diff --git a/prisma/schema.prisma b/prisma/schema.prisma
index 09d54ef02..92ae22ddd 100644
--- a/prisma/schema.prisma
+++ b/prisma/schema.prisma
@@ -1374,7 +1374,7 @@ enum MeasurementType {
   BLOOD_GLUCOSE // Canonical storage: mg/dL. Display unit (mg/dL or mmol/L) is user preference.
   TOTAL_BODY_WATER // Canonical storage: kilograms (kg of water). Matches Withings' hydration measure (type 77) and Health Connect's TotalBodyWaterRecord. Display can compute % from same-day weight if desired.
   BONE_MASS // Canonical storage: kilograms (kg). Matches Withings' bone mass measure (type 88).
-  OXYGEN_SATURATION // Canonical storage: percent (0..100). Withings ScanWatch reports type 54. Health Connect's OxygenSaturationRecord uses Percentage (already 0..100). HealthKit uses HKQuantityTypeIdentifierOxygenSaturation as a fraction (0..1) — iOS DTO must multiply by 100 before submitting.
+  OXYGEN_SATURATION // Canonical storage: percent (0..100). Withings ScanWatch reports type 54. Health Connect's OxygenSaturationRecord uses Percentage (already 0..100). HealthKit uses HKQuantityTypeIdentifierOxygenSaturation as a fraction (0..1); clients send the raw HealthKit reading and the server scales it at ingest (see `apple-health-mapping.ts`). This line used to say the opposite, and the iOS client built against it, which cost every user their oxygen-saturation and body-fat readings until the importer learned to take both.
   // ── v1.4.23 Apple Health additions ──
   HEART_RATE_VARIABILITY // ms — HealthKit `heartRateVariabilitySDNN` (typically one sample per night).
   RESTING_HEART_RATE // bpm — HealthKit `restingHeartRate` (one sample per day; distinct from spot PULSE).
diff --git a/src/lib/measurements/__tests__/apple-health-mapping.test.ts b/src/lib/measurements/__tests__/apple-health-mapping.test.ts
index 3dd28a7e9..61892652c 100644
--- a/src/lib/measurements/__tests__/apple-health-mapping.test.ts
+++ b/src/lib/measurements/__tests__/apple-health-mapping.test.ts
@@ -9,7 +9,10 @@ import {
   hkIdentifierForType,
   mapAppleHealthEntry,
 } from "../apple-health-mapping";
-import { measurementTypeEnum } from "@/lib/validations/measurement";
+import {
+  measurementTypeEnum,
+  validateMeasurementRange,
+} from "@/lib/validations/measurement";
 import type { MeasurementType } from "@/generated/prisma/client";
 
