diff --git a/src/bundledb/writer.rs b/src/bundledb/writer.rs
index 728c3c5..ae332fd 100644
--- a/src/bundledb/writer.rs
+++ b/src/bundledb/writer.rs
@@ -135,13 +135,13 @@ impl BundleWriter {
         Ok(StoredBundle { path: path, info: info })
     }
 
-    #[inline]
-    pub fn size(&self) -> usize {
-        self.data.len()
-    }
-
     #[inline]
     pub fn raw_size(&self) -> usize {
         self.raw_size
     }
+
+    #[inline]
+    pub fn estimate_final_size(&self) -> usize {
+        self.data.len() + self.chunk_count * 20 + 500
+    }
 }
diff --git a/src/repository/basic_io.rs b/src/repository/basic_io.rs
index ec316ee..78383b7 100644
--- a/src/repository/basic_io.rs
+++ b/src/repository/basic_io.rs
@@ -142,7 +142,7 @@ impl Repository {
                 BundleMode::Meta => &mut self.meta_bundle
             };
             if let Some(ref writer) = *writer {
-                (writer.size(), writer.raw_size())
+                (writer.estimate_final_size(), writer.raw_size())
             } else {
                 return Ok(())
             }