What this calculator does
Frame size is estimated from inseam rather than height, because leg length varies considerably between people of the same height and it is leg length that determines saddle position. The standard multipliers are 0.67 for road, 0.64 for trekking and the road figure less 11 cm for mountain bikes.
An 80 cm inseam gives a 53.6 cm road frame, a 51.2 cm trekking frame and a 42.6 cm mountain frame. Those three numbers describe the same rider, which is why frame sizes cannot be compared across bike types without converting.
The formula
The inseam is multiplied by the factor for the bike type: 0.67 for road, 0.64 for trekking and city bikes, and the road figure less 11 cm for mountain bikes. All three are reported regardless of which type is selected, so the equivalent size in another category is visible. The results are seat tube lengths in centimetres.
| Term | Meaning |
|---|---|
| Inseam | Crotch to floor with shoes off, measured against a wall with a book pulled up firmly. |
| Seat tube length | The traditional frame size measurement, though manufacturers measure it inconsistently. |
| Standover height | Clearance over the top tube, which mountain bikes need more of, hence the 11 cm reduction. |
| Reach and stack | Modern frame geometry measures that describe fit better than seat tube length alone. |
The inputs explained
| Field | What to enter |
|---|---|
| Inseam length (cm) | Inseam in centimetres. Measure without shoes, standing against a wall, with a book pulled firmly up between the legs and its top edge marked. |
| Bike type | Bike type, which sets the multiplier. All three results are shown regardless. |
When to use it
Buying a bike online
Without the bike present to try, inseam-based sizing is the standard starting point and narrows the choice to one or two sizes.
Comparing sizes across bike types
A road 54 and a mountain 43 are the same rider, which is not obvious from the numbers and is exactly what the three-way output shows.
Checking a second-hand frame
A frame advertised by size can be checked against your own figure before travelling to look at it.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What frame size does each inseam give?
A range of inseam measurements with all three frame types.
| Inseam | Road frame size | Mountain frame size | Trekking/city frame size |
|---|---|---|---|
| 70 cm | 46.9 cm | 35.9 cm | 44.8 cm |
| 80 cm | 53.6 cm | 42.6 cm | 51.2 cm |
| 90 cm | 60.3 cm | 49.3 cm | 57.6 cm |
Questions
How do I measure my inseam for a bike?
Stand against a wall without shoes, feet about 15 cm apart, and pull a book firmly up between your legs as though it were a saddle. Mark the top edge of the book on the wall and measure to the floor. Pulling firmly matters, since a loose measurement reads several centimetres short.
Why is inseam used rather than height?
Because riders of the same height differ considerably in leg length, and it is leg length that determines saddle height and therefore frame size. Height-based charts are a rougher guide and go wrong for people with proportionally long or short legs.
Why are mountain frames smaller than road frames?
For standover clearance. Off-road riding involves dismounting suddenly and unpredictably, and a lower top tube reduces the risk of landing on it. The 11 cm difference is a convention that has held even as frame geometry has otherwise changed a great deal.
Is seat tube length still the right measurement?
Less than it used to be. Modern frames vary in how the seat tube is measured and many use sloping top tubes that make the figure less meaningful. Reach and stack describe fit more reliably, and most manufacturers now publish both. Treat this as a starting point for which size to try.
For gearing once the frame is chosen, see the bike gear calculator. For a fuller sizing approach, see the bike sizing calculator.