Cam Sizing Calculator

The scoring formula picks the best match by minimizing distance from a target set at 62.5% of each cam's min-to-max range, rather than the 50% midpoint. That reflects common trad-climbing guidance that a cam seated slightly toward the retracted (narrower) side of its range, rather than dead center or fully expanded, tends to hold more securely against outward pulls.

Adjacent cam sizes have overlapping expansion ranges by design, so a single crack width can fall inside more than one size's min-max window at once — the 'All Matching Cams' output lists every size that fits. The 'Best Cam Size' is then whichever of those matches scores closest to the 62.5% sweet-spot target, but the others are still physically usable, just less centered in their range.

It's derived directly from the expansion percentage of the recommended cam: 25-75% is rated Optimal, 10-90% is Acceptable, and anything outside that range is Marginal. A cam expanded below about 10% risks 'walking' out of the crack as rock vibrates or the rope moves, while one above about 90% is nearly fully open and has less range left to accommodate an irregular or flaring crack.

The tool only compares a single crack-width measurement against published min/max expansion specs — it has no way to account for manufacturer differences in lobe count, camming angle, or single- versus double-axle design, nor for crack irregularity, flare, or rock texture that all affect real holding power. A cam that matches on paper can still seat poorly in an irregular crack, so placing and weighting it by hand remains the only way to confirm it will actually hold.