Lawn Mower Blade Size — How to Measure and What to Search For
Measure your blade three ways and this builds the exact search specification — length, centre hole, profile and count — then checks it against your deck width and tells you what to confirm. It does not guess a part number, and we explain why.
How this tool works
Start with what this tool will not do, because that is the honest part. It will not tell you a part number. We looked for an open, citable index that maps a mower model to its correct blade part number, and there is not one — that mapping lives inside each manufacturer's own parts catalogue. We could easily have generated a table that looked like one. We are not going to, because a wrong blade is not a wasted click: it is a blade that does not seat on the spindle, or one that is heavier than the machine's blade brake was certified against.
What it does instead is the part that is genuinely hard for people: turning a blade in your hand into a specification precise enough to search with, and catching the measurement mistake before you order.
The measurement that everyone gets wrong is the length. A mower blade is measured tip to tip, diagonally, straight across the centre bolt hole — from the leading corner at one end to the leading corner at the other. Measured along one edge, or across the flat middle, a 21 in blade reads short and you order a 20. Round the result to the nearest quarter inch, because that is how blades are catalogued: 20.9 in measured is a 21 in blade.
Then the centre hole, which is what actually decides fitment. Round, five-point star, six-point star, H-pattern or bowtie, or a shaped hole with flats. On a star or splined hole the drive goes through the hole, so a five-point blade simply will not seat on a six-point spindle — and five and six points look near-identical in a listing photograph. Measure across the flats, not the widest point, count the points, and photograph the hole flat against a ruler.
The consistency check. Deck geometry gives us an arithmetic relationship rather than a lookup: a single-blade rotary deck sweeps roughly its blade length, and a multi-blade deck divides the width between its blades with a small overlap. So a 21 in walk-behind takes a blade near 21 in, a 42 in two-blade deck takes blades near 21 in, and a 48 in three-blade deck takes blades near 16 in. We use that only to check the number you measured. If your measured length is more than about 1.5 in from what your deck implies, the tool says so — because the usual causes are measuring along the edge instead of the diagonal, a different blade count than assumed, or the wrong blade already fitted by whoever came before you.
Profile is a genuine choice, not a spec to match. Low-lift moves less air, so it draws less power and raises less dust — right for dry, sandy ground and side discharge. High-lift lofts clippings up the chute, which is what bagging needs, and it costs engine power in tall wet grass. Mulching blades carry a long curved edge that holds clippings in the deck to re-cut them, and they are a compromise in both directions. If you are not sure, match what came off.
The one hard citation on this page, and it matters more than the sizing. Under 16 CFR Part 1205, the US safety standard for walk-behind power lawn mowers, the blade control must "cause the blade motion in the normal direction of travel to come to a complete stop within 3.0 seconds after release of the control" (§ 1205.5(a)(1)(iii)). The standard exists, in CPSC's own words, "to reduce the risk of injury to consumers caused by contact, primarily of the foot and hand, with the rotating blade." A heavier or longer blade than the machine was built for carries more rotational energy, so an off-spec blade can put the machine outside the requirement it was certified against — before you even get to vibration and spindle bearing load. This is why "close enough" is the wrong standard for a mower blade.
So: use this to build the spec, then confirm it. Find the model and serial number — a sticker on the deck of a walk-behind, under the seat or on the frame of a rider — and look the blade up in the manufacturer's parts catalogue or with a dealer. That is the authoritative source, and this page is the thing that gets you there with the right numbers written down.
