Bike Fit Calculator
Saddle height, crank length and frame size from your inseam and height, with the symptom table that tells you which way to adjust.
Measure the inseam barefoot, back to a wall, with a hardback book pulled up firmly between your legs — floor to the top of the book. Guessing from trouser size will be several centimetres out.
If something hurts
| Hips rocking side to side | Saddle too high. The commonest fault, and the one that causes most injuries. |
|---|---|
| Pain behind the knee | Usually too high — the leg is over-extending at the bottom of the stroke. |
| Pain at the front of the knee | Usually too low, or the saddle too far forward. |
| Toes pointing down to reach the pedal | Too high. Your heel should stay roughly level through the stroke. |
| Feeling cramped, or knees coming up high | Too low, or the cranks are longer than suits you. |
| Sore hands and shoulders | Usually reach rather than saddle height — too long a stem, or too much drop. |
What a formula cannot do
- This is a starting point, not a fit. A fitter watches you pedal; a formula multiplies your inseam by a constant derived from a group of riders you are not in.
- The spread of comfortable saddle heights around any calculated figure is a couple of centimetres — wider than the gap between the competing methods, which is why arguing about which formula is best misses the point.
- Crank length matters less than the rule implies. Studies across a wide range of lengths find little difference in sustainable power. What it does change is hip and knee flexion at the top of the stroke, which is why riders with hip trouble often prefer shorter cranks than any chart suggests.
- Stack and reach beat a size letter. They are comparable between manufacturers where a "large" is not.
How to use
- Measure your inseam barefoot with a book pulled up firmly — not from trouser size.
- Enter your height, and armspan if you have it.
- Pick the discipline, since frames are sized differently.
- Start at the suggested saddle height and adjust one thing at a time.
Frequently asked questions
Which saddle height formula should I use?
Either. LeMond multiplies inseam by 0.883 measured from bottom bracket centre; Hamley uses 1.09 measured from the pedal at the bottom of the stroke. They describe the same position from different reference points, and the gap between them is smaller than the spread of genuinely comfortable heights around either one.
How accurate is a formula compared to a real fit?
It gets you close in one step instead of a season of guessing, and that is all. A fitter watches you pedal and sees what your body actually does; a formula multiplies your inseam by a constant derived from a group of riders you are not in. Treat the number as where to start experimenting, not where to stop.
How do I know my saddle is too high?
Your hips rock side to side as you pedal — the clearest sign and the commonest fault. Pain behind the knee and pointing your toes down to reach the bottom of the stroke both mean the same thing. Pain at the front of the knee usually means the opposite.
Does crank length actually matter?
Less than the rule of thumb implies. Studies comparing a wide range of lengths find little difference in sustainable power, which is why the traditional inseam-based figure should not be treated as a requirement. What length genuinely changes is hip and knee flexion at the top of the stroke, so riders with hip or knee trouble often do better on shorter cranks than any chart suggests.
Why measure inseam rather than use my trouser size?
Because trouser sizing is a garment measurement with ease built in and varies between manufacturers by several centimetres. Saddle height is derived from a body measurement, and being three centimetres out at the input is being three centimetres out at the saddle — enough to cause the problems in the symptom table.
What are stack and reach, and why prefer them to a size letter?
Stack is how high the head tube sits above the bottom bracket and reach is how far forward, both measured the same way by every manufacturer. A "large" is not standardised and one brand's large is another's medium, so comparing stack and reach is the only way to know whether two frames actually put you in the same position.
What is the ape index for?
Armspan minus height, which says whether your arms are long or short for your body. It matters because reach is set by your torso and arms together, so two riders with identical inseams can need noticeably different top tube lengths. Positive means you can usually carry more reach than a chart predicts.
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