Spark & Anvil Privacy
A warm window-side study overlooking a small city, jars of beans and a wooden abacus on a shelf, two tweens estimating how many beans fill a big jar

📐

FermiForge

Guess almost anything — to within a power of ten — by breaking it into small pieces.

🔮 Open the estimation console →

What a Fermi estimate is

Nobody can just know how many jelly beans fill a jar. But you can break the question into pieces you can guess — beans in one layer, and how many layers — then multiply. Make a low guess and a high guess for each piece, and the answer gets pinned to about a power of ten: a hundred, a thousand, a million. That is a Fermi estimate, named for a physicist famous for it. The exact number barely matters — knowing which power of ten is a superpower.

  1. Your gut. Drag the ladder to the decade you think the answer is in — thousands? millions? — and lock it in.
  2. Chunk. Pick the two small pieces that multiply into the answer (one card is a decoy).
  3. Bound. Slide a low guess and a high guess for each piece — you don’t need to be exact.
  4. Reveal. See your gut, your band and the real answer on one ruler, with where the real number was checked.

Everything runs on your device — no calculator, no internet, no accounts.

Meet the crew

Chunk, Bound, Round and Unit — the four estimation moves — and Sanity, the mentor who asks the honest question at the end.

What each one does
  • Chunk breaks the impossible question into two or three you can guess. Nothing gets estimated until Chunk has split it up.
  • Bound gives every piece a low guess and a high guess — never one number, always a range.
  • Round keeps the numbers friendly, rounding to one clean figure so the maths stays in your head.
  • Sanity (mentor) asks the honest question at the end: does this answer feel right? Is it in the right ballpark?
  • Unit keeps the labels straight — beans per layer, layers, breaths per minute — so nothing multiplies wrong.

📚 The long ruler: Fermi, Eratosthenes, al-Bīrūnī, Archimedes, Drake →

More ways in

What next? Pick a door

Chosen from what this app is about. What you open is remembered on this device only.