diff --git a/CHANGELOG.fr.md b/CHANGELOG.fr.md
index 3bd4801..434493b 100644
--- a/CHANGELOG.fr.md
+++ b/CHANGELOG.fr.md
@@ -2,6 +2,13 @@
 
 ## [Non publié]
 
+### Ajouté
+- **Rapport Cartes** : info-bulle d'aide sur le KPI taux d'épargne expliquant la formule — `(revenus − dépenses) ÷ revenus × 100`, calculée sur le mois de référence (#101)
+
+### Corrigé
+- **Rapport Cartes** : retrait du sélecteur de période non fonctionnel — le rapport Cartes est un instantané « mois X vs X-1 vs X-12 », seul le sélecteur de mois de référence est nécessaire (#101)
+- **Rapport Cartes** : le KPI taux d'épargne affiche maintenant « — » au lieu de « 0 % » lorsque le mois de référence n'a aucun revenu (une division par zéro est indéfinie, pas zéro) (#101)
+
 ## [0.8.2] - 2026-04-17
 
 ### Ajouté
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7f696ae..507859d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,13 @@
 
 ## [Unreleased]
 
+### Added
+- **Cartes report**: help tooltip on the savings-rate KPI explaining the formula — `(income − expenses) ÷ income × 100`, computed on the reference month (#101)
+
+### Fixed
+- **Cartes report**: removed the non-functional period selector — the Cartes report is a "month X vs X-1 vs X-12" snapshot, so only the reference-month picker is needed (#101)
+- **Cartes report**: savings-rate KPI now shows "—" instead of "0 %" when the reference month has no income (division by zero is undefined, not zero) (#101)
+
 ## [0.8.2] - 2026-04-17
 
 ### Added
diff --git a/src/components/reports/cards/KpiCard.tsx b/src/components/reports/cards/KpiCard.tsx
index 5de9af5..7f4a19f 100644
--- a/src/components/reports/cards/KpiCard.tsx
+++ b/src/components/reports/cards/KpiCard.tsx
@@ -1,4 +1,5 @@
 import { useTranslation } from "react-i18next";
+import { HelpCircle } from "lucide-react";
 import KpiSparkline from "./KpiSparkline";
 import type { CartesKpi, CartesKpiId } from "../../../shared/types";
 
@@ -9,6 +10,8 @@ export interface KpiCardProps {
   format: "currency" | "percent";
   /** When true, positive deltas are rendered in red (e.g. rising expenses). */
   deltaIsBadWhenUp?: boolean;
+  /** Optional help text shown on hover of a (?) icon next to the title. */
+  tooltip?: string;
 }
 
 function formatCurrency(amount: number, language: string): string {
@@ -102,6 +105,7 @@ export default function KpiCard({
   kpi,
   format,
   deltaIsBadWhenUp = false,
+  tooltip,
 }: KpiCardProps) {
   const { t, i18n } = useTranslation();
   const language = i18n.language;
@@ -111,9 +115,20 @@ export default function KpiCard({
       data-kpi={id}
       className="bg-[var(--card)] border border-[var(--border)] rounded-xl p-4 flex flex-col gap-3"
     >
-      <div className="text-sm text-[var(--muted-foreground)]">{title}</div>
+      <div className="text-sm text-[var(--muted-foreground)] flex items-center gap-1">
+        <span>{title}</span>
+        {tooltip && (
+          <span
+            title={tooltip}
+            aria-label={tooltip}
+            className="inline-flex items-center text-[var(--muted-foreground)] cursor-help"
+          >
+            <HelpCircle size={12} aria-hidden="true" />
+          </span>
+        )}
+      </div>
       <div className="text-2xl font-bold tabular-nums text-[var(--foreground)]">
-        {formatValue(kpi.current, format, language)}
+        {kpi.current === null ? "—" : formatValue(kpi.current, format, language)}
       </div>
       <KpiSparkline data={kpi.sparkline} />
       <div className="flex items-start justify-between gap-2 pt-1 border-t border-[var(--border)]">
diff --git a/src/hooks/useCartes.ts b/src/hooks/useCartes.ts
index d42cc15..7d82eda 100644
--- a/src/hooks/useCartes.ts
+++ b/src/hooks/useCartes.ts
@@ -1,7 +1,6 @@
 import { useReducer, useCallback, useEffect, useRef } from "react";
 import type { CartesSnapshot } from "../shared/types";
 import { getCartesSnapshot } from "../services/reportService";
-import { useReportsPeriod } from "./useReportsPeriod";
 
 interface State {
   year: number;
@@ -55,7 +54,6 @@ function reducer(state: State, action: Action): State {
 }
 
 export function useCartes() {
-  const { from, to, period, setPeriod, setCustomDates } = useReportsPeriod();
   const [state, dispatch] = useReducer(reducer, initialState);
   const fetchIdRef = useRef(0);
 
