Gear Ratio Calculator
Calculate gear ratio, output RPM, and speed change for mechanical gear trains.
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Applications
Used by cyclists tuning gear inches and development for road and mountain bikes, automotive enthusiasts selecting differential and transmission ratios, mechanical engineers prototyping gear trains for robotics and 3D printers, and hobbyists building clockwork or motor-driven projects.
The Formula
Gear Ratio (GR) = Driven Teeth ÷ Driver Teeth. Output RPM = Input RPM ÷ GR. Speed Change (%) = (Output RPM − Input RPM) ÷ Input RPM × 100. A ratio above 1.0 indicates a reduction (driven gear spins slower with more torque); below 1.0 indicates an overdrive (driven gear spins faster with less torque). Inputs are clamped between 1–10,000 teeth to keep all math well within JavaScript's safe integer range.
Fun Fact: A typical 10-speed road bike cassette paired with a 50-tooth chainring can produce gear ratios ranging from about 0.77:1 (easiest climbing gear) to 4.17:1 (fastest sprint gear) — a 5.4x spread that lets a single drivetrain handle both mountain passes and flat sprints.
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