diff --git a/bitcask.go b/bitcask.go
index 8d04596..940fb65 100644
--- a/bitcask.go
+++ b/bitcask.go
@@ -17,6 +17,7 @@ import (
 	"github.com/prologic/bitcask/internal"
 	"github.com/prologic/bitcask/internal/config"
 	"github.com/prologic/bitcask/internal/data"
+	"github.com/prologic/bitcask/internal/data/codec"
 	"github.com/prologic/bitcask/internal/index"
 	"github.com/prologic/bitcask/internal/metadata"
 )
@@ -101,6 +102,11 @@ func (b *Bitcask) Close() error {
 		return err
 	}
 
+	b.metadata.IndexUpToDate = true
+	if err := b.saveMetadata(); err != nil {
+		return err
+	}
+
 	for _, df := range b.datafiles {
 		if err := df.Close(); err != nil {
 			return err
@@ -172,29 +178,32 @@ func (b *Bitcask) Put(key, value []byte) error {
 	}
 
 	b.mu.Lock()
+	defer b.mu.Unlock()
 	offset, n, err := b.put(key, value)
 	if err != nil {
-		b.mu.Unlock()
 		return err
 	}
 
 	if b.config.Sync {
 		if err := b.curr.Sync(); err != nil {
-			b.mu.Unlock()
 			return err
 		}
 	}
 
+	// in case of successful `put`, IndexUpToDate will be always be false
 	if b.metadata.IndexUpToDate {
 		b.metadata.IndexUpToDate = false
-		if err := b.metadata.Save(filepath.Join(b.path, "meta.json"), b.config.FileFileModeBeforeUmask); err != nil {
+		if err := b.saveMetadata(); err != nil {
 			return err
 		}
 	}
 
+	if oldItem, found := b.trie.Search(key); found {
+		b.metadata.ReclaimableSpace += oldItem.(internal.Item).Size
+	}
+
 	item := internal.Item{FileID: b.curr.FileID(), Offset: offset, Size: n}
 	b.trie.Insert(key, item)
-	b.mu.Unlock()
 
 	return nil
 }
@@ -208,6 +217,9 @@ func (b *Bitcask) Delete(key []byte) error {
 		b.mu.Unlock()
 		return err
 	}
+	if item, found := b.trie.Search(key); found {
+		b.metadata.ReclaimableSpace += item.(internal.Item).Size + codec.MetaInfoSize + int64(len(key))
+	}
 	b.trie.Delete(key)
 	b.mu.Unlock()
 
@@ -221,7 +233,12 @@ func (b *Bitcask) DeleteAll() (err error) {
 
 	b.trie.ForEach(func(node art.Node) bool {
 		_, _, err = b.put(node.Key(), []byte{})
-		return err == nil
+		if err != nil {
+			return false
+		}
+		item, _ := b.trie.Search(node.Key())
+		b.metadata.ReclaimableSpace += item.(internal.Item).Size + codec.MetaInfoSize + int64(len(node.Key()))
+		return true
 	})
 	b.trie = art.New()
 
@@ -421,6 +438,7 @@ func (b *Bitcask) Merge() error {
 			return err
 		}
 	}
+	b.metadata.ReclaimableSpace = 0
 
 	// And finally reopen the database
 	return b.Reopen()
@@ -512,14 +530,19 @@ func (b *Bitcask) saveIndex() error {
 	if err := b.indexer.Save(b.trie, filepath.Join(b.path, tempIdx)); err != nil {
 		return err
 	}
-	err := os.Rename(filepath.Join(b.path, tempIdx), filepath.Join(b.path, "index"))
-	if err != nil {
-		return err
-	}
-	b.metadata.IndexUpToDate = true
+	return os.Rename(filepath.Join(b.path, tempIdx), filepath.Join(b.path, "index"))
+}
+
+// saveMetadata saves metadata into disk
+func (b *Bitcask) saveMetadata() error {
 	return b.metadata.Save(filepath.Join(b.path, "meta.json"), b.config.DirFileModeBeforeUmask)
 }
 
