How ratings work

Keiko League gives every athlete an Elo-style rating — the same idea used in chess and many competitive games. Your rating goes up when you beat someone and down when you lose, and the size of the change depends on how strong your opponent was. Beat a higher-rated opponent, gain more; lose to a lower-rated opponent, drop more.

A rating only ever changes from a confirmed randori — both partners have to agree the result. That keeps the numbers honest: every point on a leaderboard can be traced back to a result two people agreed on.

Elo calculation

Keiko League uses an Elo-style rating. Your rating changes only after both athletes submit matching results for the same QR session.

For each athlete:

• Expected score = 1 / (1 + 10 ^ ((opponent_rating - current_rating) / 400))
• Actual score = 1 for a win, 0 for a loss
• K-factor = 32
• Delta = round(K * (actual_score - expected_score))
• New rating = current_rating + delta

Quick example: if you beat a higher-rated athlete, your expected score is lower, so your rating gain is usually bigger. If you lose to a lower-rated athlete, your drop is usually bigger.

We also show rating reliability as rated-randori count beside Elo. Leaderboard rank additionally applies activity-based recency adjustments for inactive athletes.

Why only confirmed randori count

Ratings are only useful if the results behind them are trusted. That is why one person cannot record a result alone: both partners report the same outcome, and the randori is recorded only when the two reports match. A disagreement, a cancelled session, or an expired session never changes a rating — and never spends a credit.

How rank reflects activity

A single good result should not put you top forever. Leaderboard position combines your rating with how recently you have competed, so a rank reflects consistent, recent training. Athletes who have not recorded a confirmed randori for a while appear under "inactive participants" with the date of their last randori.

How many randori before a rating is reliable

A new rating starts at 1,000. Treat the first handful of randori as a settling-in period: the more confirmed randori you record, the more your rating reflects your real level. Look at the rated-randori count beside your Elo as a confidence label, not just your position on the board.

Source of truth: EloCalculator, RecordBout, and ConfirmQrSessionReport in the Rails app.