About Off By Ten

Being roughly right is an underrated superpower. Nobody needs to know that the Moon is exactly 384,400 km away — but knowing it's hundreds of thousands of kilometers, not thousands or billions, is the difference between understanding the world and just floating through it. Physicists call these Fermi estimates. We turned it into a game.

Why the name?

It comes from the scoring: guess within 5% and that's a 🎯 bullseye, but land more than ten times off — in either direction — and you score nothing. You were off by ten.

How the questions are made

Every question starts from a verifiable fact and is independently fact-checked against credible sources before it ships — and every reveal names its source. Real-world quantities are often estimates by nature; that's exactly why the game scores by closeness, not exactness. If you spot a number you can beat with a better source, we genuinely want to know.

The rules of the house

• Free to play, in any browser, no account, no app to install.
• Your stats live in your browser. There is no database of you.
• One puzzle a day. The streak is the game.

Get in touch

Bug reports, question corrections, and ideas are welcome as issues on GitHub.