diff --git a/.gx/lastpubver b/.gx/lastpubver
index 9638fb7..2dd3e53 100644
--- a/.gx/lastpubver
+++ b/.gx/lastpubver
@@ -1 +1 @@
-3.3.0: QmbQshXLNcCPRUGZv4sBGxnZNAHREA6MKeomkwihNXPZWP
+3.4.0: QmaRb5yNXKonhbkpNxNawoydk4N6es6b4fPj19sjEKsh5D
diff --git a/autobatch/autobatch.go b/autobatch/autobatch.go
index cb9c5fa..64d68bd 100644
--- a/autobatch/autobatch.go
+++ b/autobatch/autobatch.go
@@ -88,6 +88,16 @@ func (d *Datastore) Has(k ds.Key) (bool, error) {
 	return d.child.Has(k)
 }
 
+// GetSize implements Datastore.GetSize
+func (d *Datastore) GetSize(k ds.Key) (int, error) {
+	v, ok := d.buffer[k]
+	if ok {
+		return len(v), nil
+	}
+
+	return d.child.GetSize(k)
+}
+
 // Query performs a query
 func (d *Datastore) Query(q dsq.Query) (dsq.Results, error) {
 	err := d.Flush()
diff --git a/basic_ds.go b/basic_ds.go
index dbcb999..2cb7568 100644
--- a/basic_ds.go
+++ b/basic_ds.go
@@ -42,6 +42,14 @@ func (d *MapDatastore) Has(key Key) (exists bool, err error) {
 	return found, nil
 }
 
+// GetSize implements Datastore.GetSize
+func (d *MapDatastore) GetSize(key Key) (size int, err error) {
+	if v, found := d.values[key]; found {
+		return len(v), nil
+	}
+	return -1, ErrNotFound
+}
+
 // Delete implements Datastore.Delete
 func (d *MapDatastore) Delete(key Key) (err error) {
 	if _, found := d.values[key]; !found {
@@ -95,6 +103,11 @@ func (d *NullDatastore) Has(key Key) (exists bool, err error) {
 	return false, nil
 }
 
+// Has implements Datastore.GetSize
+func (d *NullDatastore) GetSize(key Key) (size int, err error) {
+	return -1, ErrNotFound
+}
+
 // Delete implements Datastore.Delete
 func (d *NullDatastore) Delete(key Key) (err error) {
 	return nil
@@ -158,6 +171,12 @@ func (d *LogDatastore) Has(key Key) (exists bool, err error) {
 	return d.child.Has(key)
 }
 
+// GetSize implements Datastore.GetSize
+func (d *LogDatastore) GetSize(key Key) (size int, err error) {
+	log.Printf("%s: GetSize %s\n", d.Name, key)
+	return d.child.GetSize(key)
+}
+
 // Delete implements Datastore.Delete
 func (d *LogDatastore) Delete(key Key) (err error) {
 	log.Printf("%s: Delete %s\n", d.Name, key)
diff --git a/datastore.go b/datastore.go
index 63263ed..c6558ef 100644
--- a/datastore.go
+++ b/datastore.go
@@ -52,6 +52,11 @@ type Datastore interface {
 	// The default implementation is found in `GetBackedHas`.
 	Has(key Key) (exists bool, err error)
 
+	// GetSize returns the size of the `value` named by `key`.
+	// In some contexts, it may be much cheaper to only get the size of the
+	// value rather than retrieving the value itself.
+	GetSize(key Key) (size int, err error)
+
 	// Delete removes the value for given `key`.
 	Delete(key Key) error
 
@@ -197,6 +202,20 @@ func GetBackedHas(ds Datastore, key Key) (bool, error) {
 	}
 }
 
