diff --git a/include/modules/clock.hpp b/include/modules/clock.hpp
index f86247a0..c3548063 100644
--- a/include/modules/clock.hpp
+++ b/include/modules/clock.hpp
@@ -48,8 +48,9 @@ class Clock final : public ALabel {
   std::string cldYearCached_;          // calendar Year mode. Cached calendar
   date::year_month cldMonShift_;       // calendar Month mode. Cached ym
   std::string cldMonCached_;           // calendar Month mode. Cached calendar
-  date::day cldBaseDay_{0};  // calendar Cached day. Is used when today is changing(midnight)
-  std::string cldText_{""};  // calendar text to print
+  date::day cldBaseDay_{0};      // calendar Cached day. Is used when today is changing(midnight)
+  std::string cldText_{""};      // calendar text to print
+  bool iso8601Calendar_{false};  // whether the calendar is in ISO8601
   CldMode cldMode_{CldMode::MONTH};
   auto get_calendar(const date::year_month_day& today, const date::year_month_day& ymd,
                     const date::time_zone* tz) -> const std::string;
diff --git a/man/waybar-clock.5.scd b/man/waybar-clock.5.scd
index 9fbad875..b4b5d5b7 100644
--- a/man/waybar-clock.5.scd
+++ b/man/waybar-clock.5.scd
@@ -132,6 +132,12 @@ View all valid format options in *strftime(3)* or have a look https://en.cpprefe
 :[ 1
 :[ Value to scroll months/years forward/backward. Can be negative. Is
    configured under *on-scroll* option
+|[ *iso8601*
+:[ bool
+:[ false
+:[ When enabled, the calendar follows the ISO 8601 standard: weeks begin on
+   Monday, and the first week of the year is numbered 1. The default week format is
+   '{:%V}'.
 
 3. Addressed by *clock: calendar: format*
 [- *Option*
diff --git a/src/modules/clock.cpp b/src/modules/clock.cpp
index f3ebcdda..2329f97f 100644
--- a/src/modules/clock.cpp
+++ b/src/modules/clock.cpp
@@ -74,6 +74,11 @@ waybar::modules::Clock::Clock(const std::string& id, const Json::Value& config)
             "using instead",
             cfgMode);
     }
+
+    if (config_[kCldPlaceholder]["iso8601"].isBool()) {
+      iso8601Calendar_ = config_[kCldPlaceholder]["iso8601"].asBool();
+    }
+
     if (config_[kCldPlaceholder]["weeks-pos"].isString()) {
       if (config_[kCldPlaceholder]["weeks-pos"].asString() == "left") cldWPos_ = WS::LEFT;
       if (config_[kCldPlaceholder]["weeks-pos"].asString() == "right") cldWPos_ = WS::RIGHT;
@@ -98,16 +103,20 @@ waybar::modules::Clock::Clock(const std::string& id, const Json::Value& config)
     } else
       fmtMap_.insert({3, "{}"});
     if (config_[kCldPlaceholder]["format"]["weeks"].isString() && cldWPos_ != WS::HIDDEN) {
+      const auto defaultFmt =
+          iso8601Calendar_ ? "{:%V}" : ((first_day_of_week() == Monday) ? "{:%W}" : "{:%U}");
       fmtMap_.insert({4, std::regex_replace(config_[kCldPlaceholder]["format"]["weeks"].asString(),
-                                            std::regex("\\{\\}"),
-                                            (first_day_of_week() == Monday) ? "{:%W}" : "{:%U}")});
+                                            std::regex("\\{\\}"), defaultFmt)});
       Glib::ustring tmp{std::regex_replace(fmtMap_[4], std::regex("</?[^>]+>|\\{.*\\}"), "")};
       cldWnLen_ += tmp.size();
     } else {
-      if (cldWPos_ != WS::HIDDEN)
-        fmtMap_.insert({4, (first_day_of_week() == Monday) ? "{:%W}" : "{:%U}"});
-      else
+      if (cldWPos_ != WS::HIDDEN) {
+        const auto defaultFmt =
+            iso8601Calendar_ ? "{:%V}" : ((first_day_of_week() == Monday) ? "{:%W}" : "{:%U}");
+        fmtMap_.insert({4, defaultFmt});
+      } else {
         cldWnLen_ = 0;
+      }
     }
     if (config_[kCldPlaceholder]["mode-mon-col"].isInt()) {
       cldMonCols_ = config_[kCldPlaceholder]["mode-mon-col"].asInt();
@@ -218,9 +227,11 @@ const unsigned cldRowsInMonth(const year_month& ym, const weekday& firstdow) {
 
 auto cldGetWeekForLine(const year_month& ym, const weekday& firstdow, const unsigned line)
     -> const year_month_weekday {
-  unsigned index{line - 2};
-  if (weekday{ym / 1} == firstdow) ++index;
-  return ym / firstdow[index];
+  const unsigned idx = line - 2;
+  const std::chrono::weekday_indexed indexed_first_day_of_week =
+      weekday{ym / 1} == firstdow ? firstdow[idx + 1] : firstdow[idx];
+
+  return ym / indexed_first_day_of_week;
 }
 
 auto getCalendarLine(const year_month_day& currDate, const year_month ym, const unsigned line,
@@ -279,7 +290,7 @@ auto getCalendarLine(const year_month_day& currDate, const year_month ym, const
     }
     // Print non-first week
     default: {
-      auto ymdTmp{cldGetWeekForLine(ym, firstdow, line)};
+      const auto ymdTmp{cldGetWeekForLine(ym, firstdow, line)};
       if (ymdTmp.ok()) {
         auto d{year_month_day{ymdTmp}.day()};
         const auto dlast{(ym / last).day()};
@@ -370,8 +381,9 @@ auto waybar::modules::Clock::get_calendar(const year_month_day& today, const yea
                                   : static_cast<const zoned_seconds&&>(zoned_seconds{
                                         tz, local_days{cldGetWeekForLine(ymTmp, firstdow, line)}})))
                    << ' ';
-              } else
+              } else {
                 os << pads;
+              }
             }
           }
 
@@ -495,6 +507,9 @@ using deleting_unique_ptr = std::unique_ptr<T, deleter_from_fn<fn>>;
 
 // Computations done similarly to Linux cal utility.
 auto waybar::modules::Clock::first_day_of_week() -> weekday {
+  if (iso8601Calendar_) {
+    return Monday;
+  }
 #ifdef HAVE_LANGINFO_1STDAY
   deleting_unique_ptr<std::remove_pointer<locale_t>::type, freelocale> posix_locale{
       newlocale(LC_ALL, m_locale_.name().c_str(), nullptr)};