How to Measure a Mower Blade Before You Order

A part number is the fastest route to the right blade. But plenty of orders start without one: a blade that came off a machine years ago, a fleet running mixed decks, a customer who photographed the old part instead of writing the number down.

When that happens, five measurements decide whether the blade fits. Four are straightforward. The fifth is the one that catches people out.

1. Length, tip to tip

Measure across the blade from one cutting tip to the other, in a straight line — not along the curve of the blade, and not around the outside edge.

Two things to keep in mind:

• A blade measuring 19-5/16" is not a 19" blade. Fractional lengths are normal, so do not round the number.
• Length is a strong indicator of deck size, but it is not the same thing. A 42" deck usually takes blades around 21" long, because two or three blades sit side by side under it. Writing down "21 inch" and "42 inch deck" as if they were the same number causes a lot of wrong orders.

2. Width, across the cutting face

Measure across the flat cutting face — the width of the metal itself, not the width of the blade assembly and not the depth of the sail.

Width tells you how much of the deck the blade sweeps and how much air it moves. Two blades of the same length with different widths behave differently: the narrower one gives less lift and less discharge, and recirculates more.

3. Thickness, at the spine

Measure the thick edge, at the spine or the flat — away from the sharpened edge, where material has been ground away.

This is where the difference between a low-cost blade and a service blade shows up most clearly. Thickness at the spine is the single best indicator of how long the blade will hold up under commercial use.

4. Centre hole — shape matters more than diameter

This decides the fit, and it is the one people skip.

Measure the diameter, but more importantly identify the shape:

• Round — a simple circular hole, often with a specific bore such as 11/16".
• Star — 5-point, 6-point or 7-point. The point count is not cosmetic; a 7-point blade will not seat on a 6-point spindle.
• Keyed or slotted — a round hole with a keyway or slots, which locates the blade at a fixed angle.

A blade with the right length, width and thickness will still refuse to mount if the centre pattern is wrong — and it will not be obvious why until someone tries to fit it.

5. Offset, the step between hole and cutting plane

Some blades are flat. Others have a step or crank between the centre hole plane and the cutting plane.

Measure from the centre hole plane down (or up) to the cutting plane. Get this wrong and the blade still mounts, but sits at the wrong height: it can foul the deck housing, or cut at an uneven height across the deck.

The three mistakes that cost a container

Rounding the length. Half an inch out is a blade that does not fit, and a container that has to come back.

Confusing blade width with deck width. A "21-inch blade" and a "21-inch deck" are different things. Always state which one you mean.

Ignoring the centre hole. Length, width and thickness are the numbers people collect. The centre hole is the number that decides the order.

If you can cross-reference instead, do that

Measuring is the fallback. If the old blade carries a stamped number, or the machine has a deck plate, use that first — a cross-reference is faster and leaves less room for interpretation. We keep a catalogue of blades by original part number for exactly this reason.

When you are not sure, send a photo and the numbers

If the blade is unusual — a step, an odd star pattern, a very narrow face — send four photos (top, bottom, edge, centre hole close-up) plus the five measurements above. That is usually enough to identify a blade without shipping a sample.

If you regularly order for a mixed fleet, tell us the machine models instead. Deck plates are easier to read than old blades, and we can map the models to the blades we already produce.