diff --git a/src/stats/zoneDistribution.test.ts b/src/stats/zoneDistribution.test.ts
new file mode 100644
index 0000000..9660d0a
--- /dev/null
+++ b/src/stats/zoneDistribution.test.ts
@@ -0,0 +1,105 @@
+import { Interval } from "../ast";
+import { Duration } from "../Duration";
+import { ConstantIntensity, RangeIntensity } from "../Intensity";
+import { zoneDistribution } from "./zoneDistribution";
+
+const testZoneDistribution = (intervals: Interval[]) =>
+  zoneDistribution(intervals).map(({ name, duration }) => [name, duration.seconds]);
+
+describe("zoneDistribution()", () => {
+  it("when no intervals, returns 0 for all zones", () => {
+    expect(testZoneDistribution([])).toEqual([
+      ["Z1: Recovery", 0],
+      ["Z2: Endurance", 0],
+      ["Z3: Tempo", 0],
+      ["Z4: Threshold", 0],
+      ["Z5: VO2 Max", 0],
+      ["Z6: Anaerobic", 0],
+      ["Freeride", 0],
+    ]);
+  });
+
+  it("sums up durations falling within single intensity range", () => {
+    const intervals: Interval[] = [
+      {
+        type: "Interval",
+        duration: new Duration(100),
+        intensity: new ConstantIntensity(0.9), // lower bound of Z4
+        comments: [],
+      },
+      {
+        type: "Interval",
+        duration: new Duration(100),
+        intensity: new ConstantIntensity(1.0), // middle of of Z4
+        comments: [],
+      },
+      {
+        type: "Interval",
+        duration: new Duration(100),
+        intensity: new ConstantIntensity(1.04), // upper bound of of Z4
+        comments: [],
+      },
+    ];
+    expect(testZoneDistribution(intervals)).toEqual([
+      ["Z1: Recovery", 0],
+      ["Z2: Endurance", 0],
+      ["Z3: Tempo", 0],
+      ["Z4: Threshold", 300],
+      ["Z5: VO2 Max", 0],
+      ["Z6: Anaerobic", 0],
+      ["Freeride", 0],
+    ]);
+  });
+
+  it("distributes interval lengths to respective zones", () => {
+    const intervals: Interval[] = [
+      {
+        type: "Interval",
+        duration: new Duration(100),
+        intensity: new ConstantIntensity(1.0), // Z4
+        comments: [],
+      },
+      {
+        type: "Interval",
+        duration: new Duration(50),
+        intensity: new ConstantIntensity(0.1), // Z1
+        comments: [],
+      },
+      {
+        type: "Interval",
+        duration: new Duration(10),
+        intensity: new ConstantIntensity(2.0), // Z6
+        comments: [],
+      },
+    ];
+    expect(testZoneDistribution(intervals)).toEqual([
+      ["Z1: Recovery", 50],
+      ["Z2: Endurance", 0],
+      ["Z3: Tempo", 0],
+      ["Z4: Threshold", 100],
+      ["Z5: VO2 Max", 0],
+      ["Z6: Anaerobic", 10],
+      ["Freeride", 0],
+    ]);
+  });
+
+  it("splits range-intensity interval duration between multiple zones", () => {
+    const intervals: Interval[] = [
+      {
+        type: "Interval",
+        duration: new Duration(100),
+        intensity: new RangeIntensity(0.6, 0.9), // Z2..Z3
+        comments: [],
+      },
+    ];
+    expect(testZoneDistribution(intervals)).toEqual([
+      ["Z1: Recovery", 0],
+      ["Z2: Endurance", 50],
+      ["Z3: Tempo", 50],
+      ["Z4: Threshold", 0],
+      ["Z5: VO2 Max", 0],
+      ["Z6: Anaerobic", 0],
+      ["Freeride", 0],
+    ]);
+  });
+});