Ask a bike fitter, an ergonomist or a cushion maker how far apart sit bones are and you will get a range, not a number, because there is no single number. The two bony points you sit on — the ischial tuberosities — vary by sex, by build and, less obviously, by how you are sitting when someone measures them. The published studies are small, but they agree on the shape of the answer, and it is worth knowing before you buy a saddle, a chair or a cushion with openings in it.
The short version: most adults measure somewhere between about 100 and 155 mm center to center — roughly 4 to 6 inches. Women average about 13 mm wider than men. Reclining pushes the contact points apart by about a centimeter. And the method used to measure changes the result by a similar amount, so a number is only meaningful with its method attached.
What the studies measured
Two ways of measuring show up in the literature, and they answer slightly different questions. The impress method has you sit on a soft material — clay in the lab, corrugated cardboard at home — and measures between the centers of the two dents. The seated-pressure method puts a pressure mat on a hard seat and measures between the two peaks of pressure. Impressions read a little narrower than pressure peaks in an upright sitter, so the table keeps them apart.
| Study | Who | Method | Men | Women |
|---|---|---|---|---|
| Chen & Yang, J. Phys. Ther. Sci., 2016 | 15 men, 15 women, Taiwan | Impress (clay) | 119.6 mm (4.7″) | 135.3 mm (5.3″) |
| Scientific Reports, 2021 | 45 men, 45 women, about 21 years old, Taiwan | Impress, sitting upright (90°) | 113.9 ± 7.8 mm (4.5″) | 127.1 ± 11.4 mm (5.0″) |
| Scientific Reports, 2021 | same 90 people | Pressure mat, sitting upright (90°) | 120.7 ± 13.4 mm (4.8″) | 134.0 ± 8.7 mm (5.3″) |
| Velagapudi & Ray, 2019 | 63 men, India (motorcycle riders) | Pressure mat, hard seat | 111.8 ± 9.3 mm (4.4″); 5th–95th percentile 97–128 mm | — |
| PLOS ONE, 2023 | 710 women, 21–50, China | MRI, bone to bone | — | 121.8 ± 11.7 mm (4.8″) |
Use this chart on your own site
Copy the snippet below; it credits the source. The image is free to reuse with the link intact.
The MRI row is a different kind of number: it is the skeletal distance between the two tuberosities, with no skin or tissue in the way. It lands close to the surface measurements anyway, which is reassuring — the dents in a piece of cardboard really are tracking the bones.
Four things the numbers say
1. The typical adult is 4 to 6 inches
Every mean in the table sits between 112 and 136 mm — 4.4 to 5.3 inches — and the spread around each mean is about a centimeter either way. The only population with published percentiles, the 63 Indian riders, ran from 97 mm at the 5th percentile to 128 mm at the 95th. Put the studies together and a range of roughly 100 to 155 mm covers the great majority of adults, with genuine outliers on both sides.
2. Women measure wider, by about half an inch
The 2021 study put the difference at 12.6 to 13.7 mm regardless of method or posture; the 2016 pilot found a slightly larger gap by impression. A wider pelvic outlet is the reason, and it is why saddles marketed to women start wider. It is an average, not a rule — the ranges for men and women overlap heavily, which is the whole argument for measuring rather than assuming.
3. Reclining spreads the contact points
This is the finding that matters most for seats and almost nobody mentions. In the 2021 study the pressure peaks moved apart as the trunk–thigh angle opened: men went from 111 mm leaning forward at 60° to 123 mm reclined at 105°, women from 125 to 136 mm. About a centimeter, from the same bones, just by sitting back. So the number a bike fitter takes with you leaning forward is not the number your car seat sees with you reclined — a driving position is close to the wide end of your own range.
4. The method is part of the number
Upright, the pressure mat read about 7 mm wider than the clay impression for the same people. Neither is wrong; they measure different things (where pressure peaks versus where bone sinks deepest). A cardboard-at-home measurement is an impression, so compare it with impression figures — and with a product whose fit range was set for impressions, not pressure maps.
Where a cutout cushion fits into this
The Socket Seat’s two openings are 6″ apart center to center and 3.5″ wide, and the cardboard method is exactly how its designed range of about 3.6 to 6.7 inches (91 to 170 mm) was set. Every mean in the table falls inside that range, most of them near its middle; the tails are the reason the fit checker exists. If you measure and land outside, the honest answer is that the openings will not line up, and we would rather tell you than sell you.
Measure, do not estimate
Height does not predict it well. Hip width does not predict it well. Sex shifts the average by a centimeter but the overlap swallows the difference. The only reliable way to know your own number is to sit on corrugated cardboard for thirty seconds and measure between the dents — and to do it in the posture you care about. For a saddle, lean forward; for a desk chair or a car, sit as you actually sit. If you ride, the same number sizes a saddle once the fitting guides’ allowance for your riding position is added — nothing when you are fully extended, up to 4 cm sitting upright; that article explains why the two uses share one measurement. The saddle width calculator does the arithmetic by riding position.
If your number is inside the range
The Socket Seat puts open space under both sit bones and firm foam under everything else. $99.99 on Amazon, 100 days to decide, free returns.
Sources: Changes in external ischial tuberosity width at varying trunk–thigh angles between sexes using two measurement methods, Scientific Reports 11, 2021 · Chen & Yang, A preliminary study of the measurement of external ischial tuberosity width and its gender differences, J. Phys. Ther. Sci. 28(4), 2016 · Velagapudi & Ray, Measurement of the width of ischial tuberosity for Indian male motorcycle users, Research into Design for a Connected World, 2019 · Predicting mid-pelvic interspinous distance in women using height and pubic arch angle, PLOS ONE, 2023. Millimeter figures are the studies’ own; inch conversions are rounded.