Not all stools are as adjustable as the beauties in Ann's kitchen above, so it's important to get the dimensions just right in order to sit comfortably. Think about these things before you go out shopping, not after you bring home the wrong size!
Do you have a counter or a bar?
It's easy to tell: counters are 36'' high, bars are generally 42''. There should be about 10'' between the seat of your stool and your surface so make sure to measure or carefully check the dimensions of any stool you're considering (especially online where they all look the same!). And although they're not as common, anything taller than a barstool is usually called spectator height.
Use this handy graphic:
How many stools will fit along your counter?
Measure the width of your stool and add 2'' for elbow room and general comfort. Now divide the length of the counter by that number. Remember to round down (if you're answer is 3.4, buy three stools rather than four) as it's always better to have a little more space than to be overcrowded.
Phew, that was a lot of math. But worth it to get just the right fit. Now let's go shopping!