How to Measure a Bike Frame (and What Size You Actually Need)
Whether you are buying a second-hand bike, replacing one that never felt right, or just want to know what you are riding, it helps to know your frame size and what size you actually need. The good news is that it takes five minutes and a tape measure. The less good news is that bike brands don't all measure the same way, so the number on the frame is only half the answer.
We measure bikes every day, because a cover only works if it fits: our Find Your Fit database holds measurements for around 979 models. Here is how to measure a frame, how to measure yourself, and how to turn both into the right size.
The short answer
Frame size is the length of the seat tube, measured from the centre of the bottom bracket (where the cranks turn) to the top of the seat tube. Road bikes are sized in centimetres, mountain bikes and hybrids mostly in S, M, L and XL. The measurement that decides your size is your inside leg, checked against the brand's own chart.
What you need
- A tape measure (a metal one is easier to hold straight)
- A long straight edge or spirit level, for bikes with a sloping top tube
- A hardback book and a wall, for measuring yourself
- Somewhere flat to stand the bike upright
How to measure the frame on your bike
1. Check for a size first. Most frames carry a size sticker or stamp. BikeRadar notes it is often on the seat tube or under the bottom bracket area. If you find 54, 56 or M, that is your answer, and you can skip to working out whether it is the right one.
2. Measure the seat tube. Stand the bike upright and run the tape from the centre of the bottom bracket, the middle of the crank bolt, up to the top of the seat tube where the seatpost goes in. That is the classic frame size, called centre-to-top. If the seat tube is bent, as it is on many mountain bikes, hold a straight edge along its line and measure along that rather than following the curve.
3. Measure the effective top tube. Most modern frames have a sloping top tube, so brands quote an effective (or virtual) top tube instead: a level line from the top of the head tube back to the seatpost. Use a spirit level to keep it horizontal. This number tells you far more about how stretched out you will be than seat tube length does.
4. Know stack and reach. Stack is how high the top of the head tube sits above the bottom bracket; reach is how far forward it sits. Together they describe the frame's shape better than any single size number, and they are how brands such as Canyon suggest comparing a new bike with one you already know fits. You will rarely measure them yourself, but they are on every brand's geometry chart.
- A. Seat tube: centre of the bottom bracket to the top of the seat tube. The traditional frame size.
- B. Effective top tube: measured level, from the head tube to the seatpost.
- C. Stack: height of the top of the head tube above the bottom bracket.
- D. Reach: how far forward the top of the head tube sits from the bottom bracket.
Why two 56cm bikes can feel different. Brands measure frame size in different ways: centre-to-top, centre-to-centre (to the middle of the top tube), or to a virtual point on a sloping frame. Sheldon Brown famously called frame-sizing conventions "anarchy", and recorded identical frames differing by 2cm on paper after a brand changed its method. Treat the size number as a starting point and compare the top tube, stack and reach.
How bike sizes are labelled
| Bike type | How it is sized | Example |
|---|---|---|
| Road and gravel | Centimetres (seat tube or a brand's equivalent) | 52, 54, 56cm |
| Mountain bikes | Letters, sometimes inches | S, M, L, XL or 17in, 19in |
| Hybrids and city bikes | Mostly letters, some in inches or cm | S to XL |
| Children's bikes | Wheel size | 16in, 20in, 24in |
| Folding bikes | Often one frame size, adjusted with seatpost and bar options | One size |
Letter sizes took over on mountain bikes because seat tube length depends so much on suspension layout and frame shape that it stopped being a useful guide. Reach is the number mountain bikers compare now.
How to measure yourself
Height. Stand against a wall in bare feet, lay a pencil or book flat on your head, mark the wall and measure to the floor.
Inside leg. This matters more than height. Stand barefoot with your back against a wall. Pull a hardback book up firmly between your legs, spine upwards and level, as a saddle would sit. Mark the top of the spine on the wall and measure to the floor. Ribble and Boardman both describe this method; it is worth doing twice.
What size bike do I need?
Start with the brand's own size chart for the exact bike you are looking at. General charts are a useful first guess, but they vary. As an example, at 178cm tall BikeRadar's general road chart suggests 56 to 58cm, Specialized's road chart a 56, and Giant's an M or M/L. These are BikeRadar's general ranges:
Road bikes
| Your height | Typical frame size |
|---|---|
| 148 to 152cm (4ft 10in to 5ft) | 47 to 48cm |
| 152 to 160cm (5ft to 5ft 3in) | 49 to 50cm |
| 160 to 168cm (5ft 3in to 5ft 6in) | 51 to 53cm |
| 168 to 175cm (5ft 6in to 5ft 9in) | 54 to 55cm |
| 175 to 183cm (5ft 9in to 6ft) | 56 to 58cm |
| 183 to 191cm (6ft to 6ft 3in) | 58 to 60cm |
| 191 to 198cm (6ft 3in to 6ft 6in) | 61 to 63cm |
Mountain bikes
| Your height | Typical size |
|---|---|
| 152 to 162cm (5ft to 5ft 4in) | XS |
| 162 to 170cm (5ft 4in to 5ft 7in) | S |
| 170 to 178cm (5ft 7in to 5ft 10in) | M |
| 178 to 185cm (5ft 10in to 6ft 1in) | L |
| 185cm and over (6ft 1in and over) | XL |
Children's bikes go by wheel size, and inside leg is a better guide than age. BikeRadar's rough guide: 14in wheels from about age 3, 16in from 4, 20in from 6, 24in from 8 and 26in from about 10.
