How we test robot lawn mowers

To help you choose the right robot lawn mower for you, we pay attention to the features that matter the most. Explore our guide and find out how we test robot lawn mowers.

For every mower that reaches our lab, we run through a range of tests to make sure you can effectively look after your garden without getting too involved. Top-scoring models have to soar through our grass cutting tests and ease-of-use checks, plus they need to have an impressive battery life.

Keep scrolling as we explain more on what a robot lawn mower has to do to be a Which? Best Buy. 

Alternatively, head straight to our guide on the best robot lawn mowers to find our pick of the top models.

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How well does it cut the grass?

We test each robot lawn mower on a range of grass types over a period of four weeks, assessing the efficiency of the cut and the neatness of the finish. This way, we can compare how well the mower cuts over time as well as at immediately out the box.

We see how well each robot mower handles long grass, rough grass, damp grass and a standard lawn, as well as how long it runs and charges for, on average.

Several of our testers rate the mower’s grass cutting ability on the overall appearance of two lawns it has cut of each type, the evenness and cleanness of the cut, how many spots it misses and any wear marks the wheels or blades leave behind.

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How evenly does it spread grass cuttings?

Unlike conventional lawn mowers, robot lawn mowers don't collect up the grass cuttings. Instead, they finely chop the clippings and redistribute them on the lawn in a thin layer that gives it nutrients and keeps it healthy. This process is called mulching.

The best robot mowers will cut the grass into small bits and distribute them in an even layer. You’ll want to avoid robots that struggle to chop grass properly or leave clumps of cut grass, as when the clumps decompose they can cause your grass to rot.

We also test manual lawn mowers with mulching modes, and rate them for the quality and ease of using mulching modes.

If you're looking for a conventional model instead, head to our lawn mower reviews to see which ones we recommend.

Can it handle difficult terrain and obstacles?

Your robot mower is sure to frustrate you if it gets stuck whenever it hits an obstacle or a patch of uneven ground, which is why we test them in a range of difficult scenarios to see how well they can cope.

We run each mower around areas with fences, flowerbeds, patios, trees and narrow passages, and assess how well it navigates each.

Some mowers will smartly avoid obstacles, and handle slopes and uneven ground without difficulty, whereas others will get stuck and need you to go and lift it up to get it out of trouble.

Is it easy to set up and program?

The best robot lawn mowers are easy to set up and program, from laying the boundary wire, setting it to run for the first time and connecting it up to any accompanying apps.

As part of our ease-of-use tests, we note:

• How easy it is to construct any boundary wires and pegs to establish the mowing area
• How straightforward it is to use the Android or iOS smartphone apps to program the mowers to run at certain times each day or week
• The convenience of using any manual controls, setting up the base station, setting the cutting height, lifting and carrying, changing the blades and cleaning the mower
• Any important limitations or interesting features the robot mowers have, such as the total number of zones you can program it to mow, the maximum size lawn you can cut, and the presence of temperature and rain sensors.

Our overall convenience and construction ratings also take into account the feedback of our gardening experts.

Head to our robot lawn mower reviews to see which ones are easy to use and will smartly navigate whatever difficult terrain your garden might have.

Can I replace the battery?

Lithium-ion batteries always deplete and eventually die over time.

At Which?, we believe cordless products should last longer than the batteries they come with. That's why we don’t give robot lawn mowers Best Buy recommendations if spare batteries can't be bought from the manufacturer – no matter how good the mower might be.

Is it safe to use?

Though all robot mowers sold in the UK are subject to European safety standards, we carry out additional safety tests that we believe better reflect consumer usage. 

Manufacturers would recommend, and common sense would dictate, that you should take every precaution to keep any people, animals or pets off your lawn whilst a robot mower is in use. However, this can’t always be guaranteed, and thus it’s imperative that a robot mower stops and immediately shuts off its blades when it encounters an object in a garden. 

To ensure they do this, we use a specially designed foot safety test. In this assessment we place a trainer-wearing foot probe into the path of the mower, utilising a range of different insertion angles and orientations, so that we can assess how it reacts in a number of different scenarios. The robot’s interactions with the foot probe are then rated on a scale - with the best result seeing the robot stop travelling, stop cutting and reverse away, and the worst failures seeing the robot mount the foot whilst continuing to cut. Should the latter happen then we downgrade the robot’s score.

What is the compatible app like?

Apps are an increasingly important part of using a robot mower, with newer models often coming with proprietary apps. Our test has been updated to take a deeper look at these, so that we can give you a detailed overview on their capabilities and assessments of how easy they are to set up and use with your robot mower. 

We also put these apps through an in-depth security assessment conducted by trained experts, in order to find any weaknesses that could make your data or home network vulnerable. 

Our app assessments and security testing don’t currently contribute to the robot mower's score, but if we find any concerning issues in our testing, we will contact the manufacturer to get them fixed. Serious issues might prevent a robot from being a Best Buy or lead to a Don't Buy rating unless resolved by the manufacturer.

Should I buy it?

All the tests we run make up a part of each robot mower’s total test score, which is the percentage figure we award each model.

Some tests are more important than others, though, and so carry different weights:

• 60% grass cutting ability and efficiency 
• 20% ease of use and convenience 
• 10% quietness 
• 5% instructions 
• 5% construction

For a detailed explanation of how these mowers work, and their pros and cons, head to our guide on the best robot lawn mowers.