Bike Frame Size Calculator
Road and MTB frames from inseam.
What it does
Frames start at inseam. Road multiplies by 0.67 for a long efficient perch while mountain multiplies by 0.58 for standover clearance on rough ground. Eighty one centimeters suggests fifty four road with forty seven mountain.
The tool above runs both products. Defaults give 54 cm road with 47 cm MTB. A 76 cm inseam gives 51 cm road with 44 cm MTB. The imperial mirror at 32 inches gives 21.4 road with 18.6 MTB. Standover and reach confirm before money moves.
Bodies bend baselines. Long torsos size up for cockpit room while short arms size down, and aggressive racers differ from upright commuters. Makers also slope top tubes and name sizes by seat tube, top tube, or small through large.
This page estimates starting sizes only. It does not fit saddles, stems, or crank arms. Shops size the rider on the bike before selling.
How to use
- Pick metric (cm) or imperial (in).
- Measure cycling inseam barefoot.
- Read road frame first.
- Read MTB frame.
- Round to stocked sizes.
- Test ride with saddle set.
Formula
- inseam
- Floor to crotch cycling inseam.
- 0.67
- Road factor for drop bar reach.
- 0.58
- Mountain factor for standover clearance.
Cycling inseam baseline for starting sizes. At very short or tall inseams stock frames step in 2 cm jumps, so round with the shop. Torso, arms, and style move the pick.
Worked example
Example: 81 cm inseam
Defaults: metric 81 cm cycling inseam barefoot to floor. Measure with a book snug.
- Inputs (metric default): 81 cm inseam.
- Road: 81 times 0.67 is 54 cm road.
- MTB: 81 times 0.58 is 47 cm MTB.
- Imperial mirror (32 in) gives 21.4 in road with 18.6 in MTB.
- Test ride before buying. Tool shows the same.
Reference table
| Inseam | Road | MTB |
|---|---|---|
| 81 cm | 54 cm road | 47 cm MTB |
| 32 in | 21.4 in road | 18.6 in MTB |
| 76 cm | 51 cm road | 44 cm MTB |
| 70 cm | 47 cm road | 41 cm MTB |
| 35 in | 23.5 in road | 20.3 in MTB |
Limits
- Starting sizes. Fit confirms.
- One formula. Geometry varies.
- No parts fit.
- Stock steps. Exact math rounds.
Common mistakes
- Measuring pants inseam instead of cycling.
- Skipping standover checks.
- Buying by height alone.
- Ignoring torso differences.
- Ordering without a test ride.
Try it yourself
Quick checks. Answers sit tucked under each question.
32 in inseam?
Road is 32 times 0.67 is 21.4 in with 18.6 in MTB.
76 cm inseam?
Road is 76 times 0.67 is 51 cm with 44 cm MTB.
Questions
How are frame sizes figured?
Road inseam times 0.67, MTB times 0.58. Here 54 cm road with 47 cm MTB.
Why do road and MTB differ?
Road leans long for reach, MTB sits small for clearance. Same legs, different jobs.
How do I measure inseam?
Barefoot with a book snug at the crotch to the floor, standing tall.
Do proportions matter?
Yes. Long torso or arms size up while short reach sizes down.
Are sizes standard across brands?
No. Makers slope, list, and name sizes differently. Standover and reach confirm.
Is math enough?
Yes. Test ride with the saddle set before deciding.