How to measure your sit bones

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Of all the numbers in a bike fit — saddle height, reach, drop, setback, bar width — exactly one can be measured at home, with no fitter and no jig: saddle width. The reason is anatomical. Saddle width is set by a pair of bones you can physically locate at the base of your pelvis, and a bone leaves a mark you can put a ruler across. Everything else in a fit is an angle or a position that changes as you move; width is a fixed piece of you.

That makes this the one fit measurement worth doing yourself before you spend money. It also comes with a warning we’ll repeat, because the saddle industry tends to bury it: the measurement narrows the field, it does not guarantee comfort.We haven’t measured anyone or ridden anything. What follows is the published method from saddle makers’ and bike-fitters’ own guides, quoted and linked, plus the honest limits of what a number can tell you.

Your sit bones carry you, not the padding

The two bones are the ischial tuberosities— “the sit bones, sitting bones, or sitz bones,” as the bike-fit trade puts it. They are the lowest points of the pelvis, and when you sit upright they are what makes contact with the surface. On a bike they are supposed to be the load path: your weight goes down through your pelvis, onto the sit bones, onto the shell of the saddle. SQlab, one of the brands that built a business around this, calls the sit bones “the key contact point between your body and the saddle” and is explicit about what happens when they miss: if the saddle is too narrow to support them, “your body weight shifts to more sensitive areas like the perineum – which can lead to numbness, pain, or even long-term issues.”

Read that mechanism carefully, because it inverts the intuitive fix. When a saddle hurts, the instinct is to add padding. But padding doesn’t change where your skeleton lands — if the saddle is too narrow, a thicker pad just puts a plusher surface under a pelvis that is still bridging the gap and dropping its weight onto soft tissue. The pad is a comfort layer over the bones; it is not a substitute for a shell wide enough to catch them. That’s the whole reason width is worth measuring, and it’s the same reason our chamois pad guidekeeps saying a pad fixes friction and shock, not a saddle that’s the wrong shape for your anatomy.

How to take the measurement at home

Every home method is the same idea in a different medium: press your loaded sit bones into something that keeps an impression, then measure between the two deepest marks center to center. There are three common media, and the choice between them is mostly what you have in the house.

• Corrugated cardboard on a hard surface.This is the version most saddle guides describe. Ergon says corrugated cardboard “works exceptionally well”; bike-components’ guide has you place the cardboard corrugated side up on a “flat and hard seating surface”, sit so your knee and hip angles are about 90°, then “arch your back and pull yourself firmly against the sitting surface with your hands” to drive the bones in. Stand up, circle the two imprints, mark each center, and measure between the centers.
• Aluminium foil on a soft surface. A bike-shop version swaps cardboard for foil: set a sheet of foil “on something soft like carpeting or a yoga mat,” ideally on a low table or a couple of steps, “sit on the foil for about 30 seconds and then stand up,” and “measure from center to center on each of the dimples.” The soft base lets the foil wrap the bones and leaves two clear low points.
• Memory foam.The same trick again — a firm memory-foam cushion holds an impression long enough to mark and measure. It’s the least precise of the three because foam rebounds, so read it quickly.

Whichever you use, a few things decide whether the number is any good. Sit the way the guides say — upright, knees and hips near 90°, weight genuinely on the bones — and do it two or three times to make sure the marks repeat. Watch what’s between you and the surface: a stiff pair of jeans with back-pocket rivets can leave an impression that isn’t your skeleton at all. And treat the online “sit-bone calculators” that ask for a trouser size with suspicion — Singletracks notes plainly that such tools “provide an estimate, and not an exact measurement of your sit bone width.” Two dents and a ruler beat a questionnaire.

