How this instrument works
Bike frame size is conventionally measured from the center of the bottom bracket to the top of the seat tube, and getting it right matters for both comfort and power transfer — too large a frame cramps your reach and standover clearance, too small and you're stretched out with the seat post maxed. This calculator uses the Guimard formula, created by French cycling coach Cyrille Guimard in the 1980s: road frame size (cm) equals inseam length (cm) multiplied by 0.67. It remains one of the most widely reproduced quick-sizing formulas in cycling fit guides today.
Mountain and trekking/city bikes use adjusted versions of the same idea, since their frame geometry and standover requirements differ from a road bike's. This calculator subtracts 11 cm from the road-bike figure for mountain bikes (accounting for the extra standover clearance needed for off-road riding) and uses a slightly lower multiplier of 0.64 for trekking/city bikes, which tend to have a more upright, relaxed geometry than a road racing frame.
Inseam-only formulas like this one give a solid starting point, but they're deliberately simple — they don't account for torso length, arm length, riding style, or a particular manufacturer's own geometry chart, all of which shift what actually fits and feels right. Bike brands often size frames in small (S/M/L) or numeric increments that don't map perfectly onto a continuous centimeter figure, so treat the output as a target to test-ride around, not a guaranteed final answer — especially for a serious road or race bike purchase where a proper fit session is worth the time.
- Measure your Inseam length in centimeters — stand against a wall in socks, place a book firmly in the crotch, and measure from the floor to the top of the book.
- Select your Bike type — road, mountain, or trekking/city.
- Read Recommended frame size — the calculated size in centimeters for that bike type.
- Compare the result against the manufacturer's own size chart, since brands sometimes label frames by S/M/L rather than exact centimeters.
- Treat this as a starting point for a road or performance bike purchase — a proper bike fit accounts for torso and arm length too, which this formula does not.
Worked example — an 80 cm inseam, road bike
Enter 80 cm for inseam and select Road bike. The Guimard formula gives 80 x 0.67 = 53.6 cm, so this rider would be steered toward road frames sized around 53-54 cm.
If the same 80 cm inseam were used for a mountain bike instead, the calculator would first compute the same 53.6 cm road-equivalent figure, then subtract 11 cm for the extra standover clearance mountain geometry requires — landing at 42.6 cm. For a trekking/city bike, the lower 0.64 multiplier applies directly: 80 x 0.64 = 51.2 cm, reflecting that upright city bikes are sized somewhat differently than a leaned-forward road frame.
Questions
What is the Guimard formula and where does the 0.67 come from?
It's a road-bike sizing rule of thumb created by Cyrille Guimard, a French professional cyclist turned coach, in the 1980s. Multiplying inseam length in centimeters by 0.67 estimates the frame size (bottom bracket to top of seat tube) that puts a rider's saddle height and standover clearance in a reasonable range for road riding. It's become one of the most commonly cited quick-reference formulas across cycling fit guides, though it predates modern bike-fit software and doesn't account for every rider's proportions.
Why do mountain bikes use a different formula than road bikes?
Mountain bikes need more standover clearance than road bikes — riders need to be able to put both feet flat with real clearance over the top tube when dismounting on rough terrain, and modern MTB geometry (sloping top tubes, dropper posts) is built around that requirement. Subtracting roughly 11 cm from the road-bike-equivalent figure accounts for that extra clearance need, giving a smaller recommended frame size than the same inseam would suggest for a road bike.
How accurate is an inseam-only frame size formula?
It's a solid starting point but not a substitute for a proper fit. Two riders with identical inseam measurements can need meaningfully different frame sizes if their torso length, arm length, or flexibility differ — factors this formula doesn't see at all. For a bike you'll ride seriously or for long distances, use this as a first estimate, then check it against the specific manufacturer's geometry chart and ideally test-ride before buying.
Should I measure inseam in shoes or barefoot?
Measure in the socks or thin shoes you'd actually cycle in, standing with your feet about shoulder-width apart, and press a book firmly up into the crotch as if it were a saddle before measuring floor-to-book height. Measuring barefoot when you'll ride in cycling shoes (or vice versa) introduces a small but avoidable error, since shoe sole thickness shifts the true floor-to-crotch distance by a centimeter or more.
Why does my calculated frame size not match the S/M/L on a bike I'm considering?
Manufacturers don't standardize frame-size labeling — one brand's 'Medium' might correspond to a 54 cm frame while another's is 56 cm, and some brands size purely by rider height rather than a centimeter figure at all. Use the calculated centimeter figure as a reference point, then cross-check it against that specific manufacturer's own size chart (usually available on their website) rather than assuming size labels are interchangeable across brands.