@@ -76,17 +74,6 @@ export function useCartes() {
     fetch(state.year, state.month);
   }, [fetch, state.year, state.month]);
 
-  // Keep the reference month in sync with the URL `to` date, so navigating
-  // via PeriodSelector works as expected.
-  useEffect(() => {
-    const [y, m] = to.split("-").map(Number);
-    if (!Number.isFinite(y) || !Number.isFinite(m)) return;
-    if (y !== state.year || m !== state.month) {
-      dispatch({ type: "SET_REFERENCE_PERIOD", payload: { year: y, month: m } });
-    }
-    // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [to]);
-
   const setReferencePeriod = useCallback((year: number, month: number) => {
     dispatch({ type: "SET_REFERENCE_PERIOD", payload: { year, month } });
   }, []);
@@ -94,10 +81,5 @@ export function useCartes() {
   return {
     ...state,
     setReferencePeriod,
-    from,
-    to,
-    period,
-    setPeriod,
-    setCustomDates,
   };
 }
diff --git a/src/i18n/locales/en.json b/src/i18n/locales/en.json
index 9a81b4a..3905ec6 100644
--- a/src/i18n/locales/en.json
+++ b/src/i18n/locales/en.json
@@ -418,6 +418,7 @@
       "expenses": "Expenses",
       "net": "Net balance",
       "savingsRate": "Savings rate",
+      "savingsRateTooltip": "Formula: (income − expenses) ÷ income × 100, computed on the reference month.",
       "deltaMoMLabel": "vs last month",
       "deltaYoYLabel": "vs last year",
       "flowChartTitle": "Income vs expenses — last 12 months",
diff --git a/src/i18n/locales/fr.json b/src/i18n/locales/fr.json
index bcf7e30..2bc0c75 100644
--- a/src/i18n/locales/fr.json
+++ b/src/i18n/locales/fr.json
@@ -418,6 +418,7 @@
       "expenses": "Dépenses",
       "net": "Solde net",
       "savingsRate": "Taux d'épargne",
+      "savingsRateTooltip": "Formule : (revenus − dépenses) ÷ revenus × 100, calculée sur le mois de référence.",
       "deltaMoMLabel": "vs mois précédent",
       "deltaYoYLabel": "vs l'an dernier",
       "flowChartTitle": "Revenus vs dépenses — 12 derniers mois",
diff --git a/src/pages/ReportsCartesPage.tsx b/src/pages/ReportsCartesPage.tsx
index d6157ca..5495704 100644
--- a/src/pages/ReportsCartesPage.tsx
+++ b/src/pages/ReportsCartesPage.tsx
@@ -1,7 +1,9 @@
+// The Cartes report is intentionally a "month X vs X-1 vs X-12" snapshot, so
+// only a reference-month picker is surfaced here — a generic date-range
+// selector has no meaning on this sub-report (see issue #101).
 import { useTranslation } from "react-i18next";
 import { Link } from "react-router-dom";
 import { ArrowLeft } from "lucide-react";
-import PeriodSelector from "../components/dashboard/PeriodSelector";
 import CompareReferenceMonthPicker from "../components/reports/CompareReferenceMonthPicker";
 import KpiCard from "../components/reports/cards/KpiCard";
 import IncomeExpenseOverlayChart from "../components/reports/cards/IncomeExpenseOverlayChart";
@@ -12,19 +14,7 @@ import { useCartes } from "../hooks/useCartes";
 
 export default function ReportsCartesPage() {
   const { t } = useTranslation();
-  const {
-    year,
-    month,
-    snapshot,
-    isLoading,
-    error,
-    setReferencePeriod,
-    period,
-    setPeriod,
-    from,
-    to,
-    setCustomDates,
-  } = useCartes();
+  const { year, month, snapshot, isLoading, error, setReferencePeriod } = useCartes();
 
   const preserveSearch = typeof window !== "undefined" ? window.location.search : "";
 
@@ -41,14 +31,7 @@ export default function ReportsCartesPage() {
         <h1 className="text-2xl font-bold">{t("reports.hub.cartes")}</h1>
       </div>
 
-      <div className="flex flex-col sm:flex-row sm:items-center justify-between gap-3 mb-6 flex-wrap">
-        <PeriodSelector
-          value={period}
-          onChange={setPeriod}
-          customDateFrom={from}
-          customDateTo={to}
-          onCustomDateChange={setCustomDates}
-        />
+      <div className="flex flex-col sm:flex-row sm:items-center justify-end gap-3 mb-6 flex-wrap">
         <CompareReferenceMonthPicker year={year} month={month} onChange={setReferencePeriod} />
       </div>
 
