Bike size calculator

How the calculator works

It uses two numbers. Your inside leg is the length from your crotch to the floor. It is the better guide, because two people of the same height can have very different legs.

The calculator multiplies your inside leg by a set number for each type of bike. That number is 0.665 for a road bike, 0.66 for a hybrid and 0.57 for a mountain bike. The answer is the frame size in cm.

Your height then goes into a size chart from a UK bike shop. That gives you a letter size, such as S, M or L. Most hybrids and mountain bikes are sold this way.

How to measure your inside leg

• Take off your shoes and stand with your back against a wall.
• Put your feet about 15 cm apart.
• Hold a hardback book between your legs, with one edge on the wall.
• Pull it up firmly, about as hard as a saddle would press.
• Ask a friend to measure from the top of the book to the floor.
• Do it two or three times and take the average.

Road bike size chart by height

Find your height, then read across to your size. A road bike frame is measured in cm. This chart comes from Wheelbase, a UK bike shop. Use the calculator above for a closer fit from your inside leg.

Why sizes vary by brand

Bike makers measure their frames in different ways. Some measure to the top of the frame and some to the middle of the top tube. Those two can differ by 1 to 1.5 cm. Many frames also slope, so the same size can look quite different.

Treat the answer as a starting point. Check the maker's own size chart for the bike you like, and take a test ride if you can. When you stand over the bike, you want a gap of at least 2 cm between you and the top tube.

Where the numbers come from

• The inside leg numbers come from the ADAC guide to frame size. The ADAC is the German motoring club. It gives 0.665 for road bikes, 0.66 for town and trekking bikes, and 0.57 for mountain bikes.
• For road bikes, Colorado Cyclist uses 0.67, which gives almost the same answer. It says the method Greg LeMond used came from his coach, Cyrille Guimard. That one uses 0.65, but measures to a lower point on the frame.
• Some websites use 0.70 for road bikes. We could not find where that number first came from, so we left it out.
• The height charts are from the Wheelbase bike size guide. Its mountain bike chart matches the one from Evans Cycles, which is also where the 2 cm gap comes from.
• The book method for your inside leg comes from Colorado Cyclist.