diff --git a/scratch/run-tests.mjs b/scratch/run-tests.mjs
index fbaab3b..a3571e1 100644
--- a/scratch/run-tests.mjs
+++ b/scratch/run-tests.mjs
@@ -5,6 +5,8 @@ import path from 'node:path';
 const testDir = path.resolve(process.cwd(), 'tests');
 const nodeBin = process.execPath;
 const jitiCli = path.resolve(process.cwd(), 'node_modules/jiti/lib/jiti-cli.mjs');
+const vitestCli = path.resolve(process.cwd(), 'node_modules/vitest/vitest.mjs');
+const vitestTests = new Set(['elo-rating.test.ts']);
 
 // Authoritative offline unit, domain, security, and integration test suites
 const liveDbTests = new Set([
@@ -44,7 +46,10 @@ for (const file of files) {
   console.log(`------------------------------------------------------------`);
   
   const start = Date.now();
-  const res = spawnSync(nodeBin, [jitiCli, filePath], {
+  const args = vitestTests.has(file)
+    ? [vitestCli, 'run', filePath]
+    : [jitiCli, filePath];
+  const res = spawnSync(nodeBin, args, {
     cwd: process.cwd(),
     stdio: 'inherit',
   });
diff --git a/src/lib/gamification/elo-rating.ts b/src/lib/gamification/elo-rating.ts
new file mode 100644
index 0000000..e000d5c
--- /dev/null
+++ b/src/lib/gamification/elo-rating.ts
@@ -0,0 +1,92 @@
+/** Match score: 1 for a win, 0.5 for a draw, and 0 for a loss. */
+export type MatchOutcome = 1 | 0.5 | 0;
+
+/** Rating bands used by accountability circle leaderboards. */
+export type MemberRankTier = 'novice' | 'apprentice' | 'adept' | 'master' | 'grandmaster';
+
+/** Member statistics before a match; new members start at rating 1200. */
+export interface EloMemberProfile {
+  rating: number;
+  matchesPlayed: number;
+  winStreak: number;
+}
+
+/** Updated ratings and actual deltas after applying the rating floor. */
+export interface EloMatchResult {
+  playerNewRating: number;
+  opponentNewRating: number;
+  playerRatingDelta: number;
+  opponentRatingDelta: number;
+  playerExpectedScore: number;
+  opponentExpectedScore: number;
+  playerNewTier: MemberRankTier;
+}
+
+/** Optional tuning for accountability circle matches. */
+export interface EloOptions {
+  /** Defaults to 32. */
+  baseKFactor?: number;
+  /** Defaults to 10; provisional members use 1.5 times the base K. */
+  placementMatchesCount?: number;
+  /** Defaults to true; a winner with a pre-match streak >= 3 gets +2. */
+  enableStreakBonus?: boolean;
+}
+
+/** Calculate the standard Elo expected score, rounded to four decimals. */
+export function calculateExpectedScore(playerRating: number, opponentRating: number): number {
+  return Math.round((1 / (1 + 10 ** ((opponentRating - playerRating) / 400))) * 10000) / 10000;
+}
+
+/** Placement volatility takes precedence over elite (rating >= 2000) stability. */
+export function determineKFactor(profile: EloMemberProfile, baseK = 32, placementCount = 10): number {
+  if (profile.matchesPlayed < placementCount) return baseK * 1.5;
+  if (profile.rating >= 2000) return baseK * 0.5;
+  return baseK;
+}
+
+/** Classify a rating using inclusive lower boundaries for each rank. */
+export function getMemberTier(rating: number): MemberRankTier {
+  if (rating < 1000) return 'novice';
+  if (rating < 1400) return 'apprentice';
+  if (rating < 1800) return 'adept';
+  if (rating < 2200) return 'master';
+  return 'grandmaster';
+}
+
+/**
+ * Resolve a match without mutating either pre-match profile.
+ * Each member uses their own K-factor. Only the winner can earn the additive
+ * streak bonus, so adjustments need not sum to zero. Ratings cannot drop below 100.
