Bike Size Calculator
Enter your height and inseam to get a recommended frame size for road, mountain, city, or kids bikes. Results include the seat-tube length in centimetres and inches, the generic size label (XS through XXL), the optimal saddle height using the LeMond method, and the stand-over clearance check. Switch between metric and imperial units and the figures update instantly.
How to measure your inseam correctly
The inseam (also called the inside leg length) is the single most important measurement for bike sizing. Stand in bare feet or thin socks with your back against a wall and your feet about 15-20 cm apart. Place a hardback book or a thin board between your legs and push it firmly but comfortably upward against your crotch, as a bike saddle would sit. Have someone measure from the floor to the top edge of the book spine. Repeat twice and average the readings. This measurement drives both the frame-size formula and the saddle-height calculation, so a half-centimetre error matters - take your time getting it right.
Frame size formulas by bike type
Road and gravel bikes use the Cyrille Guimard formula, which dates from the 1980s: frame size (cm) = inseam (cm) x 0.67. City, hybrid, and trekking bikes use a slightly shorter formula (inseam x 0.64) because they carry a more upright riding position and therefore need less seat-tube extension. Mountain bikes start with the road formula and subtract 11 cm, reflecting the lower standover height that technical terrain demands. The resulting number is a starting point; manufacturers calibrate their actual frames to this region but not to the exact centimetre, so the size band table above (and the equivalent table on the manufacturer's website) is what you use to pick a specific frame.
Saddle height and the LeMond method
Once you have a frame, saddle height is the next critical adjustment. The LeMond method, named after three-time Tour de France winner Greg LeMond, gives optimal saddle height as inseam (cm) x 0.883, measured from the centre of the bottom bracket to the top of the saddle along the seat tube. This puts your knee at a slight bend (around 25-30 degrees) at the bottom of the pedal stroke, which is the range biomechanics research consistently identifies as efficient and low-injury. A saddle that is too low causes knee pain at the front of the knee; one that is too high causes pain at the back of the knee and rocking hips. Set the height first, fine-tune by riding, and only then adjust fore-aft saddle position. Two other methods - the heel-on-pedal method and the 109% inseam method - produce almost identical results for most riders.
Stand-over clearance and when to drop a size
Stand-over clearance is the gap between your crotch and the top tube when you straddle the bike flat-footed. For road bikes, 2-5 cm is typical and safe. For mountain bikes, 5-10 cm is the accepted minimum because you need to dismount quickly on rough terrain, and some riders prefer 10-15 cm. If the recommended frame size gives you less clearance than you want, drop one size - a shorter seat tube almost never creates a problem that a longer seat post cannot fix, but a top tube that is too high is a safety hazard. Compact frames (sloping top tubes) give more clearance than traditional level top tubes for the same seat-tube length, so always check the actual standover measurement in the manufacturer's geometry chart rather than relying on the size label alone.
Road / gravel bike size chart
| Inseam (cm) | Inseam (in) | Frame size (cm) | Frame size (in) | Size label |
|---|---|---|---|---|
| 64-68 | 25-27 | 43-44 | 17 | XXS |
| 69-73 | 27-29 | 44-47 | 17-18 | XS |
| 74-78 | 29-31 | 48-50 | 19 | S |
| 79-83 | 31-33 | 51-53 | 20-21 | M |
| 84-88 | 33-35 | 54-55 | 21-22 | M/L |
| 89-93 | 35-37 | 56-58 | 22-23 | L |
| 94-98 | 37-39 | 59-60 | 23-24 | XL |
| 99+ | 39+ | 61-64 | 24-25 | XXL |
Inseam measurements drive the recommendation. Heights shown are typical for that inseam but vary by body proportions.
Frequently asked questions
What is more important for bike sizing, height or inseam?
Inseam is more important. Two riders with the same height can have very different leg lengths, and the frame size is directly driven by how far the saddle needs to be from the pedals. Height is a useful cross-check, but if your height and inseam suggest different sizes, go with the inseam.
What is the difference between frame size in cm and in inches?
Both measurements refer to the length of the seat tube (from the centre of the bottom bracket to the top of the seat tube or the seat-tube junction). Road and city bike sizes are traditionally expressed in centimetres (50 cm, 52 cm, 54 cm, etc.), while mountain bikes traditionally use inches (15", 17", 19", etc.). Some brands have moved to S/M/L/XL labels for both, so always check the geometry chart for the actual seat-tube length in millimetres.
What if I am between two sizes?
If you are between sizes, consider your riding style. Choose the smaller frame if you prefer a sportier, more aggressive position (you can always use a longer stem or setback seatpost). Choose the larger frame if you prioritise comfort and a more upright position. For road bikes, erring smaller is generally safer because reach is easier to increase than decrease. For mountain bikes, a smaller frame often handles better on technical terrain.
Do frame sizes differ between bike brands?
Yes, sometimes by 1-2 cm for the same nominal size. A "medium" from one brand may have a 54 cm seat tube; from another brand it may be 56 cm. Always verify by looking at the actual geometry chart on the manufacturer's website and comparing the seat-tube length and stack-and-reach measurements to your body measurements.
How do I size a bike for a child?
For children, wheel diameter (not frame size) is the standard reference. Measure the child's inseam the same way as for adults - book pressed against the crotch, floor to book top - and match it to the wheel-size table. The minimum saddle height of the bike must be equal to or less than the child's inseam so they can place both feet on the ground when stopped. Children grow fast, so check sizing at least every six months.
Does saddle height really matter that much?
Yes. Studies on recreational and competitive cyclists consistently show that saddle height within 1-2% of the optimal value reduces injury risk and improves power output. The LeMond formula (inseam x 0.883) is a well-validated starting point for most riders. Once set, fine-tune by feel: if your hips rock at the top of the pedal stroke the saddle is too high; if your knees ache at the front (near the kneecap) the saddle is too low.