How Do You Measure a Bike Frame Size? A Simple Guide

On most bikes, the quoted frame size is the length of the seat tube, measured from the centre of the bottom bracket to the top of the seat tube. Road and gravel bikes are usually given in centimetres, mountain bikes often in inches or S/M/L, and hybrids and kids' bikes may use letters or wheel size. That number is a starting point, not a guarantee of fit: two frames with the same seat tube can feel completely different because their top tubes, reach, stack and standover differ.

• A metal tape measure marked in centimetres and millimetres. A fabric tape stretches and will give you a different number each time.
• A flat floor and a clear wall to stand against.
• A hardback book or a set square for measuring your inside leg.
• A helper to hold the tape steady at the bottom bracket.
• The bike's geometry chart, or the measurements of a bike you already know fits.
• A pencil and paper, or your phone, to write down each number with its unit.

Before you measure, decide which number you need. If a used bike is advertised as a '54', ask whether that is centre-to-top or centre-to-centre, and whether it is centimetres or inches. If you are buying new, use the maker's geometry table rather than a generic size chart; the table is the only reliable source for that frame.

Work on a clean, level floor with the bike upright. If the bike is in a stand, make sure the stand is not lifting a wheel off the ground while you measure standover.

The same frame can be quoted two ways, and the difference is not a rounding error. Ask which method was used before you compare numbers from different sellers.

Frame size only works if it puts the saddle and bars in the right place for your body. Two riders of the same height can need different frames when their inside leg, torso and arm length differ.

Use your inside leg to check standover and to set saddle height, not to pick a frame size on its own. A common starting point for saddle height is 0.883 × inside leg, measured from the bottom bracket centre along the seat tube to the top of the saddle. Treat that as a starting point and adjust for comfort. Your overall height is still useful as a first filter on a maker's size chart.

A medium is not a standard. One maker's medium can be longer in reach and shorter in the seat tube than another maker's small. Height charts assume average proportions and usually ignore inside leg, arm length and flexibility. Use them to narrow the field, then confirm with geometry.

• Measuring the seatpost instead of the seat tube. The seatpost is removable and its length says nothing about frame size. Measure the frame tube, not the post.
• Running the tape along the down tube. The down tube is longer and slanted, so the number is meaningless. Follow the seat tube's line.
• Mixing inches and centimetres. A 17 in frame and a 17 cm measurement are not the same thing. Write the unit down every time.
• Assuming a 54 is a 54 everywhere. Centre-to-top and centre-to-centre differ, and sloping frames may quote a virtual size. Ask which method the seller used.
• Ignoring standover on a sloping frame. Measure at the point where you stand over the bike, with it upright on level ground, not at the lowest point of the top tube.
• Buying from a height chart alone. Use it to shortlist, then check reach, stack and standover before committing.

Set the saddle to your starting height and sit on the bike with the cranks level. Your front knee should sit roughly over the pedal axle when the forward crank is at three o'clock; if you cannot get near that with the saddle at a sensible height, the frame's seat angle or top tube may be wrong for you.

Reach to the bars should feel natural with a slight bend in the elbows. You should not feel stretched out or cramped, and you should be able to look ahead without straining your neck.

• Too big: you are over-stretched to the bars, the front wheel feels vague, and the saddle cannot go low enough without exceeding the seatpost limit.
• Too small: the saddle is high, the bars feel close and you feel cramped; a longer stem can help, but it changes steering.
• Standover: straddle the bike with both feet flat on the floor and check that there is clear space between you and the top tube.

If you are between sizes, compare reach and stack first, then standover. A good bike fitter can adjust saddle height, stem length and bar width, but cannot lengthen a frame's reach without changing how it steers.