@@ -95,6 +78,7 @@ export default function ReportsCartesPage() {
               kpi={snapshot.kpis.savingsRate}
               format="percent"
               deltaIsBadWhenUp={false}
+              tooltip={t("reports.cartes.savingsRateTooltip")}
             />
           </section>
 
diff --git a/src/services/reportService.cartes.test.ts b/src/services/reportService.cartes.test.ts
index 4e459fb..3d31032 100644
--- a/src/services/reportService.cartes.test.ts
+++ b/src/services/reportService.cartes.test.ts
@@ -59,7 +59,8 @@ describe("getCartesSnapshot", () => {
     expect(snapshot.kpis.income.current).toBe(0);
     expect(snapshot.kpis.expenses.current).toBe(0);
     expect(snapshot.kpis.net.current).toBe(0);
-    expect(snapshot.kpis.savingsRate.current).toBe(0);
+    // Savings rate is null (renders as "—") when income is zero.
+    expect(snapshot.kpis.savingsRate.current).toBeNull();
     expect(snapshot.kpis.income.sparkline).toHaveLength(13);
     expect(snapshot.flow12Months).toHaveLength(12);
     expect(snapshot.topMoversUp).toHaveLength(0);
@@ -118,7 +119,7 @@ describe("getCartesSnapshot", () => {
     expect(snapshot.kpis.income.deltaYoYAbs).toBeNull();
   });
 
-  it("savings rate stays at 0 when income is zero (no division by zero)", async () => {
+  it("savings rate is null when income is zero (no division by zero, renders as — in UI)", async () => {
     routeSelect([
       {
         match: "strftime('%Y-%m', date)",
@@ -129,7 +130,7 @@ describe("getCartesSnapshot", () => {
     ]);
 
     const snapshot = await getCartesSnapshot(2026, 3);
-    expect(snapshot.kpis.savingsRate.current).toBe(0);
+    expect(snapshot.kpis.savingsRate.current).toBeNull();
     expect(snapshot.kpis.income.current).toBe(0);
     expect(snapshot.kpis.expenses.current).toBe(500);
     expect(snapshot.kpis.net.current).toBe(-500);
diff --git a/src/services/reportService.ts b/src/services/reportService.ts
index efe9632..5ca02a7 100644
--- a/src/services/reportService.ts
+++ b/src/services/reportService.ts
@@ -605,10 +605,10 @@ function monthKey(year: number, month: number): string {
 }
 
 function extractDelta(
-  current: number,
+  current: number | null,
   previous: number | null,
 ): { abs: number | null; pct: number | null } {
-  if (previous === null) return { abs: null, pct: null };
+  if (current === null || previous === null) return { abs: null, pct: null };
   const abs = current - previous;
   const pct = previous === 0 ? null : (abs / previous) * 100;
   return { abs, pct };
@@ -616,7 +616,7 @@ function extractDelta(
 
 function buildKpi(
   sparkline: CartesSparklinePoint[],
-  current: number,
+  current: number | null,
   previousMonth: number | null,
   previousYear: number | null,
 ): CartesKpi {
@@ -770,7 +770,9 @@ export async function getCartesSnapshot(
   const refIncome = refRow?.income ?? 0;
   const refExpenses = refRow?.expenses ?? 0;
   const refNet = refIncome - refExpenses;
-  const refSavings = refIncome > 0 ? (refNet / refIncome) * 100 : 0;
+  // Savings rate is undefined when income is zero — expose as null rather than
+  // rendering a misleading "0 %" in the UI.
+  const refSavings = refIncome > 0 ? (refNet / refIncome) * 100 : null;
 
   const momRow = flowByMonth.get(momKey);
   const momIncome = momRow ? momRow.income : null;
@@ -852,7 +854,9 @@ export async function getCartesSnapshot(
   const historicalAverage = historicalYears.length
     ? historicalYears.reduce((sum, r) => sum + r.amount, 0) / historicalYears.length
     : null;
-  const referenceAmount = expensesKpi.current;
+  // `refExpenses` is always a concrete number (never null) — unlike
+  // `savingsKpi.current` which is nullable when income is zero.
+  const referenceAmount = refExpenses;
   const deviationPct =
     historicalAverage !== null && historicalAverage > 0
       ? ((referenceAmount - historicalAverage) / historicalAverage) * 100
diff --git a/src/shared/types/index.ts b/src/shared/types/index.ts
index f315f43..4602133 100644
--- a/src/shared/types/index.ts
+++ b/src/shared/types/index.ts
@@ -368,7 +368,9 @@ export interface CartesSparklinePoint {
 }
 
 export interface CartesKpi {
-  current: number;
+  // `current` is nullable for ratio-style KPIs (e.g. savings rate) when the
+  // denominator is zero and the value is genuinely undefined rather than 0.
+  current: number | null;
   previousMonth: number | null;
   previousYear: number | null;
   deltaMoMAbs: number | null;