What to measure on a mower blade, and what each measurement decides
| Measurement / feature | Why it matters | What it is | How to get it right |
|---|---|---|---|
| Centre hole: Round | Fitment-critical | A plain circular hole. Measure the diameter. The most common pattern on walk-behind mowers, where the blade is clamped by a bolt and an adapter rather than driven by the hole itself. | Measure: Diameter |
| Centre hole: 5-point star | Fitment-critical | A five-pointed star or splined hole that keys onto a matching star on the spindle, so the hole itself transmits the drive. Count the points carefully - five and six look similar in a photograph. | Measure: Across the flats, plus the point count |
| Centre hole: 6-point star | Fitment-critical | A six-pointed star or splined hole, same idea as the five-point but a different key. A five-point blade will not seat on a six-point spindle. | Measure: Across the flats, plus the point count |
| Centre hole: H-pattern / bowtie | Fitment-critical | An elongated hole shaped like an H or a bowtie - two lobes joined in the middle. Measure the long axis and the short axis, and note which way it lies relative to the blade. | Measure: Long axis and short axis |
| Centre hole: D-shape, square, or something else | Fitment-critical | A hole with one or more flats machined on it, or a square, or a shape none of the above describes. Measure it in two directions and photograph it - this is the case where a photo next to a ruler is worth more than any number. | Measure: Two axes, plus a photograph |
| Profile: Standard / low-lift | Your choice | A shallow upturn at the wing. Less airflow, so less power draw and less dust - the right choice for dry, sandy or dusty conditions and for side-discharge cutting. It will not fill a bag well. | Best for: Side discharge, dusty or sandy ground |
| Profile: High-lift | Your choice | A tall, steep wing that moves much more air, so it lofts clippings up into the chute. This is what you want for bagging, and it is the one that struggles in tall wet grass because the extra airflow costs engine power. | Best for: Bagging, and getting a clean stand-up cut |
| Profile: Mulching (3-in-1) | Your choice | A curved cutting edge running further along the blade, designed to hold clippings in the deck and re-cut them small. Best with a mulching plug fitted and the chute closed. It bags less well than a high-lift and discharges less well than a low-lift - it is the compromise blade. | Best for: Mulching clippings back into the lawn |
| Profile: Not sure | Your choice | Hold the old blade on a flat surface and look at it end on. Barely any upturn at the ends is low-lift; a pronounced steep wing is high-lift; a long curved cutting edge that runs well down the blade rather than just at the tip is a mulching blade. | Best for: Match what came off the machine unless you are deliberately changing how you cut |
| Blade length | Fitment-critical | The single most mis-measured dimension on a mower blade. | Measure the blade OFF the mower, tip to tip, DIAGONALLY through the centre - from the leading corner at one end to the leading corner at the other, straight across the bolt hole. Do not measure along one edge and do not measure the flat middle section. Round to the nearest quarter inch: blade catalogues are dimensioned in quarter inches, so 20.9 in measured is a 21 in blade. |
| Blade width | Check it | Too wide fouls the deck baffles; too narrow flexes under load. | Measure across the flat middle of the blade, at right angles to its length. Most walk-behind blades are between 1.75 and 2.5 in wide. Width matters because too wide a blade can foul the deck baffles and too narrow a one flexes. |
| Blade thickness | Check it | Drives blade weight, and blade weight drives how long the machine takes to stop. | Measure the flat middle section with calipers. Typical is somewhere between 0.10 and 0.25 in. A thicker blade is heavier, and blade weight is directly related to how long the machine takes to stop - see the blade-stop note. |
| Outer holes or slots | Fitment-critical | On some machines the spindle drives through these rather than the centre hole, so wrong spacing means the blade will not seat at all. | If the blade has holes or slots either side of the centre, measure CENTRE TO CENTRE between them and note whether they are round holes or elongated slots. Some spindles drive through these rather than through the centre hole, in which case getting them wrong means the blade will not seat at all. |
| Blade-stop time | CITED SAFETY RULE | 16 CFR 1205.5(a)(1)(iii): the control must "cause the blade motion in the normal direction of travel to come to a complete stop within 3.0 seconds after release of the control." | This is the reason not to fit a heavier or longer blade than the machine was built for. The blade brake has to stop the blade within 3.0 seconds, and a heavier blade carries more rotational energy - so an off-spec blade can take the machine outside the requirement it was certified against, quite apart from the vibration and spindle load. |
FAQ
How do I measure a lawn mower blade?
