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Bike size calculator
What size bike do you need? Height charts only tell half the story, and size labels barely agree between brands. This calculator turns your height and inseam into a reach and stack target, the numbers that actually transfer to any manufacturer's geometry chart, plus a starting size band and which way to go if you're between sizes.
Reach and stack are the numbers that actually transfer between brands. Match these targets (±10 mm or so) against the geometry chart of the specific bike you're considering — its size label matters far less.
Recommendations are a starting band, not a bike fit. If you're spending serious money or have injuries, a professional fitting is worth every penny.
How this calculator works
Most size charts map your height to a frame size and stop there. That works fine until it doesn't. Two riders of identical height can need different frames, because height doesn't tell you where that height is. Long legs and a short torso point to a different bike than short legs and a long torso.
So this calculator uses two measurements. Your inseam drives the core recommendation, because it governs the two things a frame absolutely must get right: standover clearance and whether the seatpost can reach your saddle height. Your height acts as a cross-check for reach, the horizontal dimension of the fit. When the two agree, you'll get a confident single answer. When they disagree, the calculator says so and explains what your proportions mean for the choice, rather than silently averaging the difference away.
How to measure your inseam properly
Stand against a wall in socks, feet about 15 cm apart. Take a hardback book, spine upward, and pull it up between your legs as if it were a saddle. Press firmly, this matters. Mark where the top of the spine meets the wall and measure floor to mark. This cycling inseam is typically 2–4 cm longer than your trouser inseam, which is why borrowing your jeans size produces bikes that are too small.
Between sizes: up or down?
The classic dilemma, and the honest answer is that both can be right. Sizing down gives a lighter-handling, sportier bike with more saddle-to-bar drop; you make up the difference with a longer stem and more exposed seatpost. Sizing up gives a more stable, upright, comfortable ride. Your proportions tip the scales: long legs favour the bigger frame (you'll run out of seatpost on the small one), long torso favours the smaller frame with a longer stem. And modern geometry matters more than the label. If you're choosing between two specific bikes, compare their reach and stack figures rather than the size names.
One chart never fits all brands
A "56" from one brand can fit like another brand's 54. Size labels describe seat tubes, but modern frames with sloping top tubes and long head tubes have made seat tube length nearly meaningless as a fit measure. That's why this calculator leads with a reach and stack target instead of a size label: those two numbers are measured the same way on every brand's geometry chart, so they're the ones actually worth comparing.
Why reach and stack, and how accurate is the estimate
Reach and stack aren't pulled from any single manufacturer's chart. They're estimated from your frame-size band using the trends typical of mainstream road, gravel and mountain bike geometry, then nudged by your fit preference (endurance geometry runs shorter and taller than race geometry at the same nominal size). Treat the numbers as a ±10 mm target, not a spec. Once you've shortlisted a bike, pull up its actual geometry chart and compare its reach and stack to these targets directly, that comparison is far more reliable than matching size labels. Once the right bike is in the right size, set the contact points with our saddle height calculator, then dial in how it rides with the tyre pressure calculator.
Frequently asked questions
How do I measure my inseam for a bike?
Stand against a wall in socks with your feet about 15 cm apart. Hold a hardback book firmly up between your legs, spine up, as if it were a saddle. Mark where the top of the book meets the wall and measure from the floor to the mark. That's your cycling inseam, which is usually a little longer than your trouser inseam.
What size bike do I need for my height?
As a rough guide for road bikes: 160–168 cm tall suits a 51–53 cm frame (S), 168–175 cm suits 54–55 cm (M), 175–183 cm suits 56–58 cm (L). Mountain bikes run 15–16 inch frames for 158–168 cm, 17–18 inch for 168–178 cm, and 19–20 inch for 178–185 cm. Height alone is only half the answer though, since your inseam decides standover and saddle height. Use both.
I'm between bike sizes. Should I size up or down?
Size down for a sportier, more agile fit (you can lengthen the stem and add seatpost), size up for a more comfortable, stable, upright fit. If your legs are long for your height, lean toward the larger size for standover and seatpost reasons; if your torso is long, the deciding factor is reach, and a smaller frame with a longer stem often fits better.
Are bike sizes the same across brands?
No. A 56 cm from one brand can fit like a 54 from another, because seat tube length no longer describes modern geometry well. That's why this calculator's headline result is a reach and stack target rather than a size label, those two numbers are measured the same way on every brand's geometry chart, so they're what's actually worth comparing.
What are reach and stack, and why does this calculator show them?
Reach is the horizontal distance from the bottom bracket to the top of the head tube; stack is the vertical distance. Every brand measures them the same way, unlike a size label, so they're the most reliable way to compare fit across different bikes. This calculator estimates a reach and stack target from your height and inseam so you have real numbers to check against any bike's own geometry chart.
Do gravel bikes size the same as road bikes?
Mostly, yes. Start from your road size. But some modern gravel bikes use progressive, longer geometry, and many riders deliberately choose a slightly smaller gravel frame for standover clearance and agility on rough ground. Check the brand's geometry chart against your road benchmark.