diff --git a/cores/esp32/WString.h b/cores/esp32/WString.h
index 66d7f663009..c83184f56b7 100644
--- a/cores/esp32/WString.h
+++ b/cores/esp32/WString.h
@@ -260,6 +260,7 @@ class String
     // parsing/conversion
     long toInt(void) const;
     float toFloat(void) const;
+    double toDouble(void) const;
 
 protected:
     char *buffer;	        // the actual char array