How to Sharpen Lawn Mower Blades

Lawn mower blade sharpening

So your lawn is starting to take off this spring, but when you finally got out there and mowed it, did that thick green grass start looking ragged and fringed with brown? A dull lawn mower blade is the likely suspect. Read on to learn how to sharpen lawn mower blades for a clean cut every  time.

How to Sharpen Lawn Mower Blades in 5 Steps

Tools You Need

  • Work gloves and eye safety equipment
  • A hand metal file, angle grinder, mounted bench grinder, or dedicated lawn mower blade sharpener
  • A wrench to remove and reattach the blade
  • A wood block
  • A nail, screwdriver, or dedicated blade balancer

Prepare the Mower

lawn mower
Photo Credit: Pickpik

NOTE: Make absolutely sure the mower can’t start by accident. Human flesh and fast blades are a terrible combination.

Gas-powered mowers: Unscrew the spark plug. Most of the time, you should be able to leave it attached via the spark plug wire. Once the spark plug is disconnected, tilt the mower on its side, making sure the air filter and carburetor are facing up. That will prevent a belch of smoke on restarting. Tighten down the gas cap.

Electric mower: disengage the cord or battery. Mark the bottom of the blade with chalk or spray paint to make sure you don’t reinstall it upside-down. Some blades are also clearly stamped “bottom” to save you this step.

Detach Mower Blades

NOTE: To sharpen the blade, first remove it from the mower. Never sharpen the blade while it’s still mounted to the mower.

In general, blades are just bolted to the deck of the mower, but the precise way they are attached can differ. 

This would be a good time to head online to see if your lawn mower manufacturer has provided an online guide. Look up your mower and see if there are any quirks to detaching the blade. The guide also will likely tell you the proper size wrench to remove the nut holding the blade in place.

If the nut is stuck in too tight, try soaking it with penetrating oil or using a long-handled wrench for more torque. Wedge in the block of wood to keep the blades from turning while you loosen the nut.

Sharpen

Sharpening of Lawn Mower Blade
Photo Credit: seejanedrill / Youtube

Once you have the blade in hand, find the cutting edge on both sides of the blade and inspect them to see how much sharpening they need. A newer blade or one that has been kept up well could just need a few passes with the hand file or sharpening stone, but one that has been used frequently since its last sharpening may be better served by an angle grinder or bench grinder.

Follow the angle and grind on each cutting edge on each end of the blade to get a sharp edge on both sides. Dedicated lawn mower blade sharpeners make this easy, using a guide for the edges of the blades so they get the correct angle from the grinding wheel.

Oregon Products, which sells a variety of lawn-mowing products, says that a dedicated blade grinder is the best way to give a mower blade the perfect edge — sharp, but not too sharp.

“Mower blades should be aggressively sharp, but not as sharp as a razor’s edge,” the site says. “You should be able to touch the blade with your hand without getting cut.” Essentially, somewhere between butter-knife sharp and razor-sharp.

Balance the Blade

Grass damaged by dull mower blades
Grass blade A was cut cleanly with a sharp mower blade, but B, C and D show dull-blade damage. Grass blade B was bent (arrow); C and D exhibit partial and severe shredding. Photo University of Arkansas Division of Agriculture / Extension and Research

An unbalanced blade can shake the mower and potentially cause damage, so balance the blade with a few extra passes on the sharpener.

Once the blade is sharpened, make sure it’s balanced before you reattach it to the mower. This can be done by using a nail hammered into the wall of your garage or by using a screwdriver. 

Place the nail or screwdriver through the blade’s center hole and see if it’s heavier on one side. If it is, give it another quick pass with the hand file or angle grinder until it’s balanced.

Reattach

With your lawn mower still on its side, make sure the blade is oriented correctly, checking with the spray paint or chalk mark from Step 1. Take this chance to add some oil and lubricant to the crankshaft of the lawn mower or clean the underside of the mower deck. 

Place the blade back onto the mower and tighten the hardware down to the recommended torque from your owner’s manual. Reattach the spark plug or battery and you’re ready to go.

That is, at least until those grass clippings start to look ragged and torn again.

Benefit of a Sharp Mower Blade

Think about the difference between shaving with a new razor and getting one last shave out of your old one. With an old blade, your skin is more prone to cuts, infection, and irritation. With a new blade, you get a nice, clean shave.

That same principle applies to your grass. Dull mower blades don’t just make freshly cut rows look sloppy; they can cause the turf to brown or look ragged after mowing, because they’ll rip and tear the edge of the grass blades, leaving the grass more at risk for disease.

“With a dull blade, today’s rotary mower tears and shreds the grass rather than cutting it,” said Richard Hentschel, an Extension educator with the University of Illinois. Shredded grass blades will take on a brown, straw-like appearance after 24-48 hours. “Those shredded grass blades can’t really heal, exposing that tissue to disease pathogens.”

When to Sharpen Mower Blades

Mower Blade
Photo Credit: Project Farm / Youtube

To see whether you should sharpen your mower blade, pay attention to the lawn and how the mower is cutting. If you start to see the grass getting pulled and torn instead of cleanly cut, it’s likely time for another blade sharpening. If the blade is warped or too far gone, it may be time to purchase a new blade.

Hentschel says it only takes about a month for lawn mower blades to go dull, especially if you have seen clouds of dirt or sprays of rock shrapnel flying out from under the mower.

When to Sharpen Again

Sharpen your lawn mower’s blade at least once a year to make sure you stay on top of your lawn-care game. The frequency of resharpening depends on the level of use, but for the average person mowing a home lawn, twice a year should do the trick.

Hentschel says he’d be thrilled if folks sharpened blades at least once a year, but he recommends twice per mowing season — starting off with fresh, sharp blades in the spring and then sharpening them again around the end of June or the beginning of July to finish off the season strong.

“Once a year is way better than never,” Hentschel said.

FAQs

Sharpening My Blades Isn’t Working. How Do I Know If I Need a New Mower Blade?

Here are a few indicators to look out for:

  • Significant chips or dents in the blade
  • If the blade is thinner
  • Your lawn mower blade is bent
  • The blade is still dull after sharpening

What Angle Should a Lawn Mower Blade Be Sharpened?

The most common lawn mower blade angle is 30-35 degrees. When sharpening your mower blades, it’s important to keep this 30-35 degree angle over time.

What Does a Lawn Mower Tune-Up Consist Of?

The three main components involved in a mower tune-up are the air filter, spark plugs, and oil, all of which must be changed. All you need to do is change the oil and air filter and replace the spark plug. The needed parts and supplies are available at home centers and hardware stores.

How Much Does a Lawn Mower Cost?

It depends on the size and condition of your yard as well as the type of lawn mower you want. Look at the table below to see the types of lawn mowers available and how much they cost. 

Lawn Mower TypeAverage Cost
Reel Mower$104
Walk-Behind Mower$363
Robot Lawn Mower$1,470
Riding Lawn Mower$2,450

To dive deeper into lawn mowers and their prices, read LawnStarter’s pricing guide.

Kick Some Grass

Now that you know how to keep your blades sharp and ready to go, you’re ready to keep your grass green and neat all season long. If you’d like someone else to worry about the finer points of mowing and sharpening, contact a local lawn care pro to keep your lawn cleanly cut all season long.

Main Image Credit: isuaneye / iStockPhoto

Stuart Kushner

Stuart Kushner

Stuart Kushner is a writer and aspiring product designer based in New York City. When he isn’t doing either, Stuart enjoys heavy metal music, exercise, and trying new food and drinks.