How to Measure Sit Bone Width for a Bike Saddle Fit

Your sit bones are the two bones you can feel pressing into a hard chair. The bike industry has built a whole fitting step around measuring the distance between them, the measurement is genuinely easy to take at home, and it is worth taking.

It is also worth knowing exactly what the number does and does not settle, because that part usually goes unsaid.

What you are measuring

The bike-fitting company BikeFit puts the anatomy plainly: “The Ischial Tuberosity is commonly referred to as the ‘sit bones,’ ‘sitting bones,’ or ‘sitz bones’”. They are the lowest bony points of the pelvis, and seated on a hard surface they take the load.

Worth noting up front that BikeFit sells bike-fitting products, and says so themselves on that page: “While we’d like to remain entirely unbiased, but that’s incredibly unlikely.” We quote them because they describe the method and its limits more carefully than most, not because they are disinterested.

The cardboard method, in their words

“sitting upright on a piece of corrugated cardboard for a designated period of time. After that point, you’ll mark the main impressions of the sit bones, find the center point of the two impressions, and finally measure the distance between the two points with a ruler or tape measure.”

Three details in that sentence do the work:

Corrugated cardboard, which is what both pages specify. A folded towel is unlikely to hold two distinct marks.
Sitting upright, which we will come back to, because it is a condition and not a detail.
Centre to centre, not the outside edges of the two dents. Measuring edge to edge inflates the number, and everything downstream is built on the centre figure.

BikeFit describes the same measurement being taken in shops on a gel pad instead of cardboard. The principle is identical.

Take it twice on separate days before you trust it. This is the same habit that makes a tape measure reliable anywhere else on the body — see how to measure your bust for why one reading is not a measurement.

What the published method leaves open

Read that instruction closely and you will notice it does not say how long. “A designated period of time” is as specific as it gets, and no duration for the impression appears on that page.

Another page does give one for the sitting step. BiSaddle says to “hold your natural riding posture for 10–15 seconds”.

But those two pages do not describe the same posture. BikeFit says to sit upright. BiSaddle says to “sit down upright, but crucially, tilt your pelvis forward slightly as if you’re on the bike — don’t sit back like you’re in an office chair.”

So the honest position is not that nobody specifies the details. It is that the two pages we read specify different ones, and a forward-tilted pelvis will not press into the cardboard the same way an upright one does. That is a reason to repeat the measurement, and to note which instruction you followed, rather than to distrust it.

The same goes for what you are sitting on underneath the cardboard. BikeFit’s instruction does not say; BiSaddle’s does — “a hard chair”, with the cardboard on “the firm seat”. A padded seat is unlikely to give the same impressions, so pick a hard, flat one and use the same one both times, so that if the two readings differ you know it was you and not the furniture. If the readings are close, take the wider of the two — on a saddle, as on an exercise bike weight limit, the number that matters is the one at the edge of the range rather than the middle of it.

Now the part that is not standardised

You have a number in millimetres. A saddle is wider than your sit bones, so something gets added. How much?

BikeFit writes the relationship out exactly as the industry uses it:

“Width of sit bones + X (Defined as added width by manufacture) = the saddle made for you.”

Read that middle clause again. X is defined by the manufacturer. It is not a standard, not a published constant, not something you can look up independently of whoever is selling you a saddle.

And the figures in circulation do not agree. BikeFit reports that “Many suggest adding about 20mm”, then notes that in the example they are discussing the rider “measured 110mm, added 20mm, but choose a 145mm width saddle” — which is 35 mm, not 20. The saddle maker BiSaddle, which sells saddles, gives an example where “a 120mm sit bone width suggests a saddle in the 140mm to 150mm range” — 20 to 30 mm, as a range.

The saddle maker SQlab writes it as “Sit bone width + area of use = your individual saddle fit”, which is the same equation with the second term named after riding style rather than a number.

So: the measurement is standard, the arithmetic after it is not. We are not going to hand you a number to add, because we would be picking one manufacturer’s convention and presenting it as a fact.

The condition attached to the whole method

This is the part worth reading before you spend money on a saddle.

The measurement as BikeFit describes it is taken sitting upright. BikeFit points out that this posture is doing a lot of quiet work: the method assumes “a 90-degree spine position”, while “the majority of cyclists will have some type of pelvic bend”. Their conclusion, in their words, is that in those rotated positions “the sit bones would not be your main pressure point or measurement point for potential saddle comfort.”

That is a criticism of the method’s scope, not a claim that the measurement is useless — and the scope cuts in a direction worth noticing. The more upright you sit, the more the measurement describes what is actually happening. An upright commuter, a city e-bike, an exercise bike at home: those are the positions where your weight really is on the sit bones, and where the number is describing your contact rather than someone else’s.

What to do with the number

Take the measurement twice, centre to centre, on corrugated cardboard.
Note your riding posture honestly — upright, or leaned forward. It changes how much the number is telling you.
Use the saddle maker’s own chart, and treat the added width as that maker’s convention rather than a fact about your body.
Treat width as one input. What else it does and does not settle is covered in our guide to the most comfortable bike seat, including a published definition of usable width that is not the same as the number on the box.

We have not fitted anyone, have not used the saddles named above, and are not recommending any of them. Every quotation here is attributed and linked, including all three sources, each of which sells the products it is describing.

Frequently asked questions

How do I measure my sit bone width at home?

The bike-fitting company BikeFit describes the cardboard method as 'sitting upright on a piece of corrugated cardboard for a designated period of time. After that point, you'll mark the main impressions of the sit bones, find the center point of the two impressions, and finally measure the distance between the two points with a ruler or tape measure.' The measurement is centre-to-centre, not edge-to-edge.

How much do I add to my sit bone measurement to get a saddle width?

There is no single published figure, and the pages we read do not agree. BikeFit writes the relationship as 'Width of sit bones + X (Defined as added width by manufacture)' — the added amount is set by whoever makes the saddle. The saddle maker BiSaddle gives an example where '120mm sit bone width suggests a saddle in the 140mm to 150mm range', which is anywhere from 20 to 30 mm.

Does sit bone width settle which saddle I need?

Not by itself. BikeFit, which sells bike-fitting products, points out that the measurement method assumes a 90-degree spine position, and that 'the majority of cyclists will have some type of pelvic bend', so in those positions 'the sit bones would not be your main pressure point or measurement point for potential saddle comfort.' The more upright you ride, the more the measurement is describing your actual contact.