diff --git a/src/lib.rs b/src/lib.rs
index ce81174..87b7d82 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -63,6 +63,17 @@ impl<B: StoreBackend> Store<B> {
                       .transpose())
     }
 
+    /// how many bytes are currently used
+    ///
+    /// equally, offset of free space
+    pub fn get_bytes_used(&self) -> Result<usize, ReadError> {
+        let mut iter = Iter::new(self.backend.data().as_ref());
+        while let Some(result) = iter.next() {
+            let _ = result?;
+        }
+        Ok(iter.offset)
+    }
+
     unsafe fn append_at(&mut self, mut offset: usize,
                         key: &[u8], value: &[u8]) -> Result<usize, WriteError<B::Error>> {
         let record_size = 4 + key.len() + 1 + value.len();
@@ -122,16 +133,8 @@ impl<B: StoreBackend> Store<B> {
     }
 
     fn append(&mut self, key: &str, value: &[u8]) -> Result<(), Error<B::Error>> {
-        let free_offset = {
-            let mut iter = Iter::new(self.backend.data().as_ref());
-            while let Some(result) = iter.next() {
-                let _ = result?;
-            }
-            iter.offset
-        };
-
+        let free_offset = self.get_bytes_used()?;
         unsafe { self.append_at(free_offset, key.as_bytes(), value)? };
-
         Ok(())
     }