Cadence is how fast you turn the pedals, measured in revolutions per minute. Speed is how fast the bike moves. The link between them is your gear: the chainring at the front, the cog at the back, and the size of your wheel. This calculator connects those numbers, so you can see what speed a given cadence produces in any gear. That is useful when you are choosing gears, judging whether you are spinning too fast or grinding too slowly, or comparing two bikes with different gearing.
The maths is simple. One pedal turn moves the chain, and the rear wheel turns once for every chainring-to-cog ratio. If your chainring has 50 teeth and your cog has 17, the ratio is about 2.94, so the wheel turns 2.94 times per pedal turn. Multiply by the wheel circumference, about 2.1 metres for a road tyre, and one pedal turn moves you about 6.18 metres. At 90 pedal turns per minute, that is 556 metres per minute, or about 33.4 kilometres per hour.
Here is a worked example. A rider pedals at 90 revolutions per minute with a 50-tooth chainring and a 17-tooth cog on a wheel of 2100 millimetres circumference. The gear ratio is 50 divided by 17, which is 2.94. Development, the distance per pedal turn, is 2.94 times 2.1 metres, which is 6.18 metres. Multiply by 90 turns and 60 minutes, then divide by 1000, and you get 33.4 kilometres per hour, or about 20.7 miles per hour. Change any one number and the speed changes with it.
Cadence matters because your body has a sweet spot. Most road riders are most efficient between 80 and 100 revolutions per minute, where the legs spin smoothly and the muscles tire slowly. Below 60, you push hard on every stroke, which strains the knees and tires the muscles quickly. Above 110, you bounce in the saddle and waste energy. Beginners often pedal too slowly in too hard a gear; shifting down and spinning faster usually feels easier and lets you ride longer.
Gear inches is an older way of describing the same idea, and it is still used by many cyclists. It answers this question: if your bike were an old penny-farthing with a directly driven wheel, how big would that wheel be? You find it by multiplying the wheel diameter in inches by the gear ratio. Our example gives 27 times 2.94, or about 79 gear inches. Higher gear inches mean a harder, faster gear; lower gear inches mean an easier gear for climbing.
Development is the most practical number of the three. It tells you how far you travel per pedal turn, in metres, and it is easy to compare across bikes and wheel sizes. A development of 6 metres is a typical flat-road gear, while a climbing gear might be near 2.5 metres and a fast downhill gear near 10 metres. If two bikes give the same development, they feel the same to pedal, even if their chainrings and wheels differ. That is why development is the fairest way to compare gearing.
The calculator has clear limits. It assumes steady pedalling on flat ground with no wheel slip, and it ignores wind, gradient, tyre pressure, and your power output. In real life, the same cadence and gear can give very different speeds uphill, downhill, or into a headwind. The tool also assumes your wheel circumference is correct; if you enter the wrong tyre size, every speed will be off by the same percentage. Measure your wheel by rolling it once on the ground for the best accuracy.
You can use this tool in several ways. Before buying a cassette, check whether the new gears give you the speeds you need at your normal cadence. On a ride, if your computer shows speed but not cadence, work backwards: enter the speed you held and find the cadence that matches. Coaches use cadence drills, such as short bursts at 110 revolutions per minute, to build smooth pedalling, and this calculator shows what speed those drills should produce. In short, it turns gear numbers into plain speed, so you can ride smarter.