+// Reclaimable returns space that can be reclaimed
+func (b *Bitcask) Reclaimable() int64 {
+	return b.metadata.ReclaimableSpace
+}
+
 func loadDatafiles(path string, maxKeySize uint32, maxValueSize uint64, fileModeBeforeUmask os.FileMode) (datafiles map[int]data.Datafile, lastID int, err error) {
 	fns, err := internal.GetDatafiles(path)
 	if err != nil {
diff --git a/bitcask_test.go b/bitcask_test.go
index 9908963..4b419f1 100644
--- a/bitcask_test.go
+++ b/bitcask_test.go
@@ -314,6 +314,64 @@ func TestDeletedKeys(t *testing.T) {
 	})
 }
 
+func TestMetadata(t *testing.T) {
+	assert := assert.New(t)
+	testdir, err := ioutil.TempDir("", "bitcask")
+	assert.NoError(err)
+	defer os.RemoveAll(testdir)
+
+	db, err := Open(testdir)
+	assert.NoError(err)
+	err = db.Put([]byte("foo"), []byte("bar"))
+	assert.NoError(err)
+	err = db.Close()
+	assert.NoError(err)
+	db, err = Open(testdir)
+	assert.NoError(err)
+
+	t.Run("IndexUptoDateAfterCloseAndOpen", func(t *testing.T) {
+		assert.Equal(true, db.metadata.IndexUpToDate)
+	})
+	t.Run("IndexUptoDateAfterPut", func(t *testing.T) {
+		assert.NoError(db.Put([]byte("foo1"), []byte("bar1")))
+		assert.Equal(false, db.metadata.IndexUpToDate)
+	})
+	t.Run("Reclaimable", func(t *testing.T) {
+		assert.Equal(int64(0), db.Reclaimable())
+	})
+	t.Run("ReclaimableAfterNewPut", func(t *testing.T) {
+		assert.NoError(db.Put([]byte("hello"), []byte("world")))
+		assert.Equal(int64(0), db.Reclaimable())
+	})
+	t.Run("ReclaimableAfterRepeatedPut", func(t *testing.T) {
+		assert.NoError(db.Put([]byte("hello"), []byte("world")))
+		assert.Equal(int64(26), db.Reclaimable())
+	})
+	t.Run("ReclaimableAfterDelete", func(t *testing.T) {
+		assert.NoError(db.Delete([]byte("hello")))
+		assert.Equal(int64(73), db.Reclaimable())
+	})
+	t.Run("ReclaimableAfterNonExistingDelete", func(t *testing.T) {
+		assert.NoError(db.Delete([]byte("hello1")))
+		assert.Equal(int64(73), db.Reclaimable())
+	})
+	t.Run("ReclaimableAfterDeleteAll", func(t *testing.T) {
+		assert.NoError(db.DeleteAll())
+		assert.Equal(int64(158), db.Reclaimable())
+	})
+	t.Run("ReclaimableAfterMerge", func(t *testing.T) {
+		assert.NoError(db.Merge())
+		assert.Equal(int64(0), db.Reclaimable())
+	})
+	t.Run("IndexUptoDateAfterMerge", func(t *testing.T) {
+		assert.Equal(true, db.metadata.IndexUpToDate)
+	})
+	t.Run("ReclaimableAfterMergeAndDeleteAll", func(t *testing.T) {
+		assert.NoError(db.DeleteAll())
+		assert.Equal(int64(0), db.Reclaimable())
+	})
+}
+
 func TestConfigErrors(t *testing.T) {
 	assert := assert.New(t)
 
diff --git a/internal/data/codec/encoder.go b/internal/data/codec/encoder.go
index c2ce784..221d187 100644
--- a/internal/data/codec/encoder.go
+++ b/internal/data/codec/encoder.go
@@ -13,6 +13,7 @@ const (
 	keySize      = 4
 	valueSize    = 8
 	checksumSize = 4
+	MetaInfoSize = keySize + valueSize + checksumSize
 )
 
 // NewEncoder creates a streaming Entry encoder.
diff --git a/internal/metadata/metadata.go b/internal/metadata/metadata.go
index a14ed60..2d93746 100644
--- a/internal/metadata/metadata.go
+++ b/internal/metadata/metadata.go
@@ -7,7 +7,8 @@ import (
 )
 
 type MetaData struct {
-	IndexUpToDate bool `json:"index_up_to_date"`
+	IndexUpToDate    bool  `json:"index_up_to_date"`
+	ReclaimableSpace int64 `json:"reclaimable_space"`
 }
 
 func (m *MetaData) Save(path string, mode os.FileMode) error {