diff --git a/packages/api/src/__tests__/chargeability-report-router.test.ts b/packages/api/src/__tests__/chargeability-report-router.test.ts
index 7e28888..03d75a5 100644
--- a/packages/api/src/__tests__/chargeability-report-router.test.ts
+++ b/packages/api/src/__tests__/chargeability-report-router.test.ts
@@ -132,6 +132,38 @@ describe("chargeability report router", () => {
 
     expect(strictMonth).toBeDefined();
     expect(proposedMonth).toBeDefined();
+    expect(strict.explainability).toEqual({
+      locationFields: [
+        "country",
+        "federalState",
+        "city",
+        "orgUnit",
+        "managementLevelGroup",
+        "managementLevel",
+      ],
+      monthDerivationFields: [
+        "baseAvailableHours",
+        "publicHolidayCount",
+        "publicHolidayWorkdayCount",
+        "publicHolidayHoursDeduction",
+        "absenceDayEquivalent",
+        "absenceHoursDeduction",
+        "effectiveAvailableHours",
+      ],
+      activeFilters: [],
+      formulas: {
+        sah: "baseAvailableHours - publicHolidayHoursDeduction - absenceHoursDeduction = effectiveAvailableHours",
+        chargeabilityPct: "chargeabilityHours / sahHours",
+        targetHours: "sahHours * targetPct",
+        gapHours: "chargeabilityHours - targetHours",
+      },
+      notes: [
+        "Location fields explain why two resources can have different SAH in the same month because country, federal state, and city holidays may differ.",
+        "Holiday deductions and absence deductions are tracked separately; absence does not deduct days that are already public holidays.",
+        "Include proposed work changes chargeability ratios and hours, but it does not change holiday or absence-based SAH derivation.",
+      ],
+    });
+    expect(withProposed.explainability.activeFilters).toEqual(["includeProposed"]);
     expect(strictMonth?.chg).toBeGreaterThan(0);
     expect(proposedMonth?.chg).toBeGreaterThan(strictMonth?.chg ?? 0);
     expect(proposedMonth?.chg).toBeCloseTo((strictMonth?.chg ?? 0) * 2, 5);
@@ -352,6 +384,13 @@ describe("chargeability report router", () => {
     const augsburg = report.resources.find((resource) => resource.city === "Augsburg");
     const munich = report.resources.find((resource) => resource.city === "Munich");
 
+    expect(augsburg?.federalState).toBe("BY");
+    expect(augsburg?.months[0]?.derivation?.publicHolidayCount).toBe(
+      (munich?.months[0]?.derivation?.publicHolidayCount ?? 0) + 1,
+    );
+    expect(augsburg?.months[0]?.derivation?.publicHolidayHoursDeduction).toBe(
+      (munich?.months[0]?.derivation?.publicHolidayHoursDeduction ?? 0) + 8,
+    );
     expect(augsburg?.months[0]?.sah).toBe((munich?.months[0]?.sah ?? 0) - 8);
   });
 
@@ -513,11 +552,43 @@ describe("chargeability report router", () => {
     expect(result.resourceCount).toBe(1);
     expect(result.returnedResourceCount).toBe(1);
     expect(result.truncated).toBe(false);
+    expect(result.explainability).toEqual({
+      locationFields: [
+        "country",
+        "federalState",
+        "city",
+        "orgUnit",
+        "managementLevelGroup",
+        "managementLevel",
+      ],
+      monthDerivationFields: [
+        "baseAvailableHours",
+        "publicHolidayCount",
+        "publicHolidayWorkdayCount",
+        "publicHolidayHoursDeduction",
+        "absenceDayEquivalent",
+        "absenceHoursDeduction",
+        "effectiveAvailableHours",
+      ],
+      activeFilters: ["resourceQuery"],
+      formulas: {
+        sah: "baseAvailableHours - publicHolidayHoursDeduction - absenceHoursDeduction = effectiveAvailableHours",
+        chargeabilityPct: "chargeabilityHours / sahHours",
+        targetHours: "sahHours * targetPct",
+        gapHours: "chargeabilityHours - targetHours",
+      },
+      notes: [
+        "Location fields explain why two resources can have different SAH in the same month because country, federal state, and city holidays may differ.",
+        "Holiday deductions and absence deductions are tracked separately; absence does not deduct days that are already public holidays.",
+        "Include proposed work changes chargeability ratios and hours, but it does not change holiday or absence-based SAH derivation.",
+      ],
+    });
     expect(result.resources).toEqual([
       expect.objectContaining({
         displayName: "Alice",
         targetPct: 80,
         country: "ES",
+        federalState: null,
         city: "Barcelona",
         managementLevelGroup: "Senior",
         managementLevel: "L7",
@@ -527,6 +598,10 @@ describe("chargeability report router", () => {
             sah: expect.any(Number),
             chargeabilityPct: expect.any(Number),
             gapPct: expect.any(Number),
+            derivation: expect.objectContaining({
+              baseAvailableHours: expect.any(Number),
+              effectiveAvailableHours: expect.any(Number),
+            }),
           }),
         ],
       }),
diff --git a/packages/api/src/router/chargeability-report-procedure-support.ts b/packages/api/src/router/chargeability-report-procedure-support.ts
index acee810..5c4a212 100644
--- a/packages/api/src/router/chargeability-report-procedure-support.ts
+++ b/packages/api/src/router/chargeability-report-procedure-support.ts
@@ -168,12 +168,62 @@ export const chargeabilityReportDetailInputSchema = chargeabilityReportInputSche
 