+ */
+export function calculateEloMatch(
+  player: EloMemberProfile,
+  opponent: EloMemberProfile,
+  outcome: MatchOutcome,
+  options: EloOptions = {},
+): EloMatchResult {
+  const { baseKFactor = 32, placementMatchesCount = 10, enableStreakBonus = true } = options;
+  const playerExpectedScore = calculateExpectedScore(player.rating, opponent.rating);
+  const opponentExpectedScore = calculateExpectedScore(opponent.rating, player.rating);
+  const playerK = determineKFactor(player, baseKFactor, placementMatchesCount);
+  const opponentK = determineKFactor(opponent, baseKFactor, placementMatchesCount);
+  const playerBonus = enableStreakBonus && outcome === 1 && player.winStreak >= 3 ? 2 : 0;
+  const opponentBonus = enableStreakBonus && outcome === 0 && opponent.winStreak >= 3 ? 2 : 0;
+  const playerNewRating = Math.max(
+    100,
+    player.rating + Math.round(playerK * (outcome - playerExpectedScore)) + playerBonus,
+  );
+  const opponentNewRating = Math.max(
+    100,
+    opponent.rating + Math.round(opponentK * (1 - outcome - opponentExpectedScore)) + opponentBonus,
+  );
+
+  return {
+    playerNewRating,
+    opponentNewRating,
+    playerRatingDelta: playerNewRating - player.rating,
+    opponentRatingDelta: opponentNewRating - opponent.rating,
+    playerExpectedScore,
+    opponentExpectedScore,
+    playerNewTier: getMemberTier(playerNewRating),
+  };
+}
diff --git a/tests/elo-rating.test.ts b/tests/elo-rating.test.ts
new file mode 100644
index 0000000..a959e3c
--- /dev/null
+++ b/tests/elo-rating.test.ts
@@ -0,0 +1,137 @@
+// @vitest-environment node
+import { describe, expect, it } from 'vitest';
+import {
+  calculateEloMatch,
+  calculateExpectedScore,
+  determineKFactor,
+  getMemberTier,
+  type EloMemberProfile,
+} from '../src/lib/gamification/elo-rating';
+
+const member = (rating = 1200, matchesPlayed = 10, winStreak = 0): EloMemberProfile => ({
+  rating, matchesPlayed, winStreak,
+});
+
+describe('Elo expected scores', () => {
+  it('returns exactly 0.5 for equals and rounds unequal scores to four decimals', () => {
+    expect(calculateExpectedScore(1200, 1200)).toBe(0.5);
+    expect(calculateExpectedScore(1000, 1400)).toBe(0.0909);
+    expect(calculateExpectedScore(1400, 1000)).toBe(0.9091);
+  });
+
+  it('stays bounded for extreme rating gaps', () => {
+    expect(calculateExpectedScore(100, 1000000)).toBe(0);
+    expect(calculateExpectedScore(1000000, 100)).toBe(1);
+  });
+});
+
+describe('dynamic K-factors', () => {
+  it('uses placement, regular, and elite factors at their boundaries', () => {
+    expect(determineKFactor(member(1200, 9))).toBe(48);
+    expect(determineKFactor(member(1999, 10))).toBe(32);
+    expect(determineKFactor(member(2000, 10))).toBe(16);
+    expect(determineKFactor(member(2200, 9))).toBe(48);
+  });
+
+  it('honors custom factors and placement counts, including zero', () => {
+    expect(determineKFactor(member(1200, 4), 20, 5)).toBe(30);
+    expect(determineKFactor(member(1200, 5), 20, 5)).toBe(20);
+    expect(determineKFactor(member(2000, 5), 20, 5)).toBe(10);
+    expect(determineKFactor(member(1200, 0), 20, 0)).toBe(20);
+  });
+});
+
+describe('rank tiers', () => {
+  it.each([
+    [100, 'novice'], [999, 'novice'], [1000, 'apprentice'],
+    [1399, 'apprentice'], [1400, 'adept'], [1799, 'adept'],
+    [1800, 'master'], [2199, 'master'], [2200, 'grandmaster'],
+  ] as const)('classifies %i as %s', (rating, tier) => {
+    expect(getMemberTier(rating)).toBe(tier);
+  });
+});
+
+describe('Elo match results', () => {
+  it('awards 16 points for a win between established equals', () => {
+    expect(calculateEloMatch(member(), member(), 1)).toEqual({
+      playerNewRating: 1216, opponentNewRating: 1184,
+      playerRatingDelta: 16, opponentRatingDelta: -16,
+      playerExpectedScore: 0.5, opponentExpectedScore: 0.5,
+      playerNewTier: 'apprentice',
+    });
+  });
+
+  it('awards more for an upset than an expected win', () => {
