How do lawnmower blade sizes work?

Question

I went to my local big box store this past weekend to find a replacement for my lawnmower blade (it's a Craftsman Precision Plus if that makes a difference, but it shouldn't).

When I got to the lawnmower blade section, I was impressed to find about 50 different sizes, although they all looked to be the same length.

However, despite very little variance in length, some were billed as being "42 in." (inches?) while others were billed as "21 in.". Although some were definitely longer than others, the variance was no more than a few inches. No way one was actually 21" and another was 42".

How do blade sizes work? Is "21 in" actually 21 inches? If so, how does that explain variances of only a few inches?

Answer 1

Mower blades are generally sold with the length designation matching the mower deck width. A larger riding mower may have a swathe width of 42", but they'll usually use two or more blades in a side-by-side or staggered configuration rather than one large blade.

So before buying, figure out what brand your mower is, and what the deck width is. Then when you're looking in the store, match the brand and deck width with the mower blade set. The blades won't actually be 42" long - probably more along the lines of 21.5" or 22", since they have to overlap a bit for you to not end up with a center stripe of unmowed grass.

For example, this pair of blades meant for a 42" riding mower are 21.5" long each, and this pair of blades meant for a different brand of riding mower are 25.25" long each. The reason for the different lengths but the same overall width, is that the mowing decks have different geometries of how the blades are spaced and staggered.

The blades labelled 21" were probably for a push or walk behind mower with an actual deck width of 21".