Bike Frame Size Calculator
| Height | Road frame (cm) | MTB size |
|---|---|---|
| 5'2" (158) | 47-48 | XS |
| 5'4" (163) | 49-50 | S |
| 5'6" (168) | 51-52 | S |
| 5'8" (173) | 52-53 | M |
| 5'10" (178) | 54-55 | M |
| 6'0" (183) | 56-57 | L |
| 6'2" (188) | 58-59 | L |
| 6'4" (193) | 60-61 | XL |
Frame size is the one bike number that cannot be compensated with a seatpost or a stem swap - it sets the standover clearance and the reach, and it is measured differently by discipline: road bikes in centimeters of seat tube (your inseam times 0.70), mountain bikes in inches or the S/M/L letters that stand in for them. This calculator takes your height and inseam and hands back both, with the full chart below.
Bottom line: a 30-inch inseam (76 cm) is a 53 cm road frame - 76 x 0.70 - which is the same rider as a 54 cm if the top tube runs long; on the mountain side the same rider lands on a medium (17-18 inch) frame. When two chart sizes argue, the smaller frame with a longer stem is the safer buy: you can add reach, but you cannot remove standover.
The honest part: the x-0.70 rule is a starting point from racing geometry - endurance road bikes run taller head tubes that effectively size up, and modern mountain bikes size by reach more than by seat tube, which is why two 'large' hardtails can feel nothing alike. Test ride when it is an option; when it is not, buy the frame that lets you stand flat-footed over the top tube with an inch to spare.
How to use
- Measure inseam: stand barefoot against a wall, put a book spine-high into your crotch (mimicking a saddle), and measure floor-to-book in inches or centimeters.
- Enter height and inseam - the calculator returns the road frame in centimeters and the mountain-bike letter size.
- Check the standover rule before buying: one to two inches of clearance over the top tube on a road bike, two to four on a mountain bike.
Frequently asked questions
How do I measure my inseam for a bike?
The book method: stand barefoot 6 inches from a wall, clamp a hardcover book spine-up into your crotch to simulate sitting on a saddle, and measure from the floor to the book's top edge. That number - not your trouser inseam, which is cut long - is what every frame formula and the saddle-height rule (inseam x 0.883) expects. Do it twice; the first attempt is usually a half inch generous because the book was not level.
What size road frame for my height?
Roughly: 5'6" (167 cm) rides 51-52 cm, 5'9" (175 cm) rides 54 cm, 6'0" (183 cm) rides 56-57 cm, and 6'3" (190 cm) rides 58-60 cm - the full ladder is in the table. But inseam beats height for the frame decision, because riders of the same height carry different leg-to-torso ratios: long legs push you up a size for standover, long torsos push you down and out with a longer stem. The calculator weighs both inputs for exactly this reason.
Is a mountain bike size the same as a road size?
No - MTB frames run about 10-12 cm 'smaller' in numbers because the wheels are smaller and the geometry slacker, and most brands have abandoned inches for T-shirt letters: a 30-inch-inseam rider takes a medium (17-18 inches) where the road math says 53 cm. Modern trail bikes also size by reach, so the letter on the toptube matters less than the cockpit: if the shop lets you, compare stack and reach numbers between brands instead of the letter.
Should I size up or down between two frame sizes?
Down, with adjustments, for most riders: a smaller frame with a longer stem and raised seatpost reproduces the bigger bike's fit, while a too-big frame's standover and top-tube length cannot be fixed by any part swap. The exceptions are comfort cruisers and endurance road bikes, where sizing up gains stability and a more upright neck angle. Racers size down for agility; tourers size up for a longer wheelbase - decide which rider you are before the chart does it for you.