About the Bike Size Calculator
This bike size calculator turns two body measurements — your height and your inseam (inside-leg length) — into a recommended frame size for a road bike, a mountain bike and a hybrid or city bike. It also gives a letter size (XS to XXL) and a starting saddle height, so you can compare listings from different brands or check whether a second-hand bike is close enough to fit.
Inseam matters more than height, because frame size is really about how far the saddle and top tube sit from the ground. If you do not know your inseam, leave it at 0 and the calculator estimates it as 46% of your height, which is close to average but less precise than measuring.
There is no single official sizing standard. The frame sizes come from common bike-shop rules of thumb (inseam × 0.665 for road, × 0.685 for hybrid, and inseam in inches × 0.575 for mountain bikes), and the saddle height uses the LeMond 0.883 × inseam guideline. Frame geometry differs between manufacturers, so treat the result as a starting point. Always check the brand’s own size chart, and if you fall between two sizes, test-ride both: riders with longer arms and torsos usually prefer the larger frame, shorter-reach riders the smaller one.
How to use the bike size calculator
- 1Choose the type of bike you are buying.
- 2Pick centimetres or inches for your measurements.
- 3Enter your height, barefoot.
- 4Measure your inseam: stand against a wall with a book pressed up between your legs and measure floor to spine of the book.
- 5Read the recommended frame and letter size, then compare with the brand’s size chart.
Formula and method
Frame size is estimated from inseam with widely used fit multipliers. The saddle-height factor of 0.883 comes from the method popularised by Greg LeMond; the road factor of 0.665, the hybrid factor of 0.685 and the mountain-bike factor of 0.575 (in inches) are common bike-shop fit rules of thumb. Road and hybrid frames are quoted in centimetres, mountain bikes in inches.
The letter size is mapped from the frame measurement using typical brand bands (for example a 52–55 cm road frame is usually a Medium). If no inseam is entered, it is estimated as 46% of height. Modern frames with sloping top tubes vary by brand, so the result is a guide rather than a guarantee.
- inseam
- Inside-leg length from floor to crotch, barefoot (cm unless noted)
- 0.665
- Road frame multiplier (seat tube length, rule of thumb)
- 0.883
- LeMond saddle-height multiplier (bottom bracket to saddle top)
Worked examples
Road bike, 175 cm rider with an 81 cm inseam
An 81 cm inseam × 0.665 gives a road frame of about 53.9 cm, which falls in the Medium band (52–55 cm). The starting saddle height is 81 × 0.883 ≈ 71.5 cm from the centre of the cranks.
Mountain bike, 5 ft 9 in rider with a 31 in inseam
A 31 inch inseam × 0.575 gives a mountain bike frame of about 17.8 inches, a Medium on most brands. The same rider would ride roughly a 52.4 cm road frame (78.74 cm × 0.665).
Hybrid bike, 160 cm rider who does not know their inseam
With no inseam entered, it is estimated as 160 × 0.46 = 73.6 cm. A hybrid frame of 73.6 × 0.685 ≈ 50.4 cm is a Small. Measuring the real inseam would sharpen the estimate.
Frequently asked questions
What size bike do I need for my height?+
As a rough guide, riders around 160–170 cm (5 ft 3 in–5 ft 7 in) ride Small road frames of about 49–52 cm, 170–180 cm (5 ft 7 in–5 ft 11 in) riders ride Medium 52–55 cm, and 180–190 cm (5 ft 11 in–6 ft 3 in) riders ride Large 55–58 cm. Inseam gives a better answer than height alone.
How do I measure my inseam for a bike?+
Stand barefoot with your back against a wall, feet about 15–20 cm apart. Pull a hardback book up firmly between your legs like a saddle, keep it level, and measure from the floor to the top edge of the book.
Should I size up or down if I am between sizes?+
Choose the larger frame if you have long arms or torso, want a stretched-out position, or ride long distances. Choose the smaller frame for a more upright, manoeuvrable fit — reach can be fine-tuned with stem length.
Why are mountain bikes measured in inches and road bikes in centimetres?+
It is convention: mountain bike seat tubes are traditionally quoted in inches (16, 18, 20 in), while road frames are quoted in centimetres (52, 54, 56 cm). Many brands now simply use S, M and L for both.
How high should my saddle be?+
The LeMond method sets the distance from the centre of the bottom bracket to the top of the saddle at 0.883 × inseam. At that height your knee should be slightly bent at the bottom of the pedal stroke.