How to Sharpen a Lawn Mower Blade

A sharp mower blade is the cheapest lawn upgrade there is. It costs nothing but half an hour, and the difference shows within days.

A dull blade doesn’t cut grass. It rips it. The University of Kentucky says dull blades bruise and tear the leaves, so the tips turn brown and shredded two or three days after mowing, and that shredding encourages disease. The same guide says research has shown dull blades can increase fuel usage by as much as 20 percent on gas mowers.

I sharpen blades with an angle grinder in my workshop, where grinding steel is everyday work. You don’t need a workshop, though. A file and a vise do the job. What matters is the safety steps and the balance check at the end, so I’ll go slowly on those.

One rule before anything else: your owner’s manual wins. Every maker has its own blade removal steps, bolt torque and replacement guidance. Use what follows to understand the job, and use the manual for the numbers.

Signs your blade needs sharpening

• Ragged, torn grass tips that turn white or brown a few days after you mow.
• Uneven cut, with strips of taller grass left behind.
• Visible nicks, dents or a rounded edge when you look at the blade.
• The mower bogs down or a battery mower runs out of charge sooner than it used to.
• More vibration than usual, which can also mean a bent or unbalanced blade.

How often? Home Depot’s guide suggests twice per season or about every 25 hours of use, and right after you hit a rock. Kentucky’s extension guide says most mowers will likely need it several times a year. For a typical suburban lawn mowed weekly, I’d sharpen at the start of the season and again mid-summer, plus any time you hit something hard.

What you need

• Gloves, safety glasses or a face shield, and hearing protection if you use a grinder.
• A socket or wrench that fits the blade bolt.
• A block of scrap wood to stop the blade from turning.
• A permanent marker, grease pencil or spray paint.
• A bench vise or clamps.
• A sharpening tool: a 10-inch mill file, an angle grinder with a flap disc, a bench grinder, or a drill-mounted blade sharpener.
• A blade balancer, or a nail or screwdriver to balance on.
• A wire brush or putty knife to clean the blade and deck.
• A torque wrench, if your manual gives a torque value (most do).

Toro says in its blade sharpening tips to wear a face shield or safety glasses whenever grinding or sharpening. I’d add gloves for handling the blade, even dull. The edge is still steel, and the ends are often sharp from wear.

Step 1: Kill the power

This is the step that keeps your fingers.

• Gas mower: pull the wire off the spark plug and tuck it away from the plug. A blade turned by hand can turn the engine over, and on a gas mower that can start it.
• Battery mower: remove the battery, and the safety key if your mower has one.
• Corded mower: unplug it.

Home Depot and Tom’s Guide both put this step first, and so do most manuals. Never skip it because “it’s only for a minute.”

Step 2: Tip the mower the right way

On a gas mower, the side you tip it on matters. Home Depot says to turn the mower on its side with the air filter and carburetor facing up. Tip it the other way and oil can run into the air filter and fuel can leak from the carburetor. Some guides, Home Depot included, also suggest draining the gas tank first to avoid spills. Check what your manual says about tipping; some makers specify a direction.

Battery mowers are simpler. With the battery out, you can usually tip them on their side or rear, but again, check the manual.

Step 3: Mark the blade

Before you loosen anything, mark the bottom of the blade with a marker or a dab of spray paint. Home Depot and Tom’s Guide both recommend this, and it saves a common mistake.

A mower blade only works one way up. The sail, the raised, curved back edge, has to point up toward the deck, and the sharpened edge has to lead in the direction of rotation. Put it back upside down and the mower barely cuts, while the blade still spins at full speed.

I also take a quick photo of the blade, bolt and any washers or adapters before taking them off. It costs nothing.

Step 4: Remove the blade

Wedge a block of wood between the blade and the deck so the blade can’t turn, then loosen the bolt or nut with your wrench. Tom’s Guide describes this method. Turn slowly, keep your hand out of the path of the blade if the wrench slips, and wear gloves.

Some blade bolts are very tight. Don’t put your body weight behind a short wrench at a bad angle. A longer handle and steady pressure work better.

Keep the bolt, washers and any blade adapter together in the order they came off.

While the blade is out, scrape the grass buildup off the underside of the deck. Caked clippings hurt airflow and cut quality.

Step 5: Inspect it before you sharpen

Clean the blade with a wire brush so you can see the steel. Then lay it on a flat surface.

Replace the blade instead of sharpening it if:

• It’s bent. Toro is clear on this: don’t straighten a bent blade, and don’t weld a broken or cracked one. Fit a new blade.
• It’s cracked or missing big chunks. Home Depot says the same.
• The sail is worn thin. Toro warns that wear can form a slot between the sail and the flat of the blade, and a piece of the sail can break off. A piece of steel leaving the deck at blade speed is dangerous.
• The edge has been sharpened back so far that the blade is visibly narrower than new, or below any wear limit in your manual.

In my workshop the rule for any spinning cutter is the same: if it’s cracked or bent, it goes in the scrap bin. Blades are cheap compared to what happens when one lets go. Buy the replacement the manual lists for your mower.

Step 6: Sharpen the edge

Clamp the blade in a vise with the cutting edge facing up and toward you. Now sharpen, following three rules.

Follow the original bevel. Don’t invent a new angle. Toro says to grind only the top surface and keep the original cutting edge angle. If you’re filing, Home Depot and Tom’s Guide suggest holding the file at roughly 45 degrees to match a typical factory edge. Your blade’s own bevel is the real guide.

