Estimation tips

How to get better at estimating sizes

Guessing sizes is a skill, and like any skill it gets better with practice and a few good habits. Most size mistakes are not random. We misjudge things that are much bigger or much smaller than anything we handle every day, and we forget that area and volume grow far faster than length. The four techniques below fix most of that: compare with anchors you already know, think in powers of ten, remember how length, area and volume scale, and break a hard question into easy parts. Each one works in everyday life as well as in a game.

Use anchors you already know

The easiest way to judge a size is to compare it with something you know well. You have a feel for a door, a person or a bus without thinking about it. Ask "how many doors tall is it?" rather than "how many metres?", and a vague guess becomes a count.

An adult person1.7 m (5 ft 7 in)
A doorway2 m (6 ft 7 in)
A bus12 m (39 ft)
A tennis court, end to end23.8 m (78 ft)
A football (soccer) pitch, end to end105 m (344 ft)

Keep a small ladder of anchors, each a few times bigger than the one before, so that anything you meet falls between two of them. When a game shows you a reference object, use it the same way: size things against the reference, not against the screen.

Think in powers of ten

Sizes in the world span an enormous range. Each step of ten times is called an order of magnitude, and it is the most useful unit for a first guess. Before you pick a number, decide which power of ten the answer is closest to: 1 mm, 1 cm, 10 cm, 1 m, 10 m? Getting that right matters far more than getting the first digit right.

One anchor for each power of ten
Power of ten	An anchor near it
10 µm	A red blood cell, 7.5 µm
100 µm	The width of a human hair, 70 µm
1 mm	A grain of sand, 500 µm
1 cm	A grain of rice, 6 mm (0.24 in)
10 cm	A smartphone, 15 cm (5.91 in)
1 m	An adult person, 1.7 m (5 ft 7 in)
10 m	A bus, 12 m (39 ft)
100 m	A football (soccer) pitch, 105 m (344 ft)

This is also why Magnitudle scores by how many times off you are, not by how many metres. Guessing 2 m for something 1 m tall is as wrong as guessing 2 km for something 1 km long, and a guess twice too big scores the same as a guess half the true size. Read how scoring works for the exact points.

Length, area and volume grow differently

Make something twice as long in every direction and its surface grows four times (2 × 2), while its volume grows eight times (2 × 2 × 2). Weight follows volume, so an animal twice as tall as another of the same shape and build weighs about eight times as much. This is known as the square–cube law.

A small cube beside a cube twice as tall, which takes eight of the small cubes to build — Twice as tall, eight times the volume.

It is the most common estimation trap. Our eyes react to bulk, so a thing that looks "eight times bigger" may be only twice as tall. Ten times as long means a hundred times the area and a thousand times the volume. When the question is about height or length, compare heights or lengths, not how much space something seems to fill.

Break big numbers into parts

Some questions look impossible until you split them up. This is Fermi estimation, named after the physicist Enrico Fermi, who was famous for good estimates made from very little information. Write down the few quantities the answer depends on, estimate each one on its own, then multiply. Errors in the parts tend to partly cancel out.

Worked example: how many people could stand on a football pitch?

A pitch is roughly 100 m long and 70 m wide (about 330 × 230 ft), so its area is about 7,000 m² (75,000 sq ft).
A person standing in a tight crowd takes up about 50 × 50 cm (20 × 20 in), so about 4 people fit in each square metre.
7,000 × 4 = 28,000. Rounded: about 30,000 people.

Each step could be a little off, but you are unlikely to be ten times off. In an estimate round, an answer twice too big or half the truth still scores 59 points out of 100.

Common mistakes

Anchoring on the first number

Once a number is in your head, from a headline or from the question before, your next guess drifts towards it. Make your own estimate from your anchors before you look at any other number.

Confusing length with area

A shape with twice the area is only about 1.4 times as long on each side. Judge the one dimension the question asks about: height, length or width.

Unit slips

Millimetres and centimetres differ by ten, metres and kilometres by a thousand, and a metre is a bit more than three feet. Check the unit before you commit.

Practise

Techniques stick when you use them. Try a question here, or practise on real sizes by topic:

Test your sense of scale

Order from biggest to smallest

0 of 3 placed

New to the game? Read the rules first.

Or try five practice rounds in your browser.

Teaching estimation? See our classroom activities.