+// GetBackedSize provides a default Datastore.GetSize implementation.
+// It exists so Datastore.GetSize implementations can use it, like so:
+//
+// func (*d SomeDatastore) GetSize(key Key) (size int, err error) {
+//   return GetBackedSize(d, key)
+// }
+func GetBackedSize(ds Datastore, key Key) (int, error) {
+	value, err := ds.Get(key)
+	if err == nil {
+		return len(value), nil
+	}
+	return -1, err
+}
+
 type Batch interface {
 	Put(key Key, val []byte) error
 
diff --git a/delayed/delayed.go b/delayed/delayed.go
index da02a11..a8bfb18 100644
--- a/delayed/delayed.go
+++ b/delayed/delayed.go
@@ -33,6 +33,11 @@ func (dds *delayed) Has(key ds.Key) (exists bool, err error) {
 	return dds.ds.Has(key)
 }
 
+func (dds *delayed) GetSize(key ds.Key) (size int, err error) {
+	dds.delay.Wait()
+	return dds.ds.GetSize(key)
+}
+
 func (dds *delayed) Delete(key ds.Key) (err error) {
 	dds.delay.Wait()
 	return dds.ds.Delete(key)
diff --git a/examples/fs.go b/examples/fs.go
index ea59d7e..94cefab 100644
--- a/examples/fs.go
+++ b/examples/fs.go
@@ -78,6 +78,10 @@ func (d *Datastore) Has(key ds.Key) (exists bool, err error) {
 	return ds.GetBackedHas(d, key)
 }
 
+func (d *Datastore) GetSize(key ds.Key) (size int, err error) {
+	return ds.GetBackedSize(d, key)
+}
+
 // Delete removes the value for given key
 func (d *Datastore) Delete(key ds.Key) (err error) {
 	fn := d.KeyFilename(key)
diff --git a/failstore/failstore.go b/failstore/failstore.go
index 222c3be..feee789 100644
--- a/failstore/failstore.go
+++ b/failstore/failstore.go
@@ -56,6 +56,16 @@ func (d *Failstore) Has(k ds.Key) (bool, error) {
 	return d.child.Has(k)
 }
 
+// GetSize returns the size of the value in the datastore, if present.
+func (d *Failstore) GetSize(k ds.Key) (int, error) {
+	err := d.errfunc("getsize")
+	if err != nil {
+		return -1, err
+	}
+
+	return d.child.GetSize(k)
+}
+
 // Delete removes a key/value from the datastore.
 func (d *Failstore) Delete(k ds.Key) error {
 	err := d.errfunc("delete")
diff --git a/keytransform/keytransform.go b/keytransform/keytransform.go
index 8fb7c41..be7db57 100644
--- a/keytransform/keytransform.go
+++ b/keytransform/keytransform.go
@@ -48,6 +48,12 @@ func (d *ktds) Has(key ds.Key) (exists bool, err error) {
 	return d.child.Has(d.ConvertKey(key))
 }
 
+// GetSize returns the size of the value named by the given key, transforming
+// the key first.
+func (d *ktds) GetSize(key ds.Key) (size int, err error) {
+	return d.child.GetSize(d.ConvertKey(key))
+}
+
 // Delete removes the value for given key
 func (d *ktds) Delete(key ds.Key) (err error) {
 	return d.child.Delete(d.ConvertKey(key))
diff --git a/mount/mount.go b/mount/mount.go
index d6aa920..8f2a89e 100644
--- a/mount/mount.go
+++ b/mount/mount.go
@@ -106,6 +106,14 @@ func (d *Datastore) Has(key ds.Key) (exists bool, err error) {
 	return cds.Has(k)
 }
 
+func (d *Datastore) GetSize(key ds.Key) (size int, err error) {
+	cds, _, k := d.lookup(key)
+	if cds == nil {
+		return -1, ds.ErrNotFound
+	}
+	return cds.GetSize(k)
+}
+
 func (d *Datastore) Delete(key ds.Key) error {
 	cds, _, k := d.lookup(key)
 	if cds == nil {
diff --git a/package.json b/package.json
index 64f2ea2..d0b8951 100644
--- a/package.json
+++ b/package.json
@@ -37,6 +37,6 @@
   "license": "MIT",
   "name": "go-datastore",
   "releaseCmd": "git commit -a -m \"gx publish $VERSION\"",
-  "version": "3.3.0"
+  "version": "3.4.0"
 }
 
