Bike Size Calculator

Why height alone is not enough

Height charts assume average proportions. A rider who is 178 cm tall with long legs and a short torso needs a taller frame but a shorter reach than a rider of the same height with a long torso. Inseam captures leg length – the measurement that decides standover clearance and saddle height – so the calculator uses it whenever you have it.

How to measure your inseam for bike size

1. Stand barefoot with your back against a wall and feet about 15–20 cm apart.
2. Push a hardback book up between your legs, spine upward, as a saddle would sit.
3. Mark the top of the book on the wall, keeping it level.
4. Measure from the floor to the mark. Take it twice and use the average.

Cycling inseam is usually a few centimetres longer than your trouser inseam, because it is measured to the pubic bone rather than to the seam.

Bike frame size formulas by type

These factors are long-standing fitting rules of thumb rather than a standard, and frame sizes are measured differently by different brands – some from the bottom-bracket centre to the top of the seat tube (c-t), others centre to centre (c-c), and many modern bikes just use S/M/L. Treat the number as a starting point and compare it with the size chart of the model you want.

Bike size chart by height

Road and gravel bikes

Mountain bikes

Hybrid and city bikes

Charts overlap at the edges on purpose: if you sit on a boundary, your inseam, flexibility and riding style decide which way to go.

Choosing the right size for each type of bike

Road bike size calculator

Road bikes put you in a longer, lower position for efficiency. Fit depends on two numbers in the geometry chart: stack (how high the handlebars sit) and reach (how far forward). Endurance road bikes have more stack and less reach than race bikes of the same size, so a less flexible rider may be happier on an endurance frame rather than a smaller race frame.

Mountain bike size calculator

Mountain bikes are now sized mostly by reach rather than seat-tube length, and many brands label them S to XL only. You want enough standover clearance to step off quickly on rough ground and a dropper post that fits your inseam. Wheel size also matters: 29-inch wheels roll over obstacles more easily, while 27.5-inch wheels feel more agile and are common on XS and S frames for shorter riders.

Hybrid, city and gravel bikes

Hybrids and city bikes have a more upright position, so reach is less critical and comfort comes first. Step-through frames make standover almost irrelevant. Gravel bikes use road-style sizing, but many riders choose slightly more stack for long days on rough surfaces.

Kids' bikes: sized by wheel, not frame

Age is the least reliable guide – use height and inseam. A child should be able to put both feet flat or on tiptoe when sitting on the saddle while learning, and reach the brake levers comfortably. Buying a bike 'to grow into' makes it harder to control and less fun.

Saddle height and standover checks

Once you have a frame, set the saddle. The LeMond method puts the top of the saddle at inseam × 0.883 above the centre of the bottom bracket – 74.2 cm for an 84 cm inseam. An alternative, the 109% method, sets the distance from the pedal axle (at the bottom of the stroke) to the saddle top at 109% of inseam. In practice, your knee should be slightly bent – about 25–35° – at the bottom of the pedal stroke, and your hips should not rock.

Standover is the gap between your body and the top tube when you stand over the bike with feet flat. Sloping top tubes on modern frames often give more clearance than these minimums, which is fine.

What if you are between two sizes?

• Choose the smaller frame if you have a shorter torso or arms, prefer an upright position or want a more agile bike.
• Choose the larger frame if you have long arms and torso, want a stretched-out, stable position or ride long distances on smooth roads.
• Fine-tune with a shorter or longer stem (usually 10–20 mm changes), saddle position and handlebar width – but do not rely on extreme changes to fix a frame that is the wrong size.

Signs your bike is the wrong size

• Too big: you cannot stand over the top tube, you stretch to reach the bars, numb hands, sore lower back or shoulders.
• Too small: knees come close to the handlebars, cramped feeling, saddle post pulled out past its minimum-insertion mark, toe overlap with the front wheel in tight turns.

Buying a used bike or ordering online

Ask the seller how the seat tube is measured (c-t or c-c) and for the model year, then look up the official geometry chart – reach and stack are the most comparable numbers across brands. A difference of 1–2 cm in seat-tube length matters less than 1–2 cm in reach. If possible, test-ride before you buy or check the shop's return policy.

Frame size conversion: cm to inches

Worked example: one rider, three bikes

A rider is 178 cm tall with an 84 cm inseam. The road formula gives 84 × 0.67 = 56.3 cm, so a 56 cm frame – usually labelled medium/large. For a mountain bike, 84 × 0.226 = 19.0 inches, a large. For a hybrid, 84 × 0.65 = 54.6 cm, roughly 21.5 inches, a medium/large. A good starting saddle height is 84 × 0.883 = 74.2 cm from the centre of the bottom bracket to the top of the saddle. All three results agree with the height charts above, which gives extra confidence. If this rider had an unusually short inseam for their height – say 78 cm – the road result would drop to 52 cm, and the calculator would suggest checking a smaller frame with a longer stem.

Reach and stack explained

Modern geometry charts use two numbers that compare well across brands. Stack is the vertical distance from the centre of the bottom bracket to the top of the head tube; reach is the horizontal distance between the same two points. A higher stack lifts the handlebars and gives a more upright, comfortable position; a longer reach stretches you out. Two 56 cm road frames from different brands can differ by 2 cm in reach – more than the difference between two sizes from the same brand – so once you know your size, compare the stack and reach of the specific models you are considering.

Other fit details that matter

• Handlebar width: road bars are usually close to shoulder width (about 38–44 cm); mountain bike bars are much wider (about 760–800 mm) for control.
• Crank length: most adult bikes use 165–175 mm; shorter riders and those with knee problems often prefer shorter cranks.
• Saddle setback: with the pedals level, a plumb line from the front of the knee should fall roughly over the pedal axle – a starting point that fitters then adjust.
• Women's-specific models: some brands offer narrower bars, shorter reach and different saddles; many women ride standard frames happily, so choose by measurements rather than labels.
• E-bikes: size them like the non-electric equivalent; the motor does not change fit, but the extra weight makes a comfortable standover more important.
• Folding bikes and BMX: folding bikes are mostly one-size with adjustable posts; BMX bikes are sized by top-tube length, not seat tube.

Professional bike fitting

If you ride long distances, have pain while riding or are buying an expensive bike, a professional bike fit can be worth the cost. A fitter measures your flexibility, foot position and riding posture, then adjusts saddle height and setback, cleat position, stem length and handlebar height. Many shops offer a basic fit when you buy a bike. A calculator gets you to the right size; a fit makes that size fit you.

Bike size by inseam: quick table

Copy the size of a bike that already fits

If you already own a bike that feels right, measure it: the distance from the centre of the bottom bracket to the top of the saddle, from the saddle tip to the centre of the handlebars, and the handlebar height relative to the saddle. Look up its model and size on the brand's website for its stack and reach. Those numbers are the best guide to your next bike – often better than any formula.

Test-ride checklist

• Stand over the top tube with feet flat – you should have the clearance listed above.
• Sit on the saddle with hands on the bars: your elbows should be slightly bent and your back comfortable, not locked straight.
• Pedal a few minutes: no hip rocking, and the knee slightly bent at the bottom of the stroke.
• Turn tightly at low speed: your toes should not hit the front wheel (some overlap is normal on small road frames).
• Ride out of the saddle and over a kerb or bump: the bike should feel stable and easy to control.

Sources

1. LeMond G., Gordis K. – Greg LeMond's Complete Book of Bicycling (saddle height = inseam × 0.883) – book
2. Hamley E.J., Thomas V. (1967) – Physiological and postural factors in the calibration of the bicycle ergometer (109% method) – J Physiol 191: 55P–57P