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How to Calculate the Square Key Length for Shafts

The key length for a shaft is known as the “keyway” and is a hub or bushing that is used for a keyset. A keyset is the square or rectangular groove of the shaft that is sized for the key. To calculate a key length for the shaft, you will need to obtain precise measurements for the shaft. You will then need to input the measurements into an equation that needs to be solved to obtain the key length.

Measure the width or diameter of the shaft using the ruler. Note this measurement as “D” on a piece of paper.

Measure the width of the keyway opening in the shaft. Note this measurement as “W” on the paper.


Measure the distance from the top of the shaft to the bottom of the bore for the key. Note this as “H” on the paper.


Write out the equation “A = D x W” on the paper. Input the measurements obtained for “D” and “W” to solve equation.


Add the result for “A” in Step 4 to the measurement for “H” to calculate the key length of the shaft.

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