Inside-leg formulas
You will see formulas online that multiply your inside leg by a fixed number, such as 0.665 or 0.70 for a road frame in centimetres. They come from calculator sites rather than bike makers, the multipliers disagree, and they ignore frame shape. Use one to sanity-check a chart, not instead of it. Don't confuse them with the 0.885 figure you may also see: that is a starting point for saddle height, not frame size.
Standover clearance
Straddle the bike with both feet flat, just in front of the saddle. You want a gap between you and the top tube: REI suggests about 2.5cm on a road bike and at least 5cm on a mountain bike, more for rough riding. Modern sloping frames usually clear this easily, so it is a check rather than a way to choose a size.
Between sizes?
Most advice, from Cycling Weekly, BikeRadar and Ribble among others, says choose the smaller frame: it is easier to make a slightly small bike fit with a longer stem or more seatpost than to shrink a big one. There are exceptions. Boardman suggests the larger size if you are tall for your inside leg, and Trek points out that a longer bike is more stable and a shorter one more nimble, so riding style matters too. If you can, ride both.
Signs your bike is the wrong size
- You feel stretched out, or ride holding the tops of the bars because the hoods are too far away
- Aching lower back, neck or shoulders on longer rides
- Numb hands, or wrists that roll inwards
- Your knees come close to the handlebars when you turn, a sign the frame is too small
- You can't get the saddle high enough, or have to drop it right down, without running out of seatpost
What you can change: saddle height and fore-aft position, stem length, handlebar width, spacers under the stem, and brake lever reach. What you can't: the frame itself. Those adjustments fine-tune a frame that is roughly right; they won't rescue one that is a size out.
Measuring your bike for a cover
A bike cover works on different measurements from frame size, and handlebar width is the one people forget. To choose the right BikeParka, measure:
- Handlebar width: end to end, including the grips. This most often decides the size.
- Length: front tyre to back tyre, including any rack or deck.
- Height: ground to the highest point, usually the bars or the saddle.
Road, hybrid, commuter and standard e-bikes with handlebars up to 63cm take the STASH or URBAN. Mountain bikes, e-MTBs and anything with bars up to 100cm take the XL / MTB. Longtails and front-loader cargo bikes have their own covers, the Longtail and Cargo, and children's bikes the KIDS / BMX. If you are close to a limit, size up: a slightly generous cover cinches down with the toggles. Or search your bike in Find Your Fit.
Bike sizing: common questions
What size is my bike frame?
Look for a size printed on the frame first, usually on the seat tube or under the bottom bracket. If there isn't one, measure the seat tube from the centre of the bottom bracket to the top of the seat tube. Road bikes are sized in centimetres; mountain bikes and hybrids are usually sold as S, M, L and XL.
Is frame size measured centre-to-centre or centre-to-top?
Both are used, which is why two bikes marked with the same size can measure differently. The traditional method is centre of the bottom bracket to the top of the seat tube (centre-to-top), but some brands measure to the centre of the top tube instead. Compare stack and reach, or the brand's own size chart, rather than the number alone.
What size bike do I need if I am 5ft 10in (178cm)?
On typical charts that is around a 56cm road bike or a medium to large mountain bike, but brands differ. BikeRadar's general road chart puts 175 to 183cm on 56 to 58cm, Specialized's road chart puts 175 to 180cm on a 56, and Giant's puts it between M and M/L. Always check the chart for the exact bike you are looking at.
Is bike size the same as wheel size?
No, except for children's bikes, which are sized by wheel: 12in, 14in, 16in, 20in, 24in and so on. For adult bikes, wheel size (700c, 29in, 27.5in) is separate from frame size.
What if I am between two sizes?
Most sizing advice, including Cycling Weekly, BikeRadar and Ribble, says go for the smaller frame, because a smaller bike is easier to extend with a longer stem or seatpost than a big one is to shrink. If you are tall for your inside leg, or prefer a longer, more stable position, the larger size can suit better.
How do I measure my bike for a bike cover?
Three measurements: handlebar width from end to end including the grips, length from the front tyre to the back tyre including any rack, and height from the ground to the highest point. Handlebar width is the one that usually decides the size. If you are close to a limit, size up.
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