Derailleur Gear Ratio Calculator

Following a number of requests over in the forum, we have decided to add a tool to enable you to easily calculate the gear ratios fitted to your bike.

Use the top row of drop downs to select the number of teeth on each of your rear sprockets, and the left hand selectors to enter the number of teeth on each of your chainrings. Don't forget to enter the diameter of your wheel!

Most bikes with derailleur gears have either 5, 6, 7, 8, 9 or even 10 sprockets on the hub, along with 1, 2 or 3 chainrings.

Gear ratios on a bicycle are expressed in inches (or cm) as an "effective wheel diameter". The way we look at a bike gearing system is to imagine the pedals fixed directly to the wheel such that we look at how big a wheel we are turning for each turn of the pedals.

Most derailleur systems with more than one front chainring have a ratio overlap such that the higher gears of a smaller chainring are similar to the lower gears of the next largest chainring.

Even though a bike is "21 speed", it will not have 21 gear ratios with consistent steps between them. You are likely to find that the ratios on the middle chainring are closely duplicated by the gears on the smaller, and larger chainrings.

For a "21 speed" bike, you have 3 ranges of 7 gears, NOT 21 gears. The ranges exist to minimise the amount of front derailleur shifting that is required.

Many shops will suggest, quite sensibly, that you avoid using the "extremes", ie the smallest chainring with the smallest rear sprocket and the largest chainring with the largest sprocket. Using these combinations can lead to premature wear of your chain and sprockets.

Regular lubrication with a suitable chain lubricant is essential (ask your local bike shop for advice on suitable lubricants) and you should also consider replacing the chain on a regular basis to prevent it from wearing out the chainrings and sprockets as quickly. If you leave your chain until it is worn, you will likely find that you have to replace the sprockets and chainrings at the same time.