Once a season for an ordinary lawn, and again mid-season if the ground is sandy or you hit something. The lawn tells you before the blade does.
What a dull blade does
It stops cutting and starts tearing. A torn grass stem does not seal cleanly, and the damaged tissue at the tip dries and discolours. That is the pale, greyish cast a lawn gets a day or two after being mown with a blunt blade — visible from a window, invisible on the blade itself.
A dull blade also costs energy. More force for a worse result, which on a cordless machine comes directly out of the runtime.
How long an edge lasts
It depends far more on what the blade meets than on hours. Sand is the enemy: a lawn on sandy soil, or one that has been top-dressed with sand, blunts a blade in a fraction of the time a clay lawn does. Stones, a buried sprinkler head or an edging strip end an edge in one pass.
A season is a reasonable default. A blade that has struck something should be looked at that day.
Sharpening, and the part people skip
Grinding the edge back is straightforward and any competent workshop will do it. What matters as much is balance. Metal removed from one end and not the other leaves the blade heavier on one side, and at mowing speed that is a vibration the whole machine and its bearings absorb.
A simple balancer is cheap. Sharpening without checking balance is how a mower that cut badly starts running badly as well.
When to replace instead
When the trailing lift wing is worn thin or eroded — that wing is what makes the airflow, and no amount of sharpening restores it. When the edge has a notch deeper than it can be ground out. When the centre hole has elongated.
How this site handles it
The blade family here lists replacements by the machines the maker names and states the length, because those are the two things that decide whether a blade fits. Where a listing is a multi-machine compatibility list — and on this shelf most are — the page reports the list as the seller’s claim rather than as a verified fit.
The questions that keep coming in
How do I know the blade is blunt without taking it off?
Look at the lawn two days after a cut. Sharp cuts heal invisibly; torn ones brown or whiten at the tip, and a lawn with a pale cast a day or two after mowing is a lawn that was torn.
Does an unbalanced blade really matter?
It is the reason a sharpened blade sometimes runs worse than a dull one. An out-of-balance blade at mowing speed shakes the whole machine and works on the motor bearings every second it runs.
Last reviewed 26 September 2026