diff --git a/format.h b/format.h
index 6cffc105..20aac4cb 100644
--- a/format.h
+++ b/format.h
@@ -1832,7 +1832,7 @@ class FormatBuf : public std::basic_streambuf<Char> {
     return ch;
   }
 
-  size_t size() {
+  size_t size() const {
     return this->pptr() - start_;
   }
 };