Ever seen a "guess how many sweets in the jar" competition? You can't tip them all out and count, so you take a quick, sensible guess — and the closest guess wins. That good-enough guess is an estimate.

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.

Why guess instead of count? Because some groups are too big to count quickly. Counting the cars in a car park, the people in a crowd, or the sweets in a jar one by one would take forever. An estimate gives you a useful answer right away, and that is often all you need.

We even have special words for estimates: we say about, roughly, or nearly. "About twenty", "roughly fifty", "nearly a hundred" — each one says close, but not exact.

Three handy strategies

How do you make a sensible guess instead of a wild one? Try one of these:

• Count a small group, then guess the rest. Count just a corner — say 5 — then see how many corners that size would fill. Five fits about four times, so about 20.
• Compare to a known amount. Picture what 10 looks like. Is this pile a bit more than ten? Two tens? That gives you a quick "about" number.
• Round to a tidy number. If a careful count would land near 58, just say about 60. Round numbers are easy to say and easy to remember.

Worked examples

Let's estimate three different groups, each with a different strategy.

1) The bird flock — compare to ten.

Picture ten birds. This flock looks like a bit more than ten and a half of another ten — so about 15. (Counted: sixteen. Very close.)

2) The marble jar — count a small group, then guess the rest. You can see that one layer at the bottom holds about 8 marbles, and the jar is about 6 layers tall. Eight, six times, is roughly 8 \times 6 \approx 48 \approx 50. So a sensible estimate is about 50 marbles — without ever counting all of them.

3) The bookshelf — round to the nearest ten. A careful count would give 58 books. That is awkward to remember, so round it: 58 is nearer to 60 than to 50, so we say about 60 books.

Try it: guess, then reveal

A random pile of counters appears. Glance and guess "about how many?" — say it out loud — then press Next to reveal the true count and see how close you were. Press Refresh for a brand-new pile.

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: