Inseam, in cycling, is the distance from the crotch to the ground measured barefoot while standing straight. And inseam to height ratio (I/H) is defined as the inseam length divided by total height. The higher this number, the longer your legs are relative to your torso which means that you have a long-legged body type. If you have a low I/H ratio, then this may indicate that you have short legs.
Inseam and height are the two most important measurements when choosing the right bike size. Many guides and bike shops tend to ignore the inseam ratio and focus more on total height, assuming everyone has the same I/H ratio. This results in people getting the wrong bike size and finding it difficult to be comfortable on the bike.
The Average Inseam to Height Ratio (I/H)
Unlike LBR (Leg to Body Ratio), there are no studies about inseam-to-height ratio because inseam has been overlooked by many. We can’t use the available LBR research because leg length is typically measured from the hips using the total height subtracted by the sitting height method.
However, looking at available data from cyclists who have contributed their I/H number, the average inseam-to-height ratio seems to be around 0.46 to 0.47. The bigger your number, the longer your legs are compared to your torso, and vice versa.
It’s unclear if males and females have different average inseam-to-height ratios, but women seem to have proportionally shorter legs than men.
Calculate your Inseam to Height Ratio
What does this number mean for you as a cyclist?
Bikes are designed for the average human being. If your inseam-to-height ratio is very low or very high, the size chart provided by the bike manufacturer might not be the best way to decide the right bike size for you.
If your I/H number falls between 0.46-0.47, it means you have an average leg length compared to other people which will make finding a perfect fit easier.
If you have short legs (low I/H number), you have a long torso which means most bike geometry for your height might have short reach which makes you hunch on the bike and causes back pain. The easiest solution for this problem is to use longer stems.
Some people with short legs also prefer to size down their bikes to get a lower front-end area and be more aero.
If you have long legs (high I/H number), this means that you have a short torso and short reach, unless your arms are longer than average. From a bike fit perspective, people with long legs are generally the hardest to fit because most road bikes for their size will be too long. Unlike the earlier solution with short-legged people, you can’t always reduce the stem length because most road bikes come with a 9-10cm stem already, and reducing the length further will make your bike too twitchy and unstable.
Long-legged people will also have to raise their seatpost more than the average people. A higher seatpost means that the handlebar might feel too low which can cause back pain.
One solution is to get a bike one size bigger. But it’s not a perfect solution because bigger bikes have a longer reach.
The best solution is to choose an “endurance” type road bike with a high stack and a short reach or flip the stem to a positive angle and get a higher stack. Aesthetically not the best, but still better than not being comfortable on the bike. Over time, you might be able to get used to the lower handlebar position, that’s when you flip the stem back.
Road Bikes with Short Reach for People with Short Torso
A large number of performance-oriented road bikes from popular brands have a long reach, but there are non-endurance type road bikes with shorter reach for people with low I/H numbers:
- Cervélo R5
- Pinarello Dogma
- Look 785 Huez
- Time Alpe D’Huez
- Trek Émonda
- Colnago C68
- Felt FR
- Parlee Altum
- Ridley Helium
If you have the geometry sheet for the specific bike you have in mind, look at the stack and reach number. If the ratio of stack divided by reach is 1.40 or bigger, then the geometry is good for people with a shorter torso.
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