diff --git a/src/habits.test.ts b/src/habits.test.ts
index 868c60c..584117d 100644
--- a/src/habits.test.ts
+++ b/src/habits.test.ts
@@ -4,6 +4,8 @@ import {
   toggleCompletion,
   isCompletedOn,
   dayStr,
+  currentStreak,
+  bestStreak,
 } from "./habits";
 
 describe("createHabit", () => {
@@ -42,3 +44,97 @@ describe("dayStr", () => {
     expect(dayStr(new Date(2026, 5, 2))).toBe("2026-06-02");
   });
 });
+
+describe("currentStreak", () => {
+  const today = "2026-06-10";
+
+  it("returns 0 for empty habit", () => {
+    const h = createHabit("Test");
+    expect(currentStreak(h, today)).toBe(0);
+  });
+
+  it("returns 1 when only today is completed", () => {
+    let h = createHabit("Test");
+    h = toggleCompletion(h, today);
+    expect(currentStreak(h, today)).toBe(1);
+  });
+
+  it("returns 1 when only yesterday is completed and today is not", () => {
+    let h = createHabit("Test");
+    h = toggleCompletion(h, "2026-06-09");
+    expect(currentStreak(h, today)).toBe(1);
+  });
+
+  it("returns 0 when last completion was two days ago", () => {
+    let h = createHabit("Test");
+    h = toggleCompletion(h, "2026-06-08");
+    expect(currentStreak(h, today)).toBe(0);
+  });
+
+  it("counts an active streak of several days ending today", () => {
+    let h = createHabit("Test");
+    h = toggleCompletion(h, "2026-06-08");
+    h = toggleCompletion(h, "2026-06-09");
+    h = toggleCompletion(h, "2026-06-10");
+    expect(currentStreak(h, today)).toBe(3);
+  });
+
+  it("only counts the consecutive tail — ignores older completions after a gap", () => {
+    let h = createHabit("Test");
+    h = toggleCompletion(h, "2026-06-01"); // gap
+    h = toggleCompletion(h, "2026-06-09");
+    h = toggleCompletion(h, "2026-06-10");
+    expect(currentStreak(h, today)).toBe(2);
+  });
+});
+
+describe("bestStreak", () => {
+  it("returns 0 for empty habit", () => {
+    const h = createHabit("Test");
+    expect(bestStreak(h)).toBe(0);
+  });
+
+  it("returns 1 for a single completion", () => {
+    let h = createHabit("Test");
+    h = toggleCompletion(h, "2026-06-10");
+    expect(bestStreak(h)).toBe(1);
+  });
+
+  it("returns 1 when there is only one completion (yesterday)", () => {
+    let h = createHabit("Test");
+    h = toggleCompletion(h, "2026-06-09");
+    expect(bestStreak(h)).toBe(1);
+  });
+
+  it("counts a streak broken in the middle correctly", () => {
+    let h = createHabit("Test");
+    h = toggleCompletion(h, "2026-06-01");
+    h = toggleCompletion(h, "2026-06-02");
+    h = toggleCompletion(h, "2026-06-03");
+    // gap
+    h = toggleCompletion(h, "2026-06-07");
+    h = toggleCompletion(h, "2026-06-08");
+    expect(bestStreak(h)).toBe(3);
+  });
+
+  it("returns the length of a long active streak", () => {
+    let h = createHabit("Test");
+    for (let d = 5; d <= 10; d++) {
+      h = toggleCompletion(h, `2026-06-${String(d).padStart(2, "0")}`);
+    }
+    expect(bestStreak(h)).toBe(6);
+  });
+
+  it("returns the best past run when current run is shorter", () => {
+    let h = createHabit("Test");
+    // Past best: 4 days
+    h = toggleCompletion(h, "2026-05-01");
+    h = toggleCompletion(h, "2026-05-02");
+    h = toggleCompletion(h, "2026-05-03");
+    h = toggleCompletion(h, "2026-05-04");
+    // Current run: 2 days
+    h = toggleCompletion(h, "2026-06-09");
+    h = toggleCompletion(h, "2026-06-10");
+    expect(bestStreak(h)).toBe(4);
+  });
+});
diff --git a/src/habits.ts b/src/habits.ts
index 28cc33a..95eef25 100644
--- a/src/habits.ts
+++ b/src/habits.ts
@@ -45,3 +45,49 @@ export function toggleCompletion(habit: Habit, day: string): Habit {
     : [...habit.completions, day].sort();
   return { ...habit, completions };
 }
+
+function shiftDay(day: string, delta: number): string {
+  const d = new Date(day + "T00:00:00Z");
+  d.setUTCDate(d.getUTCDate() + delta);
+  return d.toISOString().slice(0, 10);
+}
+
+/**
+ * Count of consecutive completed days ending on or just before `today`.
+ * If `today` is not completed but yesterday is, the streak runs through yesterday.
+ * Returns 0 when neither today nor yesterday is completed.
+ */
+export function currentStreak(habit: Habit, today: string): number {
+  const set = new Set(habit.completions);
+  let day = today;
+  if (!set.has(day)) {
+    day = shiftDay(day, -1);
+    if (!set.has(day)) return 0;
+  }
+  let count = 0;
+  while (set.has(day)) {
+    count++;
+    day = shiftDay(day, -1);
+  }
+  return count;
+}
+
+/**
+ * Length of the longest run of consecutive completed days ever recorded.
+ * Returns 0 for a habit with no completions.
+ */
+export function bestStreak(habit: Habit): number {
+  if (habit.completions.length === 0) return 0;
+  const sorted = [...habit.completions].sort();
+  let best = 1;
+  let run = 1;
+  for (let i = 1; i < sorted.length; i++) {
+    if (sorted[i] === shiftDay(sorted[i - 1], 1)) {
+      run++;
+      if (run > best) best = run;
+    } else {
+      run = 1;
+    }
+  }
+  return best;
+}