How to sharpen your lawn mower blade

Mowing the lawn can be dull, but your mower blade definitely shouldn’t be. Whether you have a petrol, manual or cordless lawn mower, a dulled or damaged blade could leave you with more than just a messy garden.

“A dull lawn mower blade can actually damage a lawn”, explains Rhiannon Moore, digital content writer at Toolstation. “Rather than cleanly slicing the grass tips, a dull blade can cause rough cuts and tear the grass sods out of the ground completely, creating bald patches.”

That could leave you with more than just a few yellow patches, as Paul Hicks, product manager at lawn mower manufacturer Stihl, explains: “Sharpening a lawn mower blade is just as important as mowing the lawn when it comes to grass health”, says Hicks, “as a dull blade can cause the grass to fray, making it more susceptible to diseases.”

Luckily, sharpening a lawn mower blade is a straightforward job that you can do at home and only needs doing a couple of times a year.

You can, of course, have your mower blade sharpened professionally – just pop the blade off and take it to a DIY shop, garden centre, hardware store or any local professional who offers this service. You could also replace the blade when it gets dull, but that’s wasteful and an unnecessary expense.

With the right tools and a little bit of elbow grease, sharpening the blade yourself will keep your mower running efficiently, save you money on fuel and replacement blades and protect your garden.

What tools are needed to sharpen a mower blade?

• Safety goggles and gloves
• Ear protectors, if necessary
• Spanner
• Spray paint, marker, or chalk
• Metal file/drill-powered blade sharpener/angle grinder
• Bench clamp or vice

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How to sharpen a lawn mower blade

Choose your tools

Start by choosing the tool you will use to sharpen the blade. Your options are:

• A metal file – this is the cheapest option but requires the most effort. The file should be at least 10 inches long.
• Rotary blade sharpener drill attachment – this sharpener should fit into any standard electric drill, and is pretty cheap.
• Angle grinder – the most expensive option is also the most powerful, so it will do the job the quickest but you risk shaving too much off the blade.

Turn off the power

“When sharpening a mower blade, safety comes first”, says Paul Hicks. “Before you look at the blade, make sure the mower is properly switched off and can’t accidentally restart. You should always ensure that you wear gloves and safety glasses when removing, refitting and sharpening the blade.”

After popping on your safety goggles and gloves, disconnect your mower from the mains or remove the battery. If you have a petrol mower, disconnect the spark plug wire so it can’t turn on accidentally and drain the petrol tank to prevent leaks.

Turn the mower over

Carefully lay the mower on its side or upside down, so you can get hold of the blade. On petrol mowers, tilt it so the carburettor faces upwards to stop fuel spilling out.

Mark the blade

Before you take the blade off, it’s a good idea to mark the bottom with paint, chalk or a piece of tape, so you know which way round to put it back on.

Remove the blade

Grab your spanner and unscrew the central nut to take off the blade. If it’s stuck tight and the blade tries to spin, wedge the blade with a scrap piece of wood. Put the nut or bolt to one side as you definitely don’t want to lose it.

Clean the mower deck

While the blade is off, use the opportunity to clear out grass and debris from inside the mower, where you can’t normally reach.

Check the blade for damage

Take a look at the blade to see if it’s OK for sharpening. A chip or two should still be fine, but bigger cracks or dents mean it needs replacing.

Clean the blade

Use a damp cloth, hose pipe or pressure washer – if necessary – to clean the blade thoroughly.

Sharpen your blade

Fix the blade securely in a bench clamp or vice with one cutting edge facing up. It can’t be overemphasised that you wear safety goggles and gloves while sharpening your mower blade, so put them on if you haven’t already.

To sharpen, move your chosen tool along the blade’s cutting edge, tilting your tool to match the existing angle of the blade edge. Repeat until the blade is the sharpness of a butter knife. Then flip it over and do the same on the opposite cutting edge.

Using a metal file

Hold the file at a 45-degree angle and push it in one direction along the blade. This is the method that requires the most effort, so keep pressing firmly and be prepared to make as many strokes as necessary.

Using a drill-operated blade sharpener

Fit the rotary blade sharpener onto your drill like a standard drill bit. Squeeze the trigger and move the sharpener smoothly along the mower blade’s cutting edge. You should only need a few passes.

Using an angle grinder

Run the angle grinder smoothly along the edge of the mower blade. Aim to get the blade looking evenly shiny along its whole length, so adjust your grip as necessary if you notice the top or bottom edge starts to get shinier. If the blade gets hot, pause for a moment or splash some water on it.

Check the blade’s balance

The blade needs to be equally balanced on both sides or it could damage your mower. So, after you’ve sharpened both sides of the mower blade, place the centre on a nail or screwdriver to check if it’s balanced. If one side is heavier, repeat the sharpening step to take a little more off.

Only take small amounts off and keep checking the balance as you go – you don’t want to have to keep shaving more and more off each side.

Put the blade back on the mower

Take the nut or bolt that you safely stowed away earlier and reinstall the freshly sharpened blade, checking your markings to ensure it’s the right way round. Tighten it securely and flip the mower over. Refill the petrol tank or replace the battery, if necessary, and you’re good to go.

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How to tell if your lawn mower blade needs sharpening

Simply put, your blade needs to be sharpened when it gets dull or damaged. “There are three tell-tale signs that mower blades need sharpening”, says Rhiannon Moore. “Dents or nicks on the blade surface; uneven grass height after cutting; and brown, frayed grass edges.”

If your grass is ragged and unevenly cut, rather than neatly and cleanly cut, then the blade is most likely dull. A dull blade can rip your grass from the ground and damage the roots, causing your lawn to turn yellow and potentially leaving it susceptible to disease. So, it’s worth regularly flipping the mower over and checking the blade for nicks and chips.

Also keep an eye out for changes in your mower’s performance – if it becomes less efficient, vibrates unusually or makes odd noises, it could be a sign that the blade needs sharpening. And if you notice your mower spitting out lots of rock or stones, that will be damaging the blade far more than normal wear and tear, and you’ll probably need to sharpen it, if not replace it.

How often to sharpen a lawn mower blade

“Mower blades should be sharpened after every 25 hours of use which, on average, translates to twice per year”, says Rhiannon Moore. “But this will depend on your lawn size and how often you get the mower out.”

Remember to clean and sharpen the blade before storing it for any significant length of time, especially putting it away for the winter. A damaged or dirty blade can be susceptible to corrosion, while a sharpened blade will be ready to go as soon as you take it out the following spring.

When to replace a lawn mower blade

“If you find a crack in your mower blade, don’t try to sharpen it,” warns Paul Hicks. “The blade is damaged, and you need to replace it entirely.” Small nicks and chips can be sharpened out, but sharpening won’t deal with cracks and dents, and if your blade is damaged or bent then it’s time to replace it.

Remember that sharpening makes the blade thinner, which will eventually make the blade unsafe to use. You’ll need to replace the blade after sharpening it a few times.

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What about cylinder blades?

If you have a mower with cylinder blades – which sit horizontally – it’s possible to sharpen the blades yourself with grinding paste. But you should note that this means working on the sharp blades as they’re spinning, which may be a bit much for the average person. Generally, it’s easier to get cylinder blades sharpened professionally.

Are there any common mistakes to avoid?

Rhiannon Moore emphasises the importance of checking and noting which way round the blades go before you take them off because you don’t want to leave yourself guessing when it’s time to put them back on.

“However, the worst mistake would be sharpening the blade if you’re not 100% confident on how to do it”, she adds. “A lawnmower blade is sharp and you need to have the right tools, appropriate PPE and the know-how to sharpen it safely. If in doubt, ask a professional to do it for you.”