diff --git a/src/stats/zoneDistribution.ts b/src/stats/zoneDistribution.ts
index 7801a80..1305415 100644
--- a/src/stats/zoneDistribution.ts
+++ b/src/stats/zoneDistribution.ts
@@ -2,33 +2,44 @@ import { Interval } from "../ast";
 import { Duration } from "../Duration";
 import { intervalsToIntensityNumbers } from "./intervalsToIntensityNumbers";
 
-type Zone = {
-  from: number;
-  to: number;
-  name: string;
-};
+type NumericZoneDuration = { name: string; duration: number };
+type ZoneDuration = { name: string; duration: Duration };
+type ZoneIndex = 0 | 1 | 2 | 3 | 4 | 5;
 
-type NumericZoneDuration = Zone & { duration: number };
-type ZoneDuration = Zone & { duration: Duration };
+const emptyZones = (): NumericZoneDuration[] => [
+  { name: "Z1: Recovery", duration: 0 },
+  { name: "Z2: Endurance", duration: 0 },
+  { name: "Z3: Tempo", duration: 0 },
+  { name: "Z4: Threshold", duration: 0 },
+  { name: "Z5: VO2 Max", duration: 0 },
+  { name: "Z6: Anaerobic", duration: 0 },
+];
 
 // Intensity ranges based on https://zwiftinsider.com/power-zone-colors/
-const emptyZones = (): NumericZoneDuration[] => [
-  { from: 0.0, to: 0.6, name: "Z1: Recovery", duration: 0 },
-  { from: 0.6, to: 0.75, name: "Z2: Endurance", duration: 0 },
-  { from: 0.75, to: 0.9, name: "Z3: Tempo", duration: 0 },
-  { from: 0.9, to: 1.05, name: "Z4: Threshold", duration: 0 },
-  { from: 1.05, to: 1.18, name: "Z5: VO2 Max", duration: 0 },
-  { from: 1.18, to: Infinity, name: "Z6: Anaerobic", duration: 0 },
-];
+const zoneIndex = (intensity: number): ZoneIndex => {
+  if (intensity >= 1.18) {
+    return 5;
+  }
+  if (intensity >= 1.05) {
+    return 4;
+  }
+  if (intensity >= 0.9) {
+    return 3;
+  }
+  if (intensity >= 0.75) {
+    return 2;
+  }
+  if (intensity >= 0.6) {
+    return 1;
+  }
+  return 0;
+};
 
 export const zoneDistribution = (intervals: Interval[]): ZoneDuration[] => {
   const zones = emptyZones();
 
   intervalsToIntensityNumbers(intervals).forEach((intensity) => {
-    const zone = zones.find((zone) => intensity >= zone.from && intensity < zone.to);
-    if (zone) {
-      zone.duration++;
-    }
+    zones[zoneIndex(intensity)].duration++;
   });
 
   return zones.map(({ duration, ...rest }) => ({ duration: new Duration(duration), ...rest }));