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Prepare a firm measuring surface
Put fresh corrugated cardboard on a hard, flat stool or chair. Wear thin clothing without pockets, seams or rivets under the contact points. Keep a pencil and millimetre ruler nearby.
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Free measurement tool
The useful input for saddle width is not your height, weight or body type. It is the centre-to-centre spacing of two sit-bone impressions made with the same method. Measure twice on corrugated cardboard; this assistant checks whether the pair agrees before it adds a clearly labelled riding-posture convention and returns a starting range.
The result is not a diagnosis, pressure map or promise that one saddle will be comfortable. Saddle shape, usable support area, curvature, cut-out, tilt, fore-aft position, clothing and time on the bike still matter. Use the range to narrow a test list, then validate one reversible change at a time.
Before the calculator
Corrugated cardboard compresses under the two bony contact areas and leaves marks that can be measured at home. The quality comes from repeating the whole process, not from drawing increasingly precise centres on the same impression. Use fresh material for the second attempt and write down the first result before making the next one.
Put fresh corrugated cardboard on a hard, flat stool or chair. Wear thin clothing without pockets, seams or rivets under the contact points. Keep a pencil and millimetre ruler nearby.
Sit upright with your shoulders back and feet slightly raised. Press down evenly without rocking from side to side. Stand up carefully so the two compressed areas stay readable.
Outline each compressed area, estimate its centre and mark it with a cross. Measure the straight-line distance between the two centre marks in millimetres. Do not measure the outside edges.
Use a fresh area or new piece of cardboard and repeat the complete sit, mark and measure process without copying the first centres. Enter both readings in the assistant.
If the two readings differ by more than 5 mm, make one fresh third impression. The assistant uses all three readings and never selects only the closest pair. If the set remains unstable, start over or use a shop measuring tool.
Interactive measurement check
Enter millimetres between the centres of two separate cardboard impressions. The tool checks repeatability before applying the selected posture allowance. It does not infer anatomy from height, weight, sex or body type.
Choose your normal seated posture, not the most aggressive position you can briefly hold.
Manufacturer convention: +20 mm. Forward-leaning but sustainable posture used for endurance road and gravel.
Optional; use the maker's labelled overall width.
First, the assistant applies an engineering repeatability rule. Two readings may differ by no more than 5 mm. A larger mismatch produces no width result and requests one fresh third impression. With three readings, every value must remain within the same tolerance of the triplet median; all three remain in the mean. The tool never keeps the convenient pair and silently drops the awkward reading.
Second, the measured mean receives the posture allowance below. These increments are the published SQlab manufacturer sizing convention, not a universal peer-reviewed law. SQlab explains that a more aggressive posture moves the pressure point forward; its regional measuring guide maps positions from +0 mm for time trial to +40 mm for upright city riding. OpenBikeFit exposes the increment in the result instead of blending it invisibly into your anatomy.
| Posture category | Allowance | How to interpret it |
|---|---|---|
| Time trial / triathlon | +0 mm | Pelvis rotated far forward; pressure point sits further forward on the saddle. |
| Road / XC race | +10 mm | Bars clearly below the saddle with a long, low torso position. |
| Sport / endurance / gravel | +20 mm | Forward-leaning but sustainable posture used for endurance road and gravel. |
| Trail / MTB | +30 mm | Torso moderately raised, with more body weight supported at the saddle. |
| Trekking / city | +40 mm | Mostly upright torso with comparatively little weight on the hands. |
The displayed range surrounds the measured mean plus that convention with at least ±5 mm of measurement uncertainty. It intentionally does not snap to a brand's available sizes. If the range is 137–147 mm, one brand may offer 138 and 145 mm while another offers 143 mm; consult the chart for the exact model and compare its usable support surface, not only its widest external point.
Small, reversible validation
Choose one or two demo or returnable saddles whose manufacturer sizing overlaps the range. Do not assume two saddles labelled 143 mm provide the same effective support.
Photograph the rail marks and record saddle height, setback and tilt. Different shell depths can alter effective height, so follow the component maker's installation and torque instructions.
Begin with a short, easy ride in your normal posture. If the setup remains comfortable and stable, extend it across two or three ordinary rides before judging it. Keep bars, cleats and training load unchanged.
Note support, unwanted sliding, inner-thigh clearance and whether you can maintain the intended posture. A wider number is not automatically better. Restore the recorded setup if the test is worse.
This tool does not assess injury or diagnose the cause of discomfort. Stop a self-test if riding causes numbness, severe pain or worsening symptoms, and seek an appropriate qualified professional rather than trying to fit around a red flag.
Official manufacturer method
SQlab describes combining measured sit-bone distance with typical posture and distinguishes effective usable width from total external width.
Read SQlab's systemOfficial regional measuring guide
The SQlab Poland guide shows the seated impression method, centre-to-centre measurement and the five posture increments reproduced visibly above.
Open the measuring guideCross-brand caution
Specialized also asks for sit-bone width before presenting discrete saddle sizes, illustrating why the final choice must be checked against the exact manufacturer and model rather than a universal lookup.
See a manufacturer size selectorThese are manufacturer sources, not independent clinical validation of a universal saddle-width formula. OpenBikeFit therefore labels the posture addition as a convention, preserves repeatability uncertainty and does not claim that the centre value is an optimum. For how the project grades stronger evidence, directional findings and conventions, read the public methodology registry.