How we test strimmers and grass trimmers

Grass trimmers can be fantastic time-savers, or terrible time wasters if you have to struggle to make them work. 

We carry out independent, scientific tests designed to find out how well grass trimmers cope with challenging situations and how easy they are to use. 

Keep reading to find out what our grass trimmers tests involve. 

Alternatively head straight to our grass trimmers reviews.

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Video: how we test grass trimmers

Watch the video below to find out what we look for in a grass trimmer, and what makes the best grass trimmers quickest and easiest to use. 

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How well does the grass trimmer cut short and long grass?

Grass trimmers are designed to cut areas of grass that are difficult to get at with the mower. This can be along the edge of flower borders and paths, close to a fence or under garden furniture. They're also often used to cut down long grass that has been allowed to grow for some time, such as where bulbs have been naturalised in the grass. 

Our experts: 

• cut short grass growing over the edge of paths, along a flower border, and against a fence
• cut grass that had been allowed to grow to a height of 15cm 
• check how neat the trimmer leaves the grass
• check if it struggles with long grass.

Will the grass trimmer cut neatly along path and lawn edges?

Many gardeners find grass trimmers useful to keep their border and path edges looking neat, so the trimmer needs to be able to cut a horizontal line.

When we cut along path and border edges, we look at: 

• how neat the finish is
• how easy it is to guide the grass trimmer in a straight line
• whether you can see the area you're cutting
• whether you have to stand in an awkward position to do this. 

Not all grass trimmers are designed to cut path edges. For example, large petrol brushcutters are made for cutting down meadows and weed patches, and are too powerful to carefully cut a neat line. Our tests show which models will excel at the tasks you need them for. 

Can the grass trimmer cut through long grass and weeds?

Some grass trimmers are designed specifically to cut down areas that are a mix of weeds and long grass, such as meadows or ground that has been left uncultivated for some time. Others struggle with this task, being too low powered to cut through tough stems and disperse the debris. 

We test all our grass trimmers on long grass and rough vegetation to find out which ones can cope.

Our tests are designed to make sure you can do this task quickly and efficiently, without the line breaking repeatedly. 

How often does the line break and is it easy to spool out more line?

In our test, we pass the trimmer head close to the edge of a spade 20 times.

It's inevitable that the line will wear down and break, but some models spool out line easily and simply, while others are less user-friendly. 

We also assess how easy it is to open the trimmer head and add or spool out more trimmer line. 

All grass trimmers tested from 2017 onwards have been tested this way, and scores for this element of the test are not comparable with those from earlier years. 

Is the grass trimmer heavy and cumbersome to carry?

Grass trimmers vary in weight between 1.5kg and almost 6kg. But this will normally depend on the type of trimmer and the job it's designed to do. 

Our expert users assess: 

• how well balanced each grass trimmer is
• how heavy it feels
• whether it's easy to carry around
• how much effort is required to make it cut.  

Vibration and noise are also important when you use a grass trimmer for a long period, so our experts assess these as well. We use a decibel meter to measure noise, and a vibration glove to see whether the grass trimmer will be uncomfortable to use. 

Is the grass trimmer easy to use?

Our experts are able to judge which grass trimmers adjust in ways that are helpful to most users. For example: 

• many grass trimmers let you adjust the position of the handles and the length of the shaft so that they're right for your height
• a rotating head will allow you to cut vertically along lawn and path edges.

Battery life on a cordless grass trimmer is also important, so we:

• time how long we can trim for before the battery is drained (we do this three times)
• check how long the battery takes to recharge from empty. 

Petrol grass trimmers can be devilishly hard to start, although some have modified designs to reduce the number of pulls you need and the force you need to apply. We assess how easy all our petrol grass trimmers are to start. 

Is the battery replaceable?

Lithium-ion batteries always deplete and eventually die over time, and at Which?, we believe that cordless products should last longer than the batteries they come with. That's why we no longer give cordless strimmers and trimmers Best Buys if spare batteries cannot be bought from the manufacturer – no matter how good they might be.

Should I buy it?

Some grass trimmers are designed to be great at edging the lawn, and others for making short work of meadow or bramble patches. Through our testing and assessments we discover which grass trimmers are best for specific jobs. 

We categorise grass trimmers into three groups: 

• Lawn edgers - best at cutting the vertical edges of the lawn, and tidying a few spots where the mower has missed. 
• General-purpose grass trimmers - able to cope with all of our tests, from edging the lawn to whacking through tough weeds. 
• Heavy-duty grass trimmers - designed for cutting down long grass and weeds, these are far too powerful for delicate jobs such as edging the lawn.

All the grass trimmers undergo the same tests, and we then rate them by including or excluding some of the tasks and weighting others by importance. 

For example, for lawn-edging grass trimmers we don't include scores for cutting long grass and weeds, and put most of the weight on the lawn-edging task. 

Similarly, when rating the heavy-duty grass trimmers we look at how well they cut long grass and weeds, and take into consideration how well they cut along a fence line and shorter grass, but ignore the marks for edging a lawn and a path. 

General-purpose grass trimmers are those that score well in all tasks. 

The overall score ignores price. For corded and petrol grass trimmers it is based on:

• 60% how well it cuts grass
• 35% ease of use
• 5% noise

And for cordless grass trimmers it is based on:

• 60% how well it cuts grass
• 25% ease of use
• 10% battery
• 5% noise

Lawn-edging grass trimmers must score 71% or more to be a Best Buy. 

General-purpose grass trimmers have to score 73% or more to be a Best Buy . 

Heavy-duty grass trimmers must score 83% or more to be a Best Buy. 

Use our grass trimmer reviews to find the best model for your garden.