Bike Size Calculator
This bike size calculator gives you a starting frame size for a road, mountain, hybrid or gravel bike based on your inseam. Pick the bike type, enter your inseam in inches or centimeters, and the calculator applies the common fit rule of thumb for that type. Add your height to see where you land on a typical S, M, L size chart.
Frame sizes are not standardized. Brands measure seat tubes differently and use their own geometry, so two bikes labeled 54 cm or Medium can fit quite differently. Treat these numbers as a way to narrow the choice before you check the manufacturer's size chart and, ideally, take a test ride.
Bike Size Calculator
How to Use the Bike Size Calculator
- 1
Measure your inseam
Stand barefoot with your back to a wall, pull a hardcover book up firmly between your legs like a saddle, and measure from the floor to the top edge of the book.
- 2
Choose units and bike type
Leave the toggle off for inches or switch on Metric units for centimeters, then choose road, mountain, hybrid/city or gravel.
- 3
Enter inseam and height
Type your inseam. Optionally add your height in feet and inches or centimeters to get a size letter from the typical chart.
- 4
Compare the results
Read the inseam-based frame size, the size by height, and the highlighted row in the chart. If they disagree, use the manufacturer's chart and a test ride to decide.
The rules of thumb this calculator uses
Each bike type uses a widely published multiplier that turns inseam into seat tube length. These rules date from frames with level top tubes, so they are approximations for modern sloping frames.
Road: inseam (cm) x 0.665 to 0.67. MTB: inseam (in) x 0.66 - 4 to inseam (cm) x 0.226. Hybrid: inseam (cm) x 0.66- Road bike: frame size (cm) = inseam (cm) x 0.665 to 0.67
- Gravel bike: sized like a road bike, inseam (cm) x 0.665 to 0.67
- Mountain bike: frame size (in) = inseam (in) x 0.66 minus 4; a second common rule, inseam (cm) x 0.226, gives the top of the range
- Hybrid or city bike: frame size (cm) = inseam (cm) x 0.66, shown in inches and cm
Worked example: 31-inch inseam, 5 ft 8 in tall
A 31-inch inseam is 78.74 cm. Here is what the calculator returns for each bike type, and what the height chart says for a 5 ft 8 in (172.7 cm) rider.
- Road: 78.74 x 0.665 = 52.4 cm and 78.74 x 0.67 = 52.8 cm, shown as 52 to 53 cm. The height chart says M, 54 to 55 cm
- Mountain: 31 x 0.66 - 4 = 16.5 in and 78.74 x 0.226 = 17.8 in, so about 16.5 to 17.8 in. The height chart says M, 17 to 18 in
- Hybrid: 78.74 x 0.66 = 52 cm, or 20.5 in. The height chart says M, 17 to 18 in
Why inseam and height can give different answers
In the example, the mountain bike results agree, while the road and hybrid results do not. The inseam rules measure a traditional seat tube, and many modern frames have a sloping top tube with a shorter seat tube than the size on the label. Height charts follow how brands label sizes today.
Your proportions matter too. Riders with long legs for their height may prefer the size that gives more standover clearance, and riders with a long torso often want the size with more reach. When the numbers disagree, the manufacturer's chart and a test ride should decide.
Typical bike size charts by height
These are the typical ranges shown in the calculator. Individual brands vary.
| Rider height | Road and gravel | Mountain and hybrid |
|---|---|---|
| 4 ft 10 in to 5 ft 0 in (148 to 152 cm) | 47 to 48 cm (XXS) | 13 to 14 in (XS) |
| 5 ft 0 in to 5 ft 3 in (152 to 160 cm) | 49 to 50 cm (XS) | 13 to 16 in (XS to S) |
| 5 ft 3 in to 5 ft 6 in (160 to 168 cm) | 51 to 53 cm (S) | 15 to 16 in (S) |
| 5 ft 6 in to 5 ft 9 in (168 to 175 cm) | 54 to 55 cm (M) | 17 to 18 in (M) |
| 5 ft 9 in to 6 ft 0 in (175 to 183 cm) | 56 to 58 cm (L) | 17 to 20 in (M to L) |
| 6 ft 0 in to 6 ft 3 in (183 to 191 cm) | 58 to 60 cm (XL) | 19 to 22 in (L to XL) |
| 6 ft 3 in to 6 ft 6 in (191 to 198 cm) | 61 to 63 cm (XXL) | 21 in and up (XL to XXL) |
Fit tips and common mistakes
- Measure inseam barefoot and press the book up firmly. Trouser inseam is usually a couple of inches shorter than cycling inseam.
- Compare stack and reach, not just the size label. Those two numbers describe how long and tall a frame really is.
- Check standover. You should be able to straddle the top tube with both feet flat and some clearance, more on a mountain bike.
- Small fit changes are cheap. Saddle height, stem length and handlebar width can fine-tune a frame that is close.
- For bike fit problems such as knee, back or hand pain, a professional bike fitter can help more than a size chart.
Features
- Frame size from inseam for road, mountain, hybrid and gravel bikes
- Inches or centimeters
- Optional height lookup with S/M/L size
- Typical size charts by rider height
- Shows the rule of thumb used for each bike type
Frequently Asked Questions
What size bike do I need for my height?
As a typical guide, riders 5 ft 6 in to 5 ft 9 in (168 to 175 cm) fit a medium road bike around 54 to 55 cm, and riders 5 ft 6 in to 5 ft 10 in fit a medium mountain bike around 17 to 18 inches. Brands differ, so use your height and inseam together and confirm with the maker's own size chart.
How do I calculate road bike frame size from inseam?
Multiply your cycling inseam in centimeters by 0.665 to 0.67. An 81 cm inseam gives 81 x 0.665 = 53.9 cm and 81 x 0.67 = 54.3 cm, so about a 54 cm frame. This rule assumes a traditional seat tube measurement, so compare it with the manufacturer's size chart for the specific bike.
How do I measure my inseam for a bike?
Stand barefoot against a wall with feet about 6 inches apart. Pull a hardcover book up firmly between your legs, spine up, as a saddle would sit. Mark the top of the book on the wall and measure from the floor to the mark. Take two or three measurements and use the average.
What size mountain bike do I need with a 32-inch inseam?
With a 32-inch inseam, the rule inseam x 0.66 minus 4 gives 17.1 inches, and the alternative rule inseam (81.3 cm) x 0.226 gives 18.4 inches. That points to a medium or medium-large frame, around 17 to 18 inches. Mountain bike sizing varies a lot by brand, so check reach and the maker's chart.
Should I size up or down if I am between sizes?
It depends on your proportions and riding. A smaller frame is easier to handle and gives more standover clearance; a larger one is more stretched out and stable. Riders with a long torso often go up, and riders with long legs and a short torso often go down. A test ride is the best way to decide.
Are gravel bikes sized like road bikes?
Mostly, yes. Gravel bikes usually use road-style sizes in centimeters or S/M/L labels, so this calculator applies the road rule of inseam (cm) x 0.665 to 0.67. Gravel frames often have a taller stack and slightly longer wheelbase than race road bikes, so the same size can feel more upright.
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