Never grind the bottom. Toro warns that grinding the underside creates a chisel shape, and the lowest point touching the grass must be the cutting edge. The bottom of the blade stays flat.

Don’t make it razor sharp. Home Depot puts the target at about as sharp as a butter knife. A thin, razor edge chips and rolls the first time it hits a twig or pebble. Landscape Management makes the same point: an edge that’s too thin is easily damaged.

With a file

Push the file along the bevel in one direction, away from you, from the inside of the edge toward the tip. Lift it on the return. Home Depot notes most blades sharpen in fewer than 50 file strokes, and says not to saw back and forth.

With an angle grinder

This is how I do it. Fit a flap disc rather than a hard grinding disc. A flap disc removes steel more gently and leaves a smoother finish, which makes it easier to stay on the bevel.

• Rest the disc flat on the bevel, at the same angle as the existing edge.
• Move it slowly and steadily along the full length of the edge. Don’t stop in one spot.
• Use light pressure. Let the disc do the work.
• After each pass, look at the shiny strip. Tom’s Guide suggests checking the shine: if it’s uneven, adjust your angle.
• Keep the heat down. If the blade is too hot to touch, it’s too hot, Home Depot says. Dip it in water to cool it.

Heat is the trap with a grinder. Steel that turns blue or straw-colored at the edge has been overheated, and it can lose some of its temper. Short passes and frequent cooling avoid it.

With a bench grinder or drill sharpener

Same rules: original angle, top side only, light passes, keep it cool. Drill-mounted sharpeners have a guide that sets the angle for you; Home Depot suggests checking progress every four or five strokes.

Do both ends evenly

A mower blade has two cutting ends. Give both the same number of strokes or passes with the same pressure. That keeps the blade close to balanced before you even check it.

If there are deep nicks, Landscape Management suggests holding the blade perpendicular to the grinder to grind out the nick, then going back to the bevel. On a file, that takes a lot of strokes, which is why I like a grinder for nicked blades.

Step 7: Balance the blade

Skip this step and you’ll undo the good work. An unbalanced blade spinning at full speed vibrates, gives an uneven cut and adds wear to the mower.

The simple test:

1. Push a nail into a wall stud, or clamp a screwdriver horizontally in the vise.
2. Hang the blade on it through the center hole.
3. Let go. If it stays level, it’s balanced.
4. If one end drops, that end is heavier.

Toro and Landscape Management both say to remove metal from the heavy end only. Landscape Management specifies grinding from the back edge of the heavier side, not the cutting edge. Take off a little, retest, and repeat until the blade hangs level.

A cone-style blade balancer is cheap and more precise than a nail. Toro recommends checking balance after every sharpening with a balancer.

Never balance by grinding the bottom of the cutting edge. Toro is specific about that one.

Step 8: Reinstall it

1. Line the blade up using your mark: marked side facing the ground, sail pointing up toward the deck.
2. Put the washers and adapter back in the order they came off.
3. Thread the bolt by hand first.
4. Block the blade with the wood again and tighten.
5. Torque the bolt to the value in your owner’s manual. Toro says to apply final torque with a standard torque wrench, not an impact or electric wrench, and to recheck the bolt any time the blade hits a solid object.

Then turn the mower upright, reconnect the spark plug wire or put the battery back, and start it in an open area. If you feel strong vibration, stop, disconnect power again and check the blade orientation and balance.

Spare blade trick

Home Depot suggests keeping a spare blade. I agree. Swap in the spare, mow, and sharpen the dull one later at your own pace. It takes the pressure off and means you never put off sharpening because you don’t have time this weekend.

Pay a shop or do it yourself?

Most hardware stores and mower repair shops sharpen blades. It’s a good option if you don’t have a vise or grinder, or if the blade bolt is seized.

Doing it yourself is quicker once you’ve done it once, and you get to inspect the blade. For a single-blade walk-behind, it’s an easy home job. Riding mowers with two or three blades are the same process, just more of it, and the deck removal steps differ by model, so read the manual first.

Reel mower blades are different

Everything above applies to rotary mowers: push, self-propelled, riding and most robot mowers. Reel mowers shear grass between a spinning reel and a fixed bedknife. The University of Tennessee’s mower publication describes renewing worn reels and bedknives with a grinder, plus backlapping with an abrasive compound between grindings. Manual reel mowers often have a sharpening kit made for that model. Kentucky’s guide notes reel mowers usually need a specialized kit. If you’re not sure which type you have, types of lawn mowers explains the difference.

Robot mower blades

Most robot mowers use small pivoting razor-style blades on a disc rather than one long blade. Those are usually replaced, not sharpened. Follow the maker’s replacement schedule. If you’re considering one, see how robot lawn mowers work.

Keep the edge longer

• Walk the lawn before mowing. Pick up sticks, stones, dog toys and pine cones.
• Raise the deck over rough spots. Scalping bumps grinds dirt into the edge.
• Mow dry grass. Wet clippings stick to the deck and blade.
• Follow the one-third rule. Kentucky’s guide says to remove no more than one-third of the leaf at a time. Less load on the blade, healthier grass. How to mow a lawn the right way covers heights by grass type.
• Clean and sharpen before winter storage. Tom’s Guide suggests this to prevent corrosion. It’s also on my fall lawn care checklist.