 /**
@@ -1013,3 +1016,68 @@ describe("v1.10.0 categorical events (WX-B)", () => {
     expect(out!.rhythmClassification).toBeUndefined();
   });
 });
+
+describe("percent-shaped HealthKit types take a fraction or an already-scaled percent", () => {
+  const PERCENT_TYPES = [
+    "HKQuantityTypeIdentifierBodyFatPercentage",
+    "HKQuantityTypeIdentifierOxygenSaturation",
+    "HKQuantityTypeIdentifierAppleWalkingSteadiness",
+    "HKQuantityTypeIdentifierWalkingAsymmetryPercentage",
+    "HKQuantityTypeIdentifierWalkingDoubleSupportPercentage",
+  ] as const;
+
+  it("scales a raw HealthKit fraction, the documented wire form", () => {
+    for (const id of PERCENT_TYPES) {
+      const mapping = APPLE_HEALTH_TYPE_MAP[id];
+      expect(mapping.convertToDbUnit(0.97), id).toBeCloseTo(97);
+      expect(mapping.convertToDbUnit(0.25), id).toBeCloseTo(25);
+    }
+  });
+
+  it("passes a value a client already scaled straight through", () => {
+    // Shipped iOS builds multiply oxygen saturation and body fat by 100
+    // before upload. Scaling again put the row outside its plausibility
+    // range, the batch route skipped it, and the client advanced its anchor
+    // past a reading it then never read again.
+    for (const id of PERCENT_TYPES) {
+      const mapping = APPLE_HEALTH_TYPE_MAP[id];
+      expect(mapping.convertToDbUnit(97), id).toBeCloseTo(97);
+      expect(mapping.convertToDbUnit(25), id).toBeCloseTo(25);
+    }
+  });
+
+  it("keeps a stored oxygen saturation inside its plausibility range either way", () => {
+    const mapping =
+      APPLE_HEALTH_TYPE_MAP["HKQuantityTypeIdentifierOxygenSaturation"];
+    for (const wire of [0.97, 97]) {
+      expect(
+        validateMeasurementRange(
+          "OXYGEN_SATURATION",
+          mapping.convertToDbUnit(wire),
+        ),
+      ).toBeNull();
+    }
+  });
+
+  it("covers every mapping that declares the percent-fraction unit reason, and only those", () => {
+    // The x100 in this file is not one thing: waist circumference converts
+    // metres to centimetres with the same factor. A blanket change to the
+    // tolerant converter turned a 1.02 m waist into 1.02 cm, which is how
+    // this test came to exist.
+    for (const [id, mapping] of Object.entries(APPLE_HEALTH_TYPE_MAP)) {
+      const isPercentClass = PERCENT_TYPES.includes(
+        id as (typeof PERCENT_TYPES)[number],
+      );
+      const tolerant =
+        mapping.convertToDbUnit(2) === 2 && mapping.convertToDbUnit(0.5) === 50;
+      expect(tolerant, id).toBe(isPercentClass);
+    }
+  });
+
+  it("still converts a waist circumference from metres to centimetres", () => {
+    const mapping =
+      APPLE_HEALTH_TYPE_MAP["HKQuantityTypeIdentifierWaistCircumference"];
+    expect(mapping.convertToDbUnit(0.86)).toBeCloseTo(86);
+    expect(mapping.convertToDbUnit(1.02)).toBeCloseTo(102);
+  });
+});
diff --git a/src/lib/measurements/apple-health-mapping.ts b/src/lib/measurements/apple-health-mapping.ts
index f090a7065..b368c805a 100644
--- a/src/lib/measurements/apple-health-mapping.ts
+++ b/src/lib/measurements/apple-health-mapping.ts
@@ -218,7 +218,27 @@ const UNIT_FIXED_DIMENSIONLESS_COUNT =
 const UNIT_FIXED_PER_MINUTE =
   "Per-minute rate: Apple emits `count/min` in every locale and the Health app offers no alternative display unit, so there is nothing to convert from.";
 const UNIT_FIXED_PERCENT_FRACTION =
-  "Apple writes the percent unit but the VALUE rides as a 0..1 fraction; the x100 in `convertToDbUnit` already covers both spellings, so branching on the record unit would double-scale it.";
+  "Apple writes the percent unit but the VALUE rides as a 0..1 fraction; `percentFromFraction` in `convertToDbUnit` already covers both spellings, so branching on the record unit would double-scale it.";
+
+/**
+ * A HealthKit percent, from a fraction or from a client that already scaled.
+ *
+ * The convention is that HK values travel raw and the server scales. Shipped
+ * iOS builds break it for two identifiers: they multiply oxygen saturation and
+ * body fat by 100 before upload, so the server scaled a second time, the value
+ * left the plausibility range, the batch route skipped the row, and the client
+ * treated that skip as final and advanced its anchor. Every reading of those
+ * two types was lost, silently, on every install.
+ *
+ * The two domains do not overlap: a fraction is at most 1, and one percent
+ * oxygen saturation or body fat does not occur in a living person. So a value
+ * above 1 is already a percent and passes through, and anything at or below 1
+ * is a fraction and gets scaled. A wrong reading still meets the per-type
+ * range guard afterwards, which is what rejects the impossible either way.
+ */
+function percentFromFraction(value: number): number {
+  return value > 1 ? value : value * 100;
+}
 const UNIT_FIXED_EVENT_PIN =
   "Categorical event: `convertToDbUnit` pins the stored value to 1 fired event regardless of the reading, so no unit attribute can move it.";
 const UNIT_FIXED_AUDIO_SPL =
@@ -245,7 +265,7 @@ export const APPLE_HEALTH_TYPE_MAP: Record<string, AppleHealthMapping> = {
     unitFixedReason: UNIT_FIXED_PERCENT_FRACTION,
     dbUnit: "%",
     // Apple ships 0..1 fraction; HealthLog stores 0..100.
-    convertToDbUnit: (v) => v * 100,
+    convertToDbUnit: percentFromFraction,
     aggregation: "latest",
   },
   HKQuantityTypeIdentifierBodyTemperature: {
@@ -269,7 +289,7 @@ export const APPLE_HEALTH_TYPE_MAP: Record<string, AppleHealthMapping> = {
     hkUnit: "m",
     dbUnit: "cm",
     // Apple ships length in metres; HealthLog stores waist in cm.
-    convertToDbUnit: (v) => v * 100,
+    convertToDbUnit: (v) => v * 100, // metres to centimetres, NOT a percent
     aggregation: "latest",
   },
 
@@ -385,7 +405,7 @@ export const APPLE_HEALTH_TYPE_MAP: Record<string, AppleHealthMapping> = {
     unitFixedReason: UNIT_FIXED_PERCENT_FRACTION,
     dbUnit: "%",
     // Apple ships 0..1 fraction; HealthLog stores 0..100.
-    convertToDbUnit: (v) => v * 100,
+    convertToDbUnit: percentFromFraction,
     aggregation: "latest",
   },
 
@@ -455,7 +475,7 @@ export const APPLE_HEALTH_TYPE_MAP: Record<string, AppleHealthMapping> = {
     unitFixedReason: UNIT_FIXED_PERCENT_FRACTION,
     dbUnit: "%",
     // Apple ships 0..1 fraction; HealthLog stores 0..100.
-    convertToDbUnit: (v) => v * 100,
+    convertToDbUnit: percentFromFraction,
     aggregation: "latest",
   },
   // Environmental audio-exposure event — iOS 13+ category-type that
@@ -632,7 +652,7 @@ export const APPLE_HEALTH_TYPE_MAP: Record<string, AppleHealthMapping> = {
     hkUnit: "%",
     unitFixedReason: UNIT_FIXED_PERCENT_FRACTION,
     dbUnit: "%",
-    convertToDbUnit: (v) => v * 100,
+    convertToDbUnit: percentFromFraction,
     aggregation: "latest",
   },
   // Walking double-support percentage — gait companion metric.
@@ -643,7 +663,7 @@ export const APPLE_HEALTH_TYPE_MAP: Record<string, AppleHealthMapping> = {
     hkUnit: "%",
     unitFixedReason: UNIT_FIXED_PERCENT_FRACTION,
     dbUnit: "%",
-    convertToDbUnit: (v) => v * 100,
+    convertToDbUnit: percentFromFraction,
     aggregation: "latest",
   },