diff --git a/standard_precip/base_sp.py b/standard_precip/base_sp.py
index 3559db9..cf06dfd 100644
--- a/standard_precip/base_sp.py
+++ b/standard_precip/base_sp.py
@@ -213,7 +213,7 @@ def calculate(self, df: pd.DataFrame, date_col: str, precip_cols: list, freq: st
             if freq == "D":
                 self._df_copy[self.freq_col] = self._df_copy[date_col].dt.dayofyear
             elif freq == "W":
-                self._df_copy[self.freq_col] = self._df_copy[date_col].dt.week
+                self._df_copy[self.freq_col] = self._df_copy[date_col].dt.isocalendar().week
             elif freq == "M":
                 self._df_copy[self.freq_col] = self._df_copy[date_col].dt.month
             else: