It’s easy to be “out of sight, out of mind” about your lawn mower blade. After all, it keeps spinning away down there, mowing the lawn all season long. What you might not realize, however, is that as a lawn mower blade dulls, it starts to butcher your grass, making it more susceptible to disease, which can lead to bare patches and other eyesores. A dull blade also puts extra stress on your mower’s engine by forcing it to work harder.

That’s why it’s important to be vigilant about your lawn mower blade's sharpness. We recommend a few honings per season, but more might be needed if you have rocky soil conditions or extra tough turfgrass. When in doubt, inspect the lawn after mowing. If the grass blades have a lot of split ends, instead of being cleanly cropped, you probably have a dull blade; brown-tipped grass blades are another giveaway.

The actual sharpening is best left to the pros at your local hardware store or power-equipment shop, who might charge $5 to $10 per blade. It’s a good idea to own two blades so that you’ll have a sharp one handy when the other goes in for servicing.  

Here’s how to replace the lawn mower blade in five steps. To prevent a fuel spill, tackle the project after running the mower dry or siphoning out the gas (a turkey baster also does the trick).

Step 1

Remove the spark plug wire so that there’s absolutely no way the engine can jump to life. Then turn the mower on its side, with the air filter facing up to prevent oil from saturating the filter.

Step 2

Put on heavy work gloves. Secure the lawn mower blade by lodging a 2x4 block between it and the mower deck. Then use a combination wrench to loosen whatever fasteners are anchoring the blade to the drive shaft; there may be one bolt or two, with or without special washers.

Step 3

Once you’ve removed the blade assembly, take a picture of it with your phone so that you know the right way to put all the parts back together (your manual should also have a drawing of this). Then take the lawn mower blade into the shop for sharpening.

Note that if you’re an advanced DIYer, you could try sharpening the blade yourself using a metal file or a bench grinder. Peter Sawchuk, Consumer Reports’ outdoor-power-equipment expert, has had luck with Dremel's Lawn Mower & Garden Tool Sharpener attachment, available at home centers for $15.

Step 4

Put the blade assembly back together, following the picture you snapped earlier. It’s important that the blade’s cutting edge follows the direction of rotation. The wings of the lawn mower blade will be oriented up toward the mower deck and any writing on the blade will likely be facing away from the deck. If the blade bolt comes with a washer, its concave side will be facing the blade.

Step 5

Once you’re ready to tighten the bolt, put the work gloves back on. Reinsert the wooden block—on the opposite side of the mower deck this time—and tighten the bolt using a combination wrench.    

Some manufacturers recommend using a torque wrench for this task, since overtightening a bolt could cause it to crack, while undertightening it could allow the lawn mower blade to fly off while the mower is running.           

Before flipping the mower back upright, take a moment to clean out the underside of the deck and apply a light coating of silicone spray, if you have it. This will prevent the built-up clippings from compromising airflow. Along with a sharpened blade, that measure could help trim fuel costs by 25 percent, while keeping your lawn and your lawn mower in tip-top condition.

More mowers. If your mower is beyond repair, check our full lawn mower Ratings and recommendations.