diff --git a/src/double-conversion.cc b/src/double-conversion.cc
index a79fe92..febba6c 100644
--- a/src/double-conversion.cc
+++ b/src/double-conversion.cc
@@ -577,7 +577,7 @@
     const char* input,
     int length,
     int* processed_characters_count,
-    bool read_as_double) {
+    bool read_as_double) const {
   const char* current = input;
   const char* end = input + length;
 
diff --git a/src/double-conversion.h b/src/double-conversion.h
index f98edae..1c3387d 100644
--- a/src/double-conversion.h
+++ b/src/double-conversion.h
@@ -502,7 +502,7 @@
   // in the 'processed_characters_count'. Trailing junk is never included.
   double StringToDouble(const char* buffer,
                         int length,
-                        int* processed_characters_count) {
+                        int* processed_characters_count) const {
     return StringToIeee(buffer, length, processed_characters_count, true);
   }
 
@@ -511,7 +511,7 @@
   // due to potential double-rounding.
   float StringToFloat(const char* buffer,
                       int length,
-                      int* processed_characters_count) {
+                      int* processed_characters_count) const {
     return static_cast<float>(StringToIeee(buffer, length,
                                            processed_characters_count, false));
   }
@@ -526,7 +526,7 @@
   double StringToIeee(const char* buffer,
                       int length,
                       int* processed_characters_count,
-                      bool read_as_double);
+                      bool read_as_double) const;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(StringToDoubleConverter);
 };