Bike Gear Ratio Calculator

Pick a chainring, a sprocket and a tire, and the calculator below tells you three things about that gear: how fast it goes at a given cadence, how many gear inches it is, and how far one turn of the cranks actually moves you. Everything recalculates as you type.

How the calculation works

A bicycle has no gearbox in the car sense. What you change when you shift is a ratio between two toothed wheels, and everything else follows from it arithmetically. There is no hidden factor and nothing is lost: the whole of it fits on one line.

Your gear ratio is simply the chainring divided by the sprocket. A 50-tooth chainring against a 17-tooth sprocket is a ratio of 2.94, which means the rear wheel turns 2.94 times for every turn of the cranks. Multiply that by how far the wheel travels in one turn, and by how many times per minute you turn the cranks, and you have your speed.

speed = cadence × (chainring ÷ sprocket) × wheel circumference

The only term that is not printed on your bike is the last one, and the calculator works it out from the tire size for you.

Gear inches and distance per pedal turn

A bare ratio of 2.94 tells you nothing useful on its own, because it ignores the wheel. The same ratio on a 26-inch mountain bike and on a 700C road bike gives two noticeably different gears. That is why cyclists compare gears in units that already include the wheel.

Gear inches is the older of the two: the diameter a direct-drive penny-farthing wheel would need in order to roll as far per pedal stroke. It is archaic on its face and still the number people quote, because it makes two bikes with different wheels directly comparable. Distance per pedal turn says the same thing in meters and is easier to picture: a 78-inch gear moves you about 6.2 meters, or just over 20 feet, for every full turn of the cranks.

Measuring your wheel circumference exactly

The calculator derives circumference from the ISO size molded into the tire wall: π × (rim diameter + twice the tire width). A 700 × 25C tire is ISO 25-622, so π × (622 + 50) gives 2111 mm. That is transparent and checkable, which is more than can be said for the tables built into most bike computers, which differ from one another and cite no source.

It is also about one percent high, because a tire carrying a rider is not a circle. If you want the real number, do the roll-out: mark the valve at the bottom, sit on the bike, roll it forward until the valve is at the bottom again, and measure between the two marks. Repeat it three times and average. That single measurement beats every table, and it is also the number to enter into a bike computer if you want its distances to be right.

Six gears, worked out

All six use a 700 × 25C tire, so 2111 mm of circumference. The speeds assume 90 rpm, a cadence most riders can hold on the flat.

Gear	What it is for	Gear inches	Distance per pedal turn	mph at 90 rpm
34 / 28	Steep climbing	32.1	2.6 m	8.6
34 / 21	Rolling hills	42.8	3.4 m	11.5
50 / 17	Cruising on the flat	77.8	6.2 m	20.8
50 / 14	Fast group ride	94.5	7.5 m	25.3
50 / 11	Sprinting	120.2	9.6 m	32.2
42 / 16	Typical single-speed	69.4	5.5 m	18.6

Which cadence goes with which gear

The calculator will happily tell you that 50 × 11 at 110 rpm is 39 mph. It will not tell you that almost nobody can hold that, and this is where a gear calculator stops being enough. The gear you can turn is limited by the power you have, and the cadence you choose decides whether that power comes from muscular force or from your heart and lungs.

The short version: for most riders the useful range is 70 to 100 rpm, and the right gear is the one that keeps you inside it on the terrain you are riding. If you find yourself below 70 rpm on a climb, the answer is a lower gear, not more effort. The cadence guide covers the ranges, what the research actually measured, and three training sessions for widening them.

Frequently asked questions

What is a good gear ratio for a road bike? For general riding, a 50/34 chainset with an 11 to 32 cassette covers almost everything: about 28 gear inches at the bottom for steep climbs and about 120 at the top for descents.

What are gear inches? The wheel diameter a direct-drive bicycle would need to travel the same distance per pedal stroke. It is a way of comparing gears between bikes with different wheel sizes.

How do I work out speed from cadence and gear? Multiply cadence by the gear ratio and by the wheel circumference. At 90 rpm with 50/17 and a 2111 mm wheel that gives 20.8 mph.

Does tire width change my gearing? Slightly. Going from a 25 mm to a 32 mm tire adds about 44 mm of circumference, which is around two percent more speed at the same cadence and gear.

What wheel circumference should I enter into my bike computer? The one you measure by rolling the bike out with your weight on it. Tabled values are typically one percent optimistic, which is about a quarter of a mile over a 25-mile ride.

Is a bigger gear always faster? Only if you can still turn it at a sensible cadence. A gear you grind at 55 rpm produces less sustained power than a smaller one you spin at 90.

What cadence should I aim for? Most riders do best between 70 and 100 rpm, with around 90 rpm the value most often quoted as a reference point.