How to sharpen lawn mower blades

Woman looking at lawn mower blades
(Image credit: Getty Images)

Knowing how to sharpen lawn mower blades is a vital skill to help keep your lawn healthy and your mower in tip top condition. A lawn mower with dull blades doesn't just make mowing more difficult, it can also be detrimental to the health of your grass as well. 

Even the very best gas lawn mowers (opens in new tab) or electric lawn mowers (opens in new tab) can suffer if their blades aren't kept sharp, so learning how to sharpen lawn mower blades correctly will give your mower a new lease of life. 

You might be surprised to learn that the sharpness of a lawn mower's blade can affect your grass, but a dull blade can actually make your lawn more prone to disease and less able to cope with extreme weather conditions. This means that prolonged dry spells or patches of heavy rain can leave your grass beleaguered, rather than flourishing into the lush, green lawn you'd prefer. 

Why should you sharpen your lawn mower blades?

The most important aspect of mowing your lawn is actually cutting your grass with the lawn mower blades. If the blades aren't sharp, then they'll actually rip and tear the grass rather than scything through it neatly. These tears will weaken the grass and leave it vulnerable to disease and fungal growth, which can degrade the quality of your lawn.

It's also worth noting that even the best riding lawn mower (opens in new tab) will be difficult to use properly if it has dull blades. This can make the task of mowing your lawn longer and more stressful than it needs to be. 

How to sharpen lawn mower blades

David Gower, chairman of the Lawn Association (opens in new tab), says: “Sharpening is something of a skillset we’ve lost. A tool to sharpen lawn mower blades (opens in new tab) could consist of a simple file (or angle grinder if you are feeling a little braver) will take minutes to create a new sharp edge. And remember, even new blades don’t always come sharpened. But a great idea is to buy a spare blade, which gives you all week to sharpen before the next mow!’’

What the expert says…

David Gower explains what happens when lawn mower blades are not sharpened: “Blunt mowing of grass comes down to many lawnmower manufacturers not being lawn specialists and often advise sharpening a blade with a service (once per year) is sufficient. It won’t be. When we cut or mow grass with a blunt blade it literally tears the grass blades causing the grass to turn dull at the end of the tips. Do this all over your lawn and you instantly lose the color in the lawn. It will then struggle to take in food and water and ultimately be unhealthy….’’

Step 1

You can remove the blade to sharpen it, although it is possible to do it in situ. Whichever method you choose, ensure that the mower is unplugged if it is electric, or that there is no chance of the mower starting by accident (you may have to tape spark plugs for instance).

Step 2

If you remove the blade, be sure to mark which is the top or bottom edge, so that you replace it the right way up. It won’t matter how sharp your blade is - it won’t cut if it’s upside down! The best way to hold the blade is by clamping it into a vice before you start work.

Step 3

Use your file to sharpen from the top edge of the cutting blade (and remember the file will only cut on the push stroke - do not run it backwards and forwards). It should take no more than 50 strokes to get the blade sharp again - it would be quicker with an angle grinder, but unless you are very confident, you run the risk of damaging or overheating the lawn mower blade.

The lawn mower blades will have come from the factory with a specific angle on their cutting edge - make sure you keep to that angle each time you sharpen - it is usually between 35 and 45 degrees.

Step 4

When you replace the blades in your lawn mower, make sure that they are balanced correctly, or they may cause vibration and ultimately damage to your mower

Safety note: If you have a cylinder mower, it is best to leave sharpening to a professional.

How often should you sharpen lawn mower blades?

David Gower says: “This is very much down to any individual and how nice a lawn you want. Grass obsessives should be doing this each week. For many, every two to four weeks will still be beneficial. It’s down to your own effort but never blame the tools when they’re your tools!’’

You should generally sharpen your lawn mower blades after at least every 20-25 hours of mowing (although doing it more often will certainly not hurt!). However, if you find yourself struggling to mow your lawn before this time frame is up, then it's worth sharpening them a little earlier. 

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Naomi MacKay

Naomi MacKay has been a freelance writer and editor for the past 20 years. She previously made the move from local newspapers and consumer technology magazines into the gardening press as Assistant Editor at Garden Answers magazine, and has also worked for the Royal Horticultural Society, and writes garden columns for a number of publications.

With contributions from