Gears are still measured in inches, and whilst it may seem antiquated to refer all gearing arrangements (including hub gears) back to the penny farthing's directly driven wheel, it's a very simple calculation that gives us an easy way to compare all sorts of pedal cycles.
To calculate gear size you enter the diameter (in inches) of the actual wheel, and the numbers of teeth on the front chainwheel and rear sprocket, into the following formula:
Gear = Wheel × Chainwheel ÷ Sprocket
You probably know the diameter of your wheel in inches already. If not, you'll find that information in the table on our Tyre Sizes  page. To be certain take a look there anyway: some wheels commonly assumed to be 26 inches diameter are actually less than 25!
If you've got MS-Excel on your computer you may wish to download these gear charts. The first one can be tailored to your precise wheel size and printed out. There are also nominal tables for 27, 26, 24, 20 and 16in wheels.
You will sometimes read that the size of a gear in inches is the distance travelled with each turn of the pedals. THAT IS NOT TRUE. Gear inches give the equivalent diameter of the wheel. To get the distance travelled you must multiply gear inches by π (3.14).
In some other countries they do indeed describe cycle gearing in terms of the distance travelled per turn of the pedals, which is called the gear development and measured in metres. This might seem like a more useful quantity; but any measure will do for comparisons. And besides: the inches come in easy whole numbers!
If you'd nevertheless prefer to "do the continental" first you need to know the circumference, in metres, of your wheel. You'll also find that in the table on our Tyre Sizes  page. But if you'd rather measure it: extend at least 2m of a steel tape on the floor; stand your bike beside the tape with the rear wheel tyre valve at the bottom exactly adjacent to the start of the tape; trundle it forwards until the valve is at the bottom again and read the adjacent distance off the tape.
Now you've got the wheel circumference, in metres, you can use it to set your bike computer or put it into the formula I gave earlier (instead of wheel diameter) to calculate the development of each of your gears. In the following examples the metres development is given in brackets after the gear inches.
Novice and infrequent cyclists – even those who are quite fit and strong through some other activity – find it difficult to pedal quickly and prefer to push hard at a slow cadence of 60rpm or so. For this reason all mass-produced bicycles come with rather high top gears of around 100in (8m) and some customers ask for even higher ones! Resist this temptation. With practice the leg muscles naturally develop a quicker and more efficient cadence, but it helps if you know in advance that this is supposed to happen. Try to pedal at 90rpm (left-right-left in one second) and you’ll soon have little use for the top gear your bike came with.
Here’s another gearing formula you may find of interest:
Speed(mph) = Gear(in) × Cadence (rpm) ÷ 336
Pedal a 100in gear at 90rpm and you’ll do 27mph. Few people are fit enough to sustain such a speed except downhill or with a following wind; in which case they might as well freewheel and save energy for the next uphill! (If you want that formula in metric: kmph speed and metres development, multiply by 6% instead of dividing by 336.)
At least three of your gears, and preferably four or five, should lie between 40in and 70in (3m to 6m). These will be your most used gears and hence should be easy to shift between: i.e. all on the same chainwheel. I’ll say more about the spacing of gears later.
If you want to be able to ride up steep or long hills you will appreciate something much lower and any touring or mountain-bike worthy of the name will have a small inner chainwheel and largest rear sprocket, which together give a bottom gear of about 25in (2m). Do not assume that it will be quicker to walk up hills than pedal very low gears. It is not easy to push a loaded bicycle up a steep hill at 3mph, but most people can ride at this speed without wobbling. To do so while maintaining a moderate cadence of 70rpm would require a 14in (1m) gear, which is off the bottom of most gear tables. So don’t worry about gearing too low: it's virtually impossible.
You might have seen it written that gears should be spaced so that each jump is the same number of inches. Whatever may be the merits of this scheme it does not give constant gear intervals. To get that you want equal percentage jumps between gears. Most ready-made freewheels and cassettes are designed this way: with differences of only one or two teeth between the smaller sprockets, increasing in stages to three, four, five or even six teeth at the bottom.
The suggestion that you have four gears between 40in and 70in implies 15% jumps, which is quite close enough for touring and off-road use. Even jumps of 25% are acceptable – especially between the topmost and lowest gears. Finer tuning (10% or less) is needed only for fast riding in a group, i.e. racing.