Bicycle Crank Length: A Formula

This page, along with the associated pages, provides a method of estimating the correct length for a crankset for a given cyclist. Crank length is the distance from the center of the crank axle to the center of the pedal axle, and is only one of dozens of variables involved in adjusting a bicycle to "fit" a rider properly. As you will learn here, it is an often overlooked and misunderstood variable.

However, right here I must make one thing very clear: The formula presented here is not a generally accepted rule of thumb among the bicycle industry nor the cycling crowd. In fact, you will not likely hear of it anywhere but here. For these reasons, I do not present the formula alone; I present the entire reasoning behind it, its implications, some references, some examples and testimonials, as well as some insights into why the rest of the cycling world isn't using the same analysis. The cyclist, thus informed, can decide for himself whether to trust my guidance or what the bike shop is telling him.

For those simply interested in the formula itself, it is simple enough. First, the rider must measure his inseam. It is not recommended that the pants size be used as this may not be accurate enough, an inch one way or the other makes a pretty big difference. Stand barefoot on a hard floor, straight, feet fairly close together, back against a wall, and have an assistant insert a clipboard between your legs. Lift gently, sliding the clipboard as high as it will comfortably go while keeping it firmly against the wall, thereby assuring it isn't tilted. While holding the clipboard this way, measure the distance from the top edge of the clipboard to the floor, in inches. Don't round to the nearest inch; measure as accurately as possible.

L(mm) = 5.48 x I(in)

Now, multiply the inseam measurement (in inches) by 5.48. This provides a good estimate of proper crank length, in millimeters, for general road cycling or racing.

For those who prefer metric units, simply convert the equation by 2.54 cm/in.

L(mm) = 2.16 x I(cm)

Now, many of you, especially those who are significantly taller or shorter than average, have just decided that I must be an idiot -- this can't possibly be right! For you, I suggest reading all the attached discussions linked above. You may still decide I'm an idiot, but I think you're going to have a hard time supporting your contention. Before you warm up the flamethrowers and start bombing my e-mail box, however, I suggest you read this.

If you find this formula believeable but keep getting conflicting input from others, I counter some of the lies, misinformation and old wive's tales.

For those of you who are convinced: before you go and spend money on a new crankset, you should read about some fudge factors.

Finally, once you decide what crank length you need, finding it is another issue. I can offer a few tips here.

Return to Kirby Palm's home page.

Of course, if you have questions or comments, you are welcome to send e-mail to me at  "palmk at nettally dot com".