How to Measure Bike Frame Size in 5 Minutes

How to Figure Out Your Bike Frame Size

Bike sizing has gotten complicated with all the geometry charts and fitting systems flying around. As someone who’s watched too many people struggle on bikes that don’t fit — knees hitting handlebars, arms fully extended, back aching after 10 minutes — I learned everything there is to know about getting this right. Today, I’ll share the method that actually works.

The Inseam Method

Probably should have led with this section, honestly. This works for most people. Stand against a wall barefoot with feet shoulder-width apart. Take a book, spine up, and press it firmly into your crotch like a saddle would. Mark where the top of the book hits the wall. Measure from floor to mark in centimeters.

That number is your cycling inseam. It’s different from pants inseam because you’re measuring to where you actually sit on a bike.

Road Bike Sizing

Multiply your inseam by 0.65. That gives you a starting frame size in centimeters.

So if your inseam is 84cm: 84 x 0.65 = 54.6cm. You’d look at 54cm or 55cm frames depending on the brand.

Road bikes are measured center-to-center (bottom bracket to top tube) or center-to-top, and brands vary. Check the specific geometry chart. A “54cm” frame from one brand might fit differently than another’s.

Mountain Bike Sizing

Multiply your inseam by 0.59 for mountain bikes. They run smaller because you need clearance for moving around on technical terrain.

That’s what makes MTB sizing endearing to us riders who like to move around on the bike — there’s actually room to shift your weight without clipping the top tube. Most mountain bike brands use S/M/L/XL instead of centimeters. A 5’7″ to 5’11” rider typically fits medium. But modern MTB geometry has gotten longer and lower, so some people size down from what they rode a decade ago.

Height Charts Are Rough Guides

You’ll see charts saying “5’8″ to 5’10” = medium.” These work as starting points but ignore proportions. Two people the same height can have very different inseam and torso lengths.

Someone with long legs and short torso might ride a smaller frame with a longer stem. Short legs and long torso might go opposite. The inseam method gets you closer than height alone.

When in Doubt, Test Ride

Numbers get you in the ballpark. Actually sitting on the bike tells you if it fits. Most shops will let you test ride. If buying online, understand the return policy. Getting frame size wrong makes cycling miserable — it’s worth the effort to get right.