Bike Gear Ratio Calculator
Calculates gear inches — a measure of how far the bike travels per pedal stroke — from chainring, cassette, and wheel diameter. Useful for comparing gear ratios across drivetrains, planning gear upgrades, and matching cadence to terrain.
Last updated: May 2026
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About this calculator
Gear inches is the effective wheel diameter when geared up: Gear Inches = (Chainring Teeth / Cassette Teeth) × (Wheel Diameter in mm / 25.4). The first factor is the mechanical gear ratio; the second converts wheel diameter from millimeters to inches. Variables: Chainring Teeth is the front sprocket's tooth count (typical road: 50/34 compact, 53/39 standard, 52/36 mid-compact; gravel/touring: 46/30 or 48/32; mountain 32/22 double, 30 or 32 single); Cassette Teeth is the rear sprocket's tooth count (typical road cassettes: 11-25, 11-28, 11-32; gravel: 11-36, 11-42; mountain: 10-50, 10-52); Wheel Diameter is the outside diameter including tire (700×25c: ~668 mm; 700×32c: ~683 mm; 29er MTB: ~735 mm; 27.5 MTB: ~684 mm). Edge cases: gear inches is one of three common gear-comparison metrics — others are 'meters of development' (gear inches × π × 0.0254, giving meters per pedal revolution) and 'speed at cadence' (gear inches × π × cadence / 1056 for mph). All three measure the same underlying ratio. The calculator assumes the cyclist is in a single gear combination; real riders use 9-12 cassette positions and 1-3 chainrings, so total gearing range is the ratio between highest and lowest gear-inches, not a single number. Modern 1× drivetrains (one chainring) typically span 350-500% gear range; 2× drivetrains span 450-650%.
How to use
Example 1 — Standard road bike high gear. 50T chainring, 11T cassette, 700×25c tire (~668 mm diameter). (50/11) × (668/25.4) = 4.545 × 26.3 = 119.6 gear inches. Verify ✓. At 90 rpm cadence, this corresponds to ~50 km/h — a fast tailwind or descent gear. Example 2 — Mountain bike low gear. 32T chainring, 50T cassette, 29×2.3 tire (~745 mm diameter). (32/50) × (745/25.4) = 0.64 × 29.3 = 18.8 gear inches. Verify ✓. At 70 rpm climbing cadence this corresponds to ~6 km/h — typical for sustained 8-10% gradient climbs.
Frequently asked questions
What does 'gear inches' actually measure?
Gear inches is the effective wheel diameter you'd need on a single-speed (direct-drive) bike to travel the same distance per pedal revolution as your geared bike in a specific combination. A 50T chainring with 25T cassette on a 700c wheel gives ~55 gear inches — meaning each pedal revolution rolls the bike forward π × 55 ≈ 173 inches, or ~4.4 meters. Higher gear inches = harder to pedal but more distance per stroke. Penny-farthing bicycles had direct-drive wheels of 50-60 inches; modern bikes use chain-gears to achieve effective wheel sizes from ~17 inches (low MTB gear) to ~130 inches (top road sprint gear). Gear inches lets you compare disparate drivetrains (road vs MTB vs single-speed) on a single scale.
How do I choose a gear ratio for my terrain?
Match your gearing range to the steepest climbs and fastest descents you'll encounter. For mostly flat road riding: a 50/34 compact crankset with 11-28 cassette (range 30 to 122 gear inches) handles 90% of recreational riding. For hilly or mountainous routes: 46/30 or 48/32 with 11-36 or 11-42 cassette (range 19 to 113 gear inches) gives a more usable low gear without sacrificing top end. For mountain biking on technical terrain: 32T or 30T single chainring with 10-50 or 10-52 cassette (range 18 to 90 gear inches) is the modern norm. For loaded touring on hilly terrain: 46/30 or triple cranksets (50/39/30) with 11-34 or 11-36 cassette ensures you can grind up 12%+ gradients with 30+ kg of bags. Practical rule: pick a low gear that lets you climb your steepest reasonable gradient at 60-80 rpm without redlining your heart rate.
Does wheel diameter change with tire size?
Yes, slightly. A 700×25c tire is ~668 mm outside diameter; the same rim with a 700×32c tire is ~683 mm; with a 700×38c tire (gravel/touring) ~695 mm. The difference of 25-30 mm tire-to-tire shifts gear inches by about 1-2%. For mountain bikes the variation is larger: 29×2.1 ≈ 728 mm, 29×2.5 ≈ 745 mm. Wider tires roll faster on rough surfaces (less rolling resistance from suspension losses) and can be ridden at lower pressure for more grip, so the effective top speed of a gravel setup at the same cadence is slightly higher with bigger tires. For exact comparison, use the manufacturer's published outside diameter or measure with a tape measure across the inflated tire mounted on the rim.
Should I worry about cross-chaining (extreme gear combinations)?
Cross-chaining means using the smallest cog with the small chainring, or the largest cog with the large chainring — both put the chain at a steep lateral angle. On modern drivetrains it's mostly fine for short periods but causes premature wear (chain stretch, cassette pitting, chainring tooth wear) over time. SRAM AXS and other modern wireless 1× systems eliminate cross-chaining entirely (one chainring, straight chain line). On 2× systems, prefer the small chainring for cogs 5-11 and the big chainring for cogs 1-7 (counting from largest cog as 1). The gear-inches calculator doesn't flag cross-chaining; you'll have to check the chain line visually or use a chain-line gauge. Modern drivetrains are tolerant of 3-4 sprocket positions of cross-chain without immediate ill effect.
When should I not use this calculator?
Skip it for derailleur capacity planning — gear inches doesn't tell you whether a rear derailleur can handle a 36T or 42T largest cog (use the manufacturer's max-cog spec). Do not use it for chain length sizing — that requires the longest chain length math from chainstay length and cog/chainring sizes, not gear inches. Skip it for internal-hub or pinion gearbox bikes (Rohloff, Pinion, Shimano Alfine) — those drivetrains list their gearing as 'overall ratio range' directly in the manufacturer specs, and the per-step gear inches isn't a single chainring/cassette ratio. For comparing belt drives (Gates Carbon Drive), the calculation uses 'pulley teeth' instead of chainring/cassette teeth but is otherwise identical. For e-bikes, gear inches still applies but pedal-assist motors reduce the practical importance — you can pedal nearly any gear ratio comfortably with assist enabled.