 type ChargeabilityReportInput = z.infer<typeof chargeabilityReportInputSchema>;
 type ChargeabilityReportDetailInput = z.infer<typeof chargeabilityReportDetailInputSchema>;
+type ChargeabilityExplainabilityInput = ChargeabilityReportInput & {
+  resourceQuery?: string | undefined;
+  resourceLimit?: number | undefined;
+};
 
 type ChargeabilityReportDbClient = Pick<
   PrismaClient,
   "assignment" | "resource" | "project" | "vacation" | "holidayCalendar" | "systemSettings"
 >;
 
+const CHARGEABILITY_LOCATION_FIELDS = [
+  "country",
+  "federalState",
+  "city",
+  "orgUnit",
+  "managementLevelGroup",
+  "managementLevel",
+] as const;
+
+const CHARGEABILITY_DERIVATION_FIELDS = [
+  "baseAvailableHours",
+  "publicHolidayCount",
+  "publicHolidayWorkdayCount",
+  "publicHolidayHoursDeduction",
+  "absenceDayEquivalent",
+  "absenceHoursDeduction",
+  "effectiveAvailableHours",
+] as const;
+
+function buildChargeabilityExplainability(input: ChargeabilityExplainabilityInput) {
+  const activeFilters = [
+    ...(input.orgUnitId ? ["orgUnitId"] : []),
+    ...(input.managementLevelGroupId ? ["managementLevelGroupId"] : []),
+    ...(input.countryId ? ["countryId"] : []),
+    ...(input.resourceQuery ? ["resourceQuery"] : []),
+    ...(input.includeProposed ? ["includeProposed"] : []),
+  ];
+
+  return {
+    locationFields: [...CHARGEABILITY_LOCATION_FIELDS],
+    monthDerivationFields: [...CHARGEABILITY_DERIVATION_FIELDS],
+    activeFilters,
+    formulas: {
+      sah: "baseAvailableHours - publicHolidayHoursDeduction - absenceHoursDeduction = effectiveAvailableHours",
+      chargeabilityPct: "chargeabilityHours / sahHours",
+      targetHours: "sahHours * targetPct",
+      gapHours: "chargeabilityHours - targetHours",
+    },
+    notes: [
+      "Location fields explain why two resources can have different SAH in the same month because country, federal state, and city holidays may differ.",
+      "Holiday deductions and absence deductions are tracked separately; absence does not deduct days that are already public holidays.",
+      "Include proposed work changes chargeability ratios and hours, but it does not change holiday or absence-based SAH derivation.",
+    ],
+  };
+}
+
 async function queryChargeabilityReport(
   db: ChargeabilityReportDbClient,
   input: ChargeabilityReportInput,
@@ -227,6 +277,7 @@ async function queryChargeabilityReport(
         chargeabilityRatio: 0,
         targetRatio: 0,
       })),
+      explainability: buildChargeabilityExplainability(input),
     };
   }
 
@@ -388,6 +439,7 @@ async function queryChargeabilityReport(
     monthKeys,
     resources: anonymizeResources(resourceRows, directory),
     groupTotals,
+    explainability: buildChargeabilityExplainability(input),
   };
 }
 
@@ -462,6 +514,7 @@ export function buildChargeabilityReportDetail(
     resourceCount: matchingResources.length,
     returnedResourceCount: resources.length,
     truncated: resources.length < matchingResources.length,
+    explainability: buildChargeabilityExplainability(input),
     resources,
   };
 }