Guides

how estimation actually works

Estimation is a learnable skill, not a talent. Nobody is born knowing how many golf balls fit inside a school bus — the people who answer well are running a method, and the method is short enough to explain in an afternoon.

It comes down to four habits. Break a question you cannot answer into pieces you can. Anchor each piece to something you have actually seen or measured. Round hard, and round in opposite directions so the errors fight each other instead of stacking up. Then sanity-check the result against something you already know is true.

These guides cover that method and the ground around it: where the technique came from, the two dozen numbers most rough calculations eventually reduce to, the arithmetic shortcuts that let you skip the calculator, the specific ways confident estimates go wrong, and how the same habit works as a defence against bad statistics and as preparation for interview questions.

Read them in any order. If you want one to start with, take How to estimate anything — the rest are elaborations on it.

Start here How to estimate anything The four-step method: decompose, anchor, round, sanity-check — worked through on a real question, plus why multiplying rough guesses is more reliable than it sounds. Worked example The piano tuner problem, explained The most famous estimation question in physics, solved step by step using nothing but things you already know. Scoring Why being 10× off is actually good Orders of magnitude, why your brain can't tell a million from a billion, and what counts as a respectable miss. Reference Numbers worth knowing by heart The two dozen anchor figures that most rough calculations eventually reduce to — populations, times, distances and scales. Technique Mental math for estimating Handling zeros separately, rounding in opposite directions, the rule of 72, and the shortcuts that remove the arithmetic entirely. Pitfalls Six ways estimates go wrong Anchoring, scale blindness, availability, false precision, correlated errors and unit confusion — with the fix for each. Applied How to spot a fake statistic Five ten-second checks that catch impossible numbers in headlines and arguments before you repeat them. History Who was Enrico Fermi? The physicist who measured the first atomic blast with scraps of paper, and why a whole class of problems carries his name. Practical Estimation questions in job interviews What interviewers are actually scoring when they ask how many windows are in New York, and how to structure an answer out loud.