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Brier Score Calculator

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The Brier score measures how good probability forecasts were: the average of (forecast − outcome)², where the outcome is 1 if it happened and 0 if not. 0 is perfect, 0.25 is what always saying 50% scores, 1 is the worst. Example: forecasts of 90%, 70%, 60%, 20% and 80% on events that went yes, yes, no, no, yes score 0.108 — a 57% improvement on coin-flipping.

Brier score calculator

One forecast per line: probability in %, then 1 if it happened or 0 if not (e.g. 70, 1). A Polymarket price in cents works as the probability.

Brier score—

Skill vs always 50%—

Log loss—

Forecasts counted—

How to read the result

🎯 Below 0.10 — sharp, confident forecasts that were mostly right.
🟢 0.10–0.20 — useful forecasts, clearly better than guessing.
⚪ Around 0.25 — no better than saying 50% every time.
🔴 Above 0.25 — confident and wrong often enough to be worse than a coin flip.

Log loss punishes confident misses much harder: a 99% forecast that fails costs 4.6, while a 60% miss costs 0.9. Use both — Brier for overall accuracy, log loss to catch overconfidence.

Scoring yourself against Polymarket

Write down your probability and the Polymarket price at the same moment, then score both once the markets resolve. If your Brier score beats the market's across a few dozen questions, your forecasts carry information the market lacked. Current prices are on Polymarket odds today; the EV calculator shows what an edge is worth.

Frequently asked questions

What is a good Brier score?

Lower is better. Always forecasting 50% scores 0.25, so anything clearly below that shows skill; strong forecasters on hard questions often land between 0.10 and 0.20.

How is the Brier score calculated?

For each forecast, subtract the outcome (1 if it happened, 0 if not) from the probability and square it; the Brier score is the average over all forecasts.

What is the difference between the Brier score and log loss?

Both reward accurate probabilities. Log loss (−ln of the probability you gave the actual outcome) punishes confident misses far more heavily than the Brier score does.

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