From sit-bone width to saddle width: the allowance, and why it moves

Here is the step that trips people up: the number you just measured is not the saddle width you buy. The saddle has to be wider than your sit bones so the bones land on the supportive part of the shell rather than perched on the edge. The common rule-of-thumb is to add roughly 20mm— BikeFit describes many companies suggesting you “add about 20mm,” with the worked example that “if your sit bones measured 130mm, you add 20mm” for a 150mm saddle; a bike shop’s guide makes the same jump from a 135mm measurement to a 155mm saddle. Singletracks frames the floor rather than a target: keep your sit-bone width “narrower than the width of the saddle by at least 15mm” so you’re not sitting right on the edge.

But a single fixed number is where the honest guides and the lazy ones part company, because the right allowance depends on how far forward you rotate your pelvis.The more upright you sit, the more of your weight lands squarely on the sit bones, and the wider the shell needs to be to hold them. The more you rotate forward into an aggressive road or aero position, the less width you need. bike-components’ guide puts real numbers on the two ends: for an upright position the allowance “could be up to four centimeters”, while “for a fully extended position, like on a triathlon bike, your sit bone width matches the saddle width” — that is, an allowance of essentially zero.

There’s a subtler reason underneath that, and it’s the mechanism worth understanding. Your pelvis is roughly V-shaped, so the distance between your contact points isn’t constant — it changes as you tip forward. BikeFit cites Selle Royal’s research on exactly this: “ischial (sit bone) distance varies according to riding position due to the v-shape pelvic anatomy. In the more inclined spine angle, the distance will become shorter as the contact points move from the seat bones toward the pubic bones.” In other words, an aggressive rider isn’t sitting on the same wide part of the pelvis they measured while upright at 90° — the contact has migrated forward onto a narrower structure. Measure upright, buy a wide saddle, then ride slammed in the drops, and you can end up on a saddle that’s wrong for the position you actually use. This is precisely the trade-off that feeds into the bigger decision in how to choose a bike saddle, where width is one input among nose shape, cutout, length and tilt.

Brands run their own width systems — the number isn’t portable

Once you have a measurement and an allowance, you meet the next complication: a saddle width printed by one brand doesn’t mean the same thing on another. Manufacturers measure their saddles at different points along the shell and shape them differently, so “150mm” is a brand-internal size, not a universal spec. That’s why the serious brands sell you into their own sizing system rather than a raw millimeter.

SQlab was the first to build a formal one: it combines your measured sit-bone distance with your typical riding position, “since a more aggressive posture shifts the pressure point further forward on the saddle,” and offers a range of widths across almost every model so the recommendation lands on a real product. Ergon runs a Small/Medium/Large system and puts the practical dividing line around a measured 12 centimeters — near the boundary between their sizes. Specialized and Bontrager operate their own in-store fit systems on the same principle. The upshot for you: your home measurement tells you which size bandyou fall in, and after that you’re shopping inside one brand’s chart at a time, not carrying a single magic number between them.

What the measurement can and can’t do

This is where we’re going to disappoint the algorithm. Getting your sit-bone width right is necessary and it eliminates a lot of bad choices — a saddle two sizes too narrow was never going to work, and now you won’t buy it. But BikeFit says the quiet part out loud: “there’s no guarantee that a sit bone measurement based on the 90-degree riding position will provide you with a comfortable saddle.” Width is one variable. The saddle’s length, the width of its nose, whether it has a cutout or a relief channel, how you tilt it, and where you sit fore-and-aft all decide whether a correctly-sized saddle is actually comfortable in your position.

And the saddle is only half of the contact system. The other half is what you wear on it. A well-cushioned, correctly-positioned chamois can rescue a saddle that’s slightly off, and a bad one can ruin a saddle that fits perfectly — which is why we treat the saddle and the shorts as a single unit in the bib shorts guide. Measure your sit bones, use the number to shortlist two or three sizes inside a brand you can actually try, and buy where there’s a test-saddle program or a return window. The measurement gets you to the right neighbourhood. The last few millimetres of comfort are found on the road, and nobody — us included — can measure them for you at a table.