You can measure your sit bones at home using a piece of corrugated cardboard and a hard, flat surface. The process takes about five minutes and gives you a reliable starting point for choosing the right bike saddle width. Most people’s sit bone width falls somewhere between 100 and 150 mm, and knowing your number helps you pick a saddle that supports your pelvis instead of pressing into soft tissue.
What Sit Bones Actually Do
Your sit bones, formally called the ischial tuberosities, are the two bony knobs at the bottom of your pelvis. When you sit upright on a hard surface, these are the points that bear the majority of your body weight. On a bike saddle, they’re meant to act like the two feet of a bridge, resting on the wider part of the saddle so your weight stays on bone rather than on the nerves and blood vessels between them.
When you lean forward into a more aggressive riding position, your pelvis rotates and the pressure shifts away from the sit bones toward the front of the pelvis. This is why riding position matters for saddle choice, not just sit bone width alone.
The Cardboard Method
This is the most commonly recommended approach and requires only two things: a piece of corrugated cardboard (the kind with the wavy layer inside) and a hard, flat surface like a wooden stool, step, or sturdy coffee table. A soft or contoured chair won’t work because it absorbs the impression before the cardboard can capture it.
Place the cardboard on the hard surface and sit down on it with an upright posture, spine roughly vertical. Keep your feet flat on the floor with your knees at about 90 degrees. Sit for 30 to 60 seconds with your full weight pressing down, then stand up carefully without shifting the cardboard.
You should see two oval-shaped dents in the cardboard where your sit bones pressed in. If the impressions are faint, try sitting a bit longer or bouncing gently a couple of times before holding still. Once you have clear marks, find the center of each oval and measure the distance between those two center points with a ruler or tape measure. That distance, in millimeters, is your sit bone width.
Tips for a Clearer Impression
Wear thin clothing or shorts. Thick jeans or padded cycling shorts can blur the indentations. If the cardboard is too stiff to show clear marks, try a slightly thinner piece, though it still needs the corrugated inner layer to hold an impression. You can also lightly dust the cardboard with chalk or baby powder beforehand so the compressed spots are easier to spot visually.
The Aluminum Foil Method
If you can’t get a clear impression from cardboard, aluminum foil over a lightly padded surface works well as an alternative. Fold a towel in half and place it on a flat, firm surface like a coffee table or a carpeted stair step. Lay a sheet of aluminum foil smoothly over the towel, then sit straight down on it with an upright posture.
When you stand up, you’ll see two distinct depressions in the foil where your sit bones pressed through. One useful trick: place two small round objects (ball bearings, marbles, or even dried peas) into the impressions. They’ll settle into the deepest points naturally, making it easy to measure the distance between centers. That measurement is your sit bone width.
How Riding Position Changes the Number
Your sit bone width isn’t completely fixed. It changes depending on how far forward you lean. A study published in Scientific Reports measured this directly and found that the effective width between the sit bones increases as you sit more upright. Across a full range of positions, the difference was about 11.5 mm for men and 11.7 mm for women when measured by pressure mapping. At a forward-leaning 60-degree angle between torso and thigh, the average width was around 111 mm. At an upright 105-degree angle, it grew to roughly 123 mm.
This matters because a road cyclist hunched over drop bars and a commuter sitting upright on a city bike are loading their saddle in different places. Measuring while sitting upright gives you the widest reading, which works well for upright riding. If you ride in a more aggressive position, your effective contact width will be narrower.
From Measurement to Saddle Width
Your sit bone width is not the same as your ideal saddle width. Saddles need to be wider than the gap between your bones so each sit bone has a platform to rest on. Most saddle manufacturers recommend adding a specific number of millimeters to your measurement based on your riding style.
- Upright city or touring bikes: Add roughly 20 to 30 mm to your sit bone width.
- Mountain bikes: Add around 15 to 25 mm, since you’re in a moderately forward position.
- Road bikes with an aggressive fit: Add about 10 to 20 mm, accounting for the pelvic rotation that narrows your contact points.
So if your cardboard measurement comes out to 120 mm and you ride a hybrid bike in a fairly upright position, you’d start by looking at saddles in the 140 to 150 mm range. These numbers are starting points. Saddle comfort also depends on padding density, shape, and how the saddle matches your specific anatomy beyond just width.
Common Mistakes to Avoid
The most frequent error is using a surface that’s too soft. Sitting on a cushioned chair or thick carpet pad spreads your weight across a larger area and makes it impossible to isolate the two bony contact points. You need a hard, flat surface underneath whatever impression material you’re using.
Another common issue is not sitting with enough weight or shifting around during the measurement. Sit still, let your full weight settle, and keep your spine upright. Leaning forward or slouching changes which part of your pelvis makes contact with the surface and will skew the measurement.
Finally, take the measurement two or three times and average the results. Home methods aren’t precise to the millimeter, but if you’re consistently getting readings in the same range (say, 118 to 124 mm), you have a reliable enough number to guide your saddle choice. If your readings vary by more than 10 mm between attempts, the impressions probably aren’t clear enough. Try a different material or press more firmly.