.2 acres. Dreame doesn't publish a slope for the 3500 or the 5000, so we wouldn't guess one for your hill.
Dreame A3 AWD Pro, Dreame Yard Care US, checked Oct 10, 2026.
What this means when you choose
- About a quarter acre, open, we'd pick the Navimow i210 AWD, rated 0.25 acres.
- About 0.62 acres with two zones, we'd pick the Dreame A3 AWD Pro 2500.
- About 0.75 acres under trees, we'd pick the LUBA 3 AWD 3000.
- An open acre, we'd pick the Navimow X430 or the Worx WR340, then compare the slope with your hill.