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Bike Frame Size Calculator
Use this free bike frame size calculator to find the right frame before you buy. Enter your height and inseam, pick your bike type — road, mountain, hybrid, or gravel — and the calculator gives your frame size in centimeters and inches, a size label, a suggested saddle height, and the standover height to look for. If you have not measured your inseam, it can estimate it from your height.
Measure in bare feet, back against a wall, with a book pressed firmly up between your legs. Measure from the floor to the top of the book.
How bike frame size works
Frame size is really a leg-length question, not a height question. The frame is measured along the seat tube — from the bottom bracket, where the pedals turn, up to the top of the seat tube — and that length has to match your inseam so your legs extend properly when you pedal. Two riders who are both 178 cm tall can have inseams that differ by 8 cm, and they need different frames. That is why this calculator sizes from your inseam first and only uses height as a cross-check and for the estimate option.
Gravel frame (cm) = inseam (cm) × 0.66 · Hybrid frame (cm) = inseam (cm) × 0.685 · Mountain frame (cm) = inseam (cm) × 0.57
Saddle height (cm) = inseam (cm) × 0.883
Each bike type uses its own multiplier because each type is ridden in a different position. A road bike is ridden leaning forward, so its frame is sized at 0.67 times inseam. A gravel bike is close to a road bike but slightly more upright and stable on rough ground, so it uses 0.66. A hybrid is ridden nearly upright, so it uses the largest multiplier, 0.685. A mountain bike uses the smallest, 0.57, because mountain frames are compact on purpose — a smaller frame leaves room to move around on the trail, and the inches measurement that mountain bikes are sold in comes out of that same number divided by 2.54.
If you have not measured your inseam, the calculator can estimate it as 0.45 times your height — about 80 cm for a 178 cm rider. That estimate is good enough to shop with, but a real measurement is better: stand barefoot with your back against a wall, feet slightly apart, and press a book firmly up between your legs, then measure from the floor to the top of the book.
The calculator also gives you two setup numbers. Saddle height is inseam times 0.883, measured from the center of the bottom bracket to the top of the saddle — this is the classic formula used by bike fitters, and it puts your leg almost straight, with a slight bend, at the bottom of the pedal stroke. Standover is the other check: when you stand over the bike with both feet flat, there should be a gap between you and the top tube. For road, gravel, and hybrid bikes that gap should be about 2 to 5 cm, so the standover height to look for is your inseam minus 2 to 5 cm. Mountain bikes need more room — 5 to 10 cm — because you need to step off quickly on rough ground.
Finally, the handlebar position depends on the bike type. On a road bike the handlebars usually sit about 5 to 10 cm below the saddle, which is an aerodynamic position for longer rides. On a gravel bike the drop is smaller, about 2 to 5 cm. On a hybrid the bars sit roughly level with the saddle, or up to 2 cm above it, for an upright, comfortable view of the road. On a mountain bike they sit level with the saddle or slightly above, so you stay in control on descents.
Worked example
Sam is 178 cm tall with an 81 cm inseam and wants a road bike. What frame size should Sam buy?
Step 1: Take the inseam, 81 cm, and multiply by the road factor 0.67: 81 × 0.67 = 54.3 cm.
Step 2: Convert to inches: 54.3 ÷ 2.54 = 21.4 in.
Step 3: The size label for a 54 cm road frame is M (medium).
Step 4: Saddle height is 81 × 0.883 = 71.5 cm from the bottom bracket.
Step 5: Standover target is the inseam minus 2 to 5 cm, so 81 − 5 = 76 cm to 81 − 2 = 79 cm.
So Sam should look for a 54.3 cm (21.4 in) road frame, size M, set the saddle at about 71.5 cm, and check that the standover height is between 76 and 79 cm. If Sam wanted a mountain bike instead, the same inseam would give 81 × 0.57 = 46.2 cm, which is 18.2 inches — a different number from the same legs. Enter your own height and inseam above to find your size.
FAQ
Is inseam or height more important for bike frame size?
Inseam is more important, because frame size is really about leg length. Two people who are both 5 feet 10 inches tall can have inseams that differ by 3 inches, and they need different frame sizes. Height is still useful as a cross-check and for reach — the distance to the handlebars — but always size the frame from your inseam first.
What should I do if I am between two frame sizes?
In general, pick the smaller frame if you want an agile, responsive ride and you are comfortable leaning forward, and pick the larger frame if you want a more upright, relaxed position. Smaller frames are also easier to adjust: you can raise the saddle and fit a longer stem. A frame that is too big is harder to fix, because you cannot shorten the top tube.
Why are mountain bike frames measured in inches?
It is mostly tradition. Road, gravel, and hybrid frames are usually measured in centimeters, while mountain bikes are usually sold in inches — 15, 17, 19, and 21 inches are common sizes. This calculator shows both units for every bike type, so you can shop in whichever unit the bike you are looking at uses.
How much standover clearance do I need?
Standover clearance is the gap between your inseam and the top tube of the bike when you stand over it with both feet flat. Road, gravel, and hybrid bikes need about 2 to 5 cm (roughly 1 to 2 inches) of clearance. Mountain bikes need more — about 5 to 10 cm (2 to 4 inches) — because you need room to move around on rough trails and to step off quickly.
Can I trust a frame size calculator instead of a test ride?
A calculator gets you to the right size range, which is exactly what you need for shopping online or narrowing down a shortlist. But frame geometry differs between brands — two 54 cm frames can feel different — so always test ride before you buy when you can. Use the saddle height and standover numbers from this calculator as your starting setup on the test ride.