The blade is a length, and lengths run out
Every trip to the grinder takes steel off both ends of the blade. Nobody notices because a sharpening removes something like fifty thousandths of an inch across the pair, and fifty thousandths is invisible. Twenty of them is an inch, and an inch is the whole usable range on a typical blade.
With the defaults in the form — 21 in new, 20.6 in now, a 20 in limit, 0.05 in per sharpening — there are 12 sharpenings left and there were 20 in the blade when it was new. Eight have gone. At 25 mowing hours per edge that is 300 hours left and 500 in the blade from new, which on a 120 hour season is a blade that lasts a little over four years.
Where the arithmetic bites
The interesting number is not the sharpenings, it is the cost per hour, and it is remarkably small. A blade at 18 dollars plus 20 sharpenings at 6 dollars is 138 dollars over 500 hours, which is 27.6 cents an hour for one blade and 82.8 cents an hour for a three-blade deck. Across a 120 hour season that is 99.36 dollars in blades and sharpening for the whole machine.
Set against that, replacing a blade every 25 hours instead of sharpening it works out at 72 cents an hour for one blade and 2.16 dollars an hour for three — so sharpening saves 1.33 dollars an hour on those prices. That is the comparison worth having, and it flips whenever blades are cheap and sharpening is charged by a shop.
The cross-check people find uncomfortable
If you have kept a count of sharpenings, the page divides the length actually gone by that count and reports the steel per sharpening it implies. It very often comes out higher than the figure entered, which means the grinding is heavier than assumed, or that some of the missing length went to a rock rather than to a wheel. Either way it is better to know, because it changes how many sharpenings are genuinely left.
What length does not tell you
A blade can be finished while still measuring long. The lift wing along the trailing edge is what moves the air that stands the grass up, and it erodes in sandy ground far faster than the cutting edge shortens — a blade with a rounded-off lift still cuts, but the finish goes flat and the discharge packs. The trailing edge itself thins from the underside, which is what eventually leads to a blade coming apart, and a blade bent by an impact is out of service at full length.
None of those are measurable with a rule across the tips, which is why the maker limit is where to stop rather than where to aim. The page counts the one thing that can be measured and says so.
Sharpening interval, not sharpening technique
How many hours an edge lasts is entirely local. The same blade on clean loam and on sandy ground with sticks in it will not give the same figure within a factor of two, and irrigation grit is worse than either. The 25 hours in the form is a placeholder. Watch the cut quality, note the hour meter when you take the blade off, and after two cycles you have your own number — at which point everything else on the page becomes yours rather than generic.
Questions people ask
How many times can a mower blade be sharpened?
As many as the steel allows, which is the length from new down to the limit the blade maker sets, divided by what a sharpening takes off. With a 21 in blade, a 20 in limit and 0.05 in per sharpening that is 20 from new. Sharpen more heavily and the count drops in direct proportion, which is the argument for touching up a dull edge rather than reshaping a wrecked one.
What is the wear limit on a mower blade?
A figure from the blade maker or the machine documentation, which this page takes as an input and does not supply. It exists because a blade that has been ground back has less metal holding the tip on while the tip is travelling at a few hundred feet a second. Length is also not the only limit — the lift wing and the trailing edge wear independently of it.
Is sharpening cheaper than replacing mower blades?
On the prices in the form, sharpening runs 82.8 cents an hour for a three-blade deck against 2.16 dollars an hour for replacing a blade every 25 hours, so it saves about 1.33 dollars an hour. That gap closes as blade prices fall and shop sharpening charges rise, and the page recalculates it from whatever numbers you put in rather than assuming an answer.
How much does a sharpening take off a blade?
Your own grinding decides it. Measure across the tips before and after one sharpening and the difference is your figure. It will be larger for a blade that was allowed to go properly blunt or that hit something, because reshaping a damaged edge removes far more than touching up a dull one, and that is exactly why a blade that has been abused runs out of sharpenings early.
How often should mower blades be sharpened?
By hours of cutting on your own ground rather than by a calendar, and the number is local. Sand, grit and sticks blunt an edge several times faster than clean turf. Note the hour meter when you pull the blade, do it twice, and the interval is yours. The 25 hours in the form is a placeholder to replace.