Fermi Estimation

Get a defensible order-of-magnitude answer from factors you can reason about. The goal is the right power of ten and a clear view of what drives it, not a precise number.

When to use

• Capacity, load, storage, cost, headcount, market size or time estimates with little data.
• "Roughly how many", "ballpark", "back of the envelope", "is it even feasible".
• Checking whether a plan's numbers are plausible before investing in measurement.

Skip: when exact data exists (query it instead), or the user needs a precise calculation.

Before you start

1. State in one block: Quantity (with units and period), Purpose (what decision depends on it), Needed accuracy (a power of ten, or within a factor of 2), and Pass (Decompose, Estimate, Combine, Cross-check, Sensitivity).
2. Gather first. Look for real anchors before guessing: logs, dashboards, billing, docs, prior numbers. Replace any guess with data you can find. Ask up to 3 questions only for anchors tools cannot supply.
3. Light path. Small ask: 3-4 factors, one range each, a result line and one cross-check.

Steps