How-To Guide · Updated May 2026
How to Sharpen Lawn Mower Blades
A sharp blade is the single biggest thing you can do for your lawn's health. Dull blades tear grass instead of cutting it, leaving brown tips and making the lawn more vulnerable to disease. Here's how to sharpen yours in 20 minutes.
What you'll need
- Socket wrench or spanner (usually 13 mm or 15 mm)
- Flat mill file (10-inch) or angle grinder with a flap disc
- Bench vice or G-clamp
- Safety gloves and eye protection
- A nail and a marker pen
Step-by-step guide
- Disconnect the power source. Remove the battery on cordless mowers or disconnect the spark plug lead on petrol mowers. Never work on a blade that could accidentally start.
- Remove the blade. Tip the mower on its side (air filter up on petrol models). Use a socket wrench or spanner to remove the central bolt. Mark the top of the blade so you refit it the right way up.
- Clamp the blade securely. Use a bench vice or G-clamp to hold the blade steady. Never hold it in your hand while filing.
- Sharpen with a flat file or angle grinder. File in one direction only, following the existing bevel angle (usually 30-40 degrees). Remove metal evenly from both ends to maintain balance. 10-15 strokes per side is usually enough.
- Check the balance. Hang the blade on a nail through the centre hole. If one side drops, file a little more off that end. An unbalanced blade causes vibration that damages the motor and bearings.
- Refit the blade. Bolt the blade back on with the cutting edge facing the direction of rotation (usually marked with an arrow). Torque the bolt firmly - a loose blade is dangerous.
File vs angle grinder
A flat file is slower but safer and almost impossible to over-grind. An angle grinder is faster but removes metal quickly - go too far and you'll weaken the blade edge. If you've never done this before, use a file. If you're comfortable with a grinder, use a 40-grit flap disc at low speed and keep the blade cool with water.
When to replace instead of sharpen
If the blade has deep gouges, is bent, or has been sharpened so many times the cutting edge is thin, replace it. A new blade for most cordless and petrol mowers costs £10-£25. A cracked or bent blade at 3,000 RPM is genuinely dangerous - don't risk it.