Gear Ratio Calculator

Gear ratio is chainring teeth divided by cog teeth, so the cog size sits in the denominator — a larger cog means the rear wheel turns fewer times per crank revolution, which is exactly what you want on a climb. Going from a 17-tooth to a 25-tooth cog on the same 50-tooth chainring drops the ratio from 2.94 to 2.0, a noticeably easier gear at the same cadence.

Speed, gear inches, and development all depend on the wheel's actual rolling circumference, which the calculator derives directly from the diameter you enter. A wider or taller tire adds real millimeters to that rolling diameter, and since circumference scales linearly with diameter, even a small measurement error carries straight through into the speed and development results.

Both describe the same physical gearing using a different unit: Gear Inches multiplies gear ratio by wheel diameter in inches, an old but still widely used scale for comparing gears across different wheel sizes and drivetrain types. Development instead multiplies gear ratio by the wheel's circumference in meters, giving you the actual distance traveled per crank revolution — more intuitive if you think in metric distances rather than an abstract inch figure.

On a freewheel-equipped bike you can stop pedaling and coast, so this number is irrelevant. On a fixed-gear bike you skid to scrub speed, and the tire only contacts the ground at a limited number of points during those skids — calculated as cog teeth divided by the greatest common divisor of chainring and cog teeth. A low skid-patch count means those same few spots wear flat fast, which is why track and fixed-gear riders often deliberately choose gear combinations that maximize this number.