diff --git a/db/write_batch.cc b/db/write_batch.cc
index 40eed2e..23eb00f 100644
--- a/db/write_batch.cc
+++ b/db/write_batch.cc
@@ -39,7 +39,7 @@ void WriteBatch::Clear() {
   rep_.resize(kHeader);
 }
 
-size_t WriteBatch::ApproximateSize() {
+size_t WriteBatch::ApproximateSize() const {
   return rep_.size();
 }
 
diff --git a/include/leveldb/write_batch.h b/include/leveldb/write_batch.h
index 9386ace..9b319f0 100644
--- a/include/leveldb/write_batch.h
+++ b/include/leveldb/write_batch.h
@@ -52,7 +52,7 @@ class LEVELDB_EXPORT WriteBatch {
   //
   // This number is tied to implementation details, and may change across
   // releases. It is intended for LevelDB usage metrics.
-  size_t ApproximateSize();
+  size_t ApproximateSize() const;
 
   // Copies the operations in "source" to this batch.
   //