diff --git a/retrystore/retrystore.go b/retrystore/retrystore.go
index 7b3cd0e..a03daa8 100644
--- a/retrystore/retrystore.go
+++ b/retrystore/retrystore.go
@@ -83,3 +83,14 @@ func (d *Datastore) Has(k ds.Key) (bool, error) {
 	})
 	return has, err
 }
+
+// GetSize returns the size of the value in the datastore, if present.
+func (d *Datastore) GetSize(k ds.Key) (int, error) {
+	var size int
+	err := d.runOp(func() error {
+		var err error
+		size, err = d.Batching.GetSize(k)
+		return err
+	})
+	return size, err
+}
diff --git a/sync/sync.go b/sync/sync.go
index 36fd0f7..2218483 100644
--- a/sync/sync.go
+++ b/sync/sync.go
@@ -51,6 +51,13 @@ func (d *MutexDatastore) Has(key ds.Key) (exists bool, err error) {
 	return d.child.Has(key)
 }
 
+// GetSize implements Datastore.GetSize
+func (d *MutexDatastore) GetSize(key ds.Key) (size int, err error) {
+	d.RLock()
+	defer d.RUnlock()
+	return d.child.GetSize(key)
+}
+
 // Delete implements Datastore.Delete
 func (d *MutexDatastore) Delete(key ds.Key) (err error) {
 	d.Lock()
diff --git a/test/basic_tests.go b/test/basic_tests.go
index 4059756..4f87a52 100644
--- a/test/basic_tests.go
+++ b/test/basic_tests.go
@@ -29,6 +29,14 @@ func SubtestBasicPutGet(t *testing.T, ds dstore.Datastore) {
 		t.Fatal("should have key foo, has returned false")
 	}
 
+	size, err := ds.GetSize(k)
+	if err != nil {
+		t.Fatal("error getting size after put: ", err)
+	}
+	if size != len(val) {
+		t.Fatalf("incorrect size: expected %d, got %d", len(val), size)
+	}
+
 	out, err := ds.Get(k)
 	if err != nil {
 		t.Fatal("error getting value after put: ", err)
@@ -47,6 +55,14 @@ func SubtestBasicPutGet(t *testing.T, ds dstore.Datastore) {
 		t.Fatal("should have key foo, has returned false")
 	}
 
+	size, err = ds.GetSize(k)
+	if err != nil {
+		t.Fatal("error getting size after get: ", err)
+	}
+	if size != len(val) {
+		t.Fatalf("incorrect size: expected %d, got %d", len(val), size)
+	}
+
 	err = ds.Delete(k)
 	if err != nil {
 		t.Fatal("error calling delete: ", err)
@@ -60,6 +76,18 @@ func SubtestBasicPutGet(t *testing.T, ds dstore.Datastore) {
 	if have {
 		t.Fatal("should not have key foo, has returned true")
 	}
+
+	size, err = ds.GetSize(k)
+	switch err {
+	case dstore.ErrNotFound:
+	case nil:
+		t.Fatal("expected error getting size after delete")
+	default:
+		t.Fatal("wrong error getting size after delete: ", err)
+	}
+	if size != -1 {
+		t.Fatal("expected missing size to be -1")
+	}
 }
 
 func SubtestNotFounds(t *testing.T, ds dstore.Datastore) {
@@ -81,6 +109,18 @@ func SubtestNotFounds(t *testing.T, ds dstore.Datastore) {
 	if have {
 		t.Fatal("has returned true for key we don't have")
 	}
+
+	size, err := ds.GetSize(badk)
+	switch err {
+	case dstore.ErrNotFound:
+	case nil:
+		t.Fatal("expected error getting size after delete")
+	default:
+		t.Fatal("wrong error getting size after delete: ", err)
+	}
+	if size != -1 {
+		t.Fatal("expected missing size to be -1")
+	}
 }
 
 func SubtestManyKeysAndQuery(t *testing.T, ds dstore.Datastore) {