How to Measure a Bike Frame

Start with the seat tube: the centre of the bottom bracket to the top of the seat tube gives the frame size. Add the effective top tube for front-to-back room and standover for clearance when you stop. Reach and stack come from the geometry chart, and you need them to compare two models. Measure with the bike upright on a hard floor.

At the centre of the bottom bracket, the axle the cranks turn on. Not the edge of the bottom bracket shell, and not the chainring. Measure up the seat tube to the top of the tube where the seatpost enters.

Centre-to-centre stops at the middle of the top tube junction. Centre-to-top carries on to the top of the seat tube, so it always reads longer. Older charts and track frames use centre-to-centre, most modern brands publish centre-to-top. Ask which method a listing used before comparing two numbers.

Measure the effective top tube, which is level from the head tube top to the seat tube axis. A tape run along the sloping tube reads short, and the steeper the slope the bigger the difference. For the size label, measure the seat tube centre-to-top as usual; the seat tube angle does not change the method.

From the floor to the top of the top tube at the midpoint, halfway between the seat tube and the head tube, because that is where you stand when you stop. Some charts use the lowest point of the tube instead, so check the note on the chart you are reading.

Yes, with level ground, a plumb line, and two measurements. Drop the plumb line through the top of the head tube, measure level back to the bottom bracket centre for reach, then straight down for stack. A few millimetres out is normal, and for a bike you have not bought the geometry chart is the number the brand publishes.

Check the sticker on the seat tube and the number stamped under the bottom bracket first. If the frame only carries a letter size, look up the geometry chart for that model year, which lists the centimetre equivalent. If both are missing, measure the seat tube centre-to-top and compare it with the geometry table.

No. Frame size is the frame, and wheel size is the diameter of the wheel. A mountain bike with 29 inch wheels still comes in S, M, L, and XL frames. Height picks the frame size; the wheel diameter is a separate choice.

No. Frame measurements describe the frame only. Stem length, bar width, saddle position, and spacers under the stem change how that frame fits you, and a stem swap is far cheaper than a frame swap. The size stamped on the frame stays the same.

Within about 3mm is close enough. Road frame sizes step 2cm apart, so a small error rarely changes the answer on the size label. Standover is the number to take care with, because you compare it directly with your inseam when you build in clearance.

The 17 inches is the seat tube length, measured in imperial instead of centimetres. It is a common mountain bike Medium, and 16 inch or 18 inch frames sit either side of it. Measure centre-to-top in the same way and convert: 17 inches is about 43cm. Never compare the inch number with a centimetre label without converting first.

It depends what the 20 inches describes. Stamped on the seat tube, it is a large mountain bike frame: about 51cm. On a kids bike it usually describes the wheel, and a 20 inch wheel fits a rider around 6 to 9 years old. Check whether the number is on the frame or the tyre before you buy.

58 cm measured centre-to-top is a large road frame label. The bike frame size chart lists 58 cm for riders of roughly 181–190 cm (5 ft 11 in to 6 ft 3 in). Road labels step in 2cm, so 56 cm is the next size down in most ranges.

A tape measure, a spirit level or long straight edge, a plumb line, and masking tape to mark the bottom bracket centre and the head tube top. A helper holds the level and the plumb line. Work on a hard floor, not carpet, and measure with the bike upright.