Take it off the mower and measure tip to tip diagonally, straight across the centre bolt hole — from the leading corner at one end to the leading corner at the other. Do not measure along one edge and do not measure the flat middle: both read short, and that is how people end up ordering a 20 in blade for a 21 in mower. Round to the nearest quarter inch, because blades are catalogued in quarter inches. Then measure the width across the flat middle, the thickness of that middle section, the centre hole across its flats, and the centre-to-centre distance between any outer holes or slots.
What size blade does a 42 inch mower deck take?
A 42 in deck almost always runs two blades, so each one is around 21 in — the blades divide the width with a small overlap. That is deck geometry rather than a fitment lookup, though, and there are exceptions, so count the blades under your own deck before you trust it. Length also does not settle fitment on its own: two 21 in blades can have completely different centre holes, and only the one matching your spindle will seat. Look the part up against your model number.
Why won't my new mower blade fit?
Almost always the centre hole. On a star or splined hole the drive is transmitted through the hole itself, so a five-point blade will not seat on a six-point spindle — and the two are very hard to tell apart in a listing photo. The other usual causes are outer holes or slots at the wrong spacing (on machines where the spindle drives through those rather than the centre), or a blade that is correct in length but too wide and fouls the deck baffles. Measure the centre hole across the flats, count the points, and photograph it flat against a ruler.
What is the difference between high-lift and mulching blades?
Airflow and where the clippings go. A high-lift blade has a tall, steep wing that moves a lot of air to loft clippings up into the chute — that is what bagging needs, and it is also what gives the cleanest stand-up cut, at the cost of engine power in tall wet grass. A mulching blade has a longer curved cutting edge running further down the blade, designed to keep clippings circulating in the deck so they get re-cut small and drop back into the lawn. It bags less well than a high-lift and discharges less well than a low-lift. Low-lift is the third option: least airflow, least dust, best for dry sandy ground and side discharge.
Can I fit a longer blade to cut a wider strip?
No, and it is a genuinely unsafe idea rather than just an ineffective one. Under 16 CFR Part 1205 the blade has to come to a complete stop within 3.0 seconds of releasing the control, and a longer, heavier blade carries more rotational energy — so the machine can end up outside the safety requirement it was certified against. On top of that the blade tips will foul the deck, the extra mass loads the spindle bearings and the crankshaft, and the vibration will find every other weak point. Fit the length the machine specifies.
Should I sharpen or replace a mower blade?
Sharpen while there is blade left to sharpen; replace when there is not. Replace outright if it is cracked, has a chunk missing from a wing, has worn thin at the trailing edge, or has been sharpened so often that the lift wing profile is gone — the wing is what moves the air, and a blade sharpened flat cuts and bags badly no matter how keen the edge is. Whenever you sharpen, re-balance: hang the blade on a nail through the centre hole and grind the heavy end until it sits level. An unbalanced blade destroys spindle bearings.
Do I need to replace all the blades on a riding mower at once?
Yes, as a set. Mixed new and worn blades on a multi-blade deck cut at slightly different effective heights, which leaves a stepped or striped finish that looks like a deck-levelling fault, and the new blade takes more load than its neighbours. Buy two for a mid-size deck and three for a large one, fit them together, and re-check the deck level afterwards.
Why doesn't this tool just tell me the part number?
Because we could not find an open, citable source that maps mower models to blade part numbers, and inventing one would be worse than useless. A wrong blade does not seat on the spindle, or it is heavier than the blade brake was certified against — that is a safety item, not a returns hassle. So this page does the part we can do honestly: it turns your measurements into a precise search specification, catches the common measuring mistake, and points you at the authoritative source, which is your mower's model and serial number in the manufacturer's own parts catalogue or at a dealer.