Sometimes you don't need an exact answer — you just want a sensible about how many? at a glance. That quick guess is an estimate. A good estimate is close to the real number, even though it isn't exact.

Estimating builds right on counting: you glance, you guess, and then you can count to check. The two work together — the guess is fast, the count is sure.

Watch: a scattered group of frogs appears. First we make a quick estimate — about how many? — and then we count them one by one to find the true total. At the end we compare: was our guess close? Press replay to try a different group each time.

Estimating is also a handy way to catch silly answers. If you guessed about ten and then counted forty, something went wrong — time to look again. A quick estimate is a friendly check on your own work.

Khan Academy explores estimating here: