diff --git a/addons/hr_holidays/static/src/views/calendar/year/calendar_year_renderer.js b/addons/hr_holidays/static/src/views/calendar/year/calendar_year_renderer.js
index cdfd919778e..75985607dd2 100644
--- a/addons/hr_holidays/static/src/views/calendar/year/calendar_year_renderer.js
+++ b/addons/hr_holidays/static/src/views/calendar/year/calendar_year_renderer.js
@@ -44,7 +44,6 @@ export class TimeOffCalendarYearRenderer extends CalendarYearRenderer {
             weekNumbers: true,
             weekNumbersWithinDays: false,
             weekLabel: _t("Week"),
-            firstDay: 0,
         });
     }
 
diff --git a/addons/web/static/src/views/calendar/calendar_common/calendar_common_renderer.js b/addons/web/static/src/views/calendar/calendar_common/calendar_common_renderer.js
index 8e89fdd201c..aced367c18f 100644
--- a/addons/web/static/src/views/calendar/calendar_common/calendar_common_renderer.js
+++ b/addons/web/static/src/views/calendar/calendar_common/calendar_common_renderer.js
@@ -9,6 +9,7 @@ import { getColor } from "../colors";
 import { useCalendarPopover, useClickHandler, useFullCalendar } from "../hooks";
 import { CalendarCommonPopover } from "./calendar_common_popover";
 import { browser } from "@web/core/browser/browser";
+import { getWeekNumber } from "../utils";
 
 import { Component, onMounted, useEffect } from "@odoo/owl";
 
@@ -112,7 +113,7 @@ export class CalendarCommonRenderer extends Component {
             unselectAuto: false,
             weekLabel: this.props.model.scale === "month" && this.env.isSmall ? "" : _t("Week"),
             weekends: this.props.isWeekendVisible,
-            weekNumberCalculation: "ISO",
+            weekNumberCalculation: (date) => getWeekNumber(date, this.props.model.firstDayOfWeek),
             weekNumbers: true,
             weekNumbersWithinDays: !this.env.isSmall,
             windowResize: this.onWindowResizeDebounced,
diff --git a/addons/web/static/src/views/calendar/calendar_controller.js b/addons/web/static/src/views/calendar/calendar_controller.js
index fc14fdc0f38..107b291fcbe 100644
--- a/addons/web/static/src/views/calendar/calendar_controller.js
+++ b/addons/web/static/src/views/calendar/calendar_controller.js
@@ -19,6 +19,7 @@ import { useSearchBarToggler } from "@web/search/search_bar/search_bar_toggler";
 import { ViewScaleSelector } from "@web/views/view_components/view_scale_selector";
 import { CogMenu } from "@web/search/cog_menu/cog_menu";
 import { browser } from "@web/core/browser/browser";
+import { getWeekNumber } from "./utils";
 
 import { Component, useState } from "@odoo/owl";
 
@@ -137,7 +138,7 @@ export class CalendarController extends Component {
     }
 
     get currentWeek() {
-        return this.date.toFormat("W");
+        return getWeekNumber(this.model.rangeStart);
     }
 
     get rendererProps() {
diff --git a/addons/web/static/src/views/calendar/calendar_year/calendar_year_renderer.js b/addons/web/static/src/views/calendar/calendar_year/calendar_year_renderer.js
index 7673953d6d5..7ba34717b2f 100644
--- a/addons/web/static/src/views/calendar/calendar_year/calendar_year_renderer.js
+++ b/addons/web/static/src/views/calendar/calendar_year/calendar_year_renderer.js
@@ -5,6 +5,7 @@ import { useDebounced } from "@web/core/utils/timing";
 import { getColor } from "../colors";
 import { useCalendarPopover, useFullCalendar } from "../hooks";
 import { CalendarYearPopover } from "./calendar_year_popover";
+import { getWeekNumber } from "../utils";
 
 import { Component, useEffect, useRef } from "@odoo/owl";
 
@@ -58,7 +59,7 @@ export class CalendarYearRenderer extends Component {
             timeZone: luxon.Settings.defaultZone.name,
             titleFormat: { month: "long", year: "numeric" },
             unselectAuto: false,
-            weekNumberCalculation: "ISO",
+            weekNumberCalculation: (date) => getWeekNumber(date, this.props.model.firstDayOfWeek),
             weekNumbers: false,
             windowResize: this.onWindowResizeDebounced,
         };
diff --git a/addons/web/static/src/views/calendar/utils.js b/addons/web/static/src/views/calendar/utils.js
index 7378ddf104b..a22b3c7a53c 100644
--- a/addons/web/static/src/views/calendar/utils.js
+++ b/addons/web/static/src/views/calendar/utils.js
@@ -12,3 +12,33 @@ export function getFormattedDateSpan(start, end) {
             : start.toFormat("DDD") + " - " + end.toFormat("DDD");
     }
 }
+
+/**
+ * Get the week number of a given date.
+ * Returns the ISO week number of the Monday nearest to the first day of the week.
+ *
+ * @param {Date | luxon.DateTime} date
+ * @param {number} first day of the week (optional)
+ * @returns {number} week number
+ */
+export function getWeekNumber(date, firstDay) {
+    if (!date.isLuxonDateTime) {
+        date = luxon.DateTime.fromJSDate(date);
+    }
+    if (Number.isInteger(firstDay)) { // go to start of week
+        date = date.minus({ days: (date.weekday + 7 - firstDay) % 7 });
+    } else {
+        firstDay = date.weekday;
+    }
+    // go to nearest Monday, up to 3 days back- or forwards
+    date = firstDay > 1 && firstDay < 5 // if firstDay after Mon & before Fri
+        ? date.minus({ days: (date.weekday + 6) % 7 }) // then go back 1-3 days
+        : date.plus({ days: (8 - date.weekday) % 7 }); // else go forwards 0-3 days
+    date = date.plus({ days: 6 }); // go to last weekday of ISO week
+    const jan4 = luxon.DateTime.local(date.year, 1, 4);
+    // count from previous year if week falls before Jan 4
+    const diffDays = date < jan4
+        ? date.diff(jan4.minus({ years: 1 }), 'day').days
+        : date.diff(jan4, 'day').days;
+    return Math.trunc(diffDays / 7) + 1;
+}
diff --git a/addons/web/static/tests/views/calendar/calendar_view_tests.js b/addons/web/static/tests/views/calendar/calendar_view_tests.js
index 3076a7c764b..8534dc20385 100644
--- a/addons/web/static/tests/views/calendar/calendar_view_tests.js
+++ b/addons/web/static/tests/views/calendar/calendar_view_tests.js
@@ -1241,8 +1241,8 @@ QUnit.module("Views", ({ beforeEach }) => {
     });
 
     QUnit.test(`week numbering`, async (assert) => {
-        // The week is now calculated by FullCalendar (ISO week). If it's start a sunday it
-        // returns the week of the monday.
+        // Using ISO week calculation, get the ISO week number of
+        // the Monday nearest to the start of the week.
 
         patchWithCleanup(localization, { weekStart: 7 });
 
@@ -4214,7 +4214,7 @@ QUnit.module("Views", ({ beforeEach }) => {
         );
         assert.strictEqual(
             target.querySelector(".fc-head .fc-week-number").textContent,
-            "Week 36",
+            "Week 37",
             "The number of the week should be correct"
         );
     });