+    expect(calculateEloMatch(member(1000), member(1400), 1).playerRatingDelta).toBe(29);
+    expect(calculateEloMatch(member(1400), member(1000), 1).playerRatingDelta).toBe(3);
+  });
+
+  it('leaves equal draws unchanged and moves unequal draws toward each other', () => {
+    expect(calculateEloMatch(member(), member(), 0.5).playerRatingDelta).toBe(0);
+    const draw = calculateEloMatch(member(1000), member(1400), 0.5);
+    expect(draw.playerRatingDelta).toBe(13);
+    expect(draw.opponentRatingDelta).toBe(-13);
+  });
+
+  it('uses independent placement and elite K-factors', () => {
+    const result = calculateEloMatch(member(2000, 0), member(2000), 1);
+    expect(result.playerRatingDelta).toBe(24);
+    expect(result.opponentRatingDelta).toBe(-8);
+  });
+
+  it('passes custom options to both members', () => {
+    const result = calculateEloMatch(member(1200, 2), member(1200, 3), 1, {
+      baseKFactor: 20, placementMatchesCount: 3,
+    });
+    expect(result.playerRatingDelta).toBe(15);
+    expect(result.opponentRatingDelta).toBe(-10);
+    expect(calculateEloMatch(member(), member(), 1, { baseKFactor: 0 }).playerRatingDelta).toBe(0);
+    expect(calculateEloMatch(member(1200, 0), member(1200, 0), 1, {
+      placementMatchesCount: 0,
+    }).playerRatingDelta).toBe(16);
+  });
+
+  it('gives the streak bonus only to a qualifying winner on either side', () => {
+    expect(calculateEloMatch(member(1200, 10, 2), member(), 1).playerRatingDelta).toBe(16);
+    const win = calculateEloMatch(member(1200, 10, 3), member(1200, 10, 4), 1);
+    expect(win.playerRatingDelta).toBe(18);
+    expect(win.opponentRatingDelta).toBe(-16);
+    const loss = calculateEloMatch(member(1200, 10, 4), member(1200, 10, 3), 0);
+    expect(loss.playerRatingDelta).toBe(-16);
+    expect(loss.opponentRatingDelta).toBe(18);
+    expect(calculateEloMatch(member(), member(1200, 10, 2), 0).opponentRatingDelta).toBe(16);
+  });
+
+  it('does not award bonuses for draws or when disabled', () => {
+    const streak = member(1200, 10, 3);
+    const draw = calculateEloMatch(streak, streak, 0.5);
+    expect(draw.playerRatingDelta).toBe(0);
+    expect(draw.opponentRatingDelta).toBe(0);
+    expect(calculateEloMatch(streak, streak, 1, { enableStreakBonus: false }).playerRatingDelta).toBe(16);
+    expect(calculateEloMatch(streak, streak, 0, { enableStreakBonus: false }).opponentRatingDelta).toBe(16);
+  });
+
+  it('clamps both sides to 100 and reports only the actual rating change', () => {
+    const loss = calculateEloMatch(member(105), member(105), 0);
+    expect(loss.playerNewRating).toBe(100);
+    expect(loss.playerRatingDelta).toBe(-5);
+    const win = calculateEloMatch(member(105), member(105), 1);
+    expect(win.opponentNewRating).toBe(100);
+    expect(win.opponentRatingDelta).toBe(-5);
+    expect(calculateEloMatch(member(100), member(100), 0).playerRatingDelta).toBe(0);
+  });
+
+  it.each([
+    [999, 'apprentice'], [1399, 'adept'], [1799, 'master'], [2199, 'grandmaster'],
+  ] as const)('promotes a winner across the boundary above %i', (rating, tier) => {
+    expect(calculateEloMatch(member(rating), member(rating), 1).playerNewTier).toBe(tier);
+  });
+
+  it('updates the tier after a loss and does not mutate input profiles', () => {
+    const player = Object.freeze(member(1400));
+    const opponent = Object.freeze(member(1400));
+    expect(calculateEloMatch(player, opponent, 0).playerNewTier).toBe('apprentice');
+    expect(player).toEqual(member(1400));
+    expect(opponent).toEqual(member(1400));
+  });
+});
diff --git a/vitest.config.ts b/vitest.config.ts
index 26a5452..08089fb 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -13,6 +13,6 @@ export default defineConfig({
     environment: 'jsdom',
     globals: true,
     setupFiles: ['./tests/setup.ts'],
-    include: ['tests/**/*.test.tsx'],
+    include: ['tests/**/*.test.tsx', 'tests/elo-rating.test.ts'],
   },
-});
\ No newline at end of file
+});