diff --git a/format.h b/format.h
index d384cfef..6b6951ac 100644
--- a/format.h
+++ b/format.h
@@ -712,24 +712,23 @@ inline unsigned count_digits(uint32_t n) {
 // Formats a decimal unsigned integer value writing into buffer.
 template <typename UInt, typename Char>
 inline void format_decimal(Char *buffer, UInt value, unsigned num_digits) {
-  --num_digits;
+  buffer += num_digits;
   while (value >= 100) {
     // Integer division is slow so do it for a group of two digits instead
     // of for every digit. The idea comes from the talk by Alexandrescu
     // "Three Optimization Tips for C++". See speed-test for a comparison.
     unsigned index = (value % 100) * 2;
     value /= 100;
-    buffer[num_digits] = Data::DIGITS[index + 1];
-    buffer[num_digits - 1] = Data::DIGITS[index];
-    num_digits -= 2;
+    *--buffer = Data::DIGITS[index + 1];
+    *--buffer = Data::DIGITS[index];
   }
   if (value < 10) {
-    *buffer = static_cast<char>('0' + value);
+    *--buffer = static_cast<char>('0' + value);
     return;
   }
   unsigned index = static_cast<unsigned>(value * 2);
-  buffer[1] = Data::DIGITS[index + 1];
-  buffer[0] = Data::DIGITS[index];
+  *--buffer = Data::DIGITS[index + 1];
+  *--buffer = Data::DIGITS[index];
 }
 
 #ifndef _WIN32