diff --git a/returnn/tf/layers/basic.py b/returnn/tf/layers/basic.py
index de621dd7d..5a77fac92 100644
--- a/returnn/tf/layers/basic.py
+++ b/returnn/tf/layers/basic.py
@@ -1637,42 +1637,51 @@ class LengthLayer(LayerBase):
   layer_class = "length"
 
   # noinspection PyUnusedLocal
-  def __init__(self, add_time_axis=False, dtype="int32", sparse=False, **kwargs):
+  def __init__(self, axis="T", add_time_axis=False, dtype="int32", sparse=False, **kwargs):
     """
+    :param str|DimensionTag axis:
     :param bool add_time_axis:
     :param str dtype:
     :param bool sparse:
     """
     super(LengthLayer, self).__init__(**kwargs)
     assert len(self.sources) == 1, "%s: expects one source" % self
-    out = tf.cast(self.sources[0].output.get_sequence_lengths(), dtype)
+    source = self.sources[0].output
+    axis = source.get_axis_from_description(axis, allow_int=False)
+    dim = source.dim_tags[axis]
+    self.dim_tag = dim
     if add_time_axis:
-      out = tf.expand_dims(out, axis=self.output.time_dim_axis)
-    self.output.placeholder = out
+      self.output.placeholder = tf.expand_dims(dim.dyn_size, axis=self.output.time_dim_axis)
+    else:
+      self.output.placeholder = dim.dyn_size_ext.placeholder
 
   @classmethod
-  def get_out_data_from_opts(cls, name, sources, add_time_axis=False, dtype="int32", sparse=False, **kwargs):
+  def get_out_data_from_opts(cls, name, sources, axis="T", add_time_axis=False, dtype="int32", sparse=False, **kwargs):
     """
     :param str name:
     :param list[LayerBase] sources:
+    :param str|DimensionTag axis:
     :param bool add_time_axis:
     :param str dtype:
     :param bool sparse:
     :rtype: Data
     """
+    assert len(sources) == 1
+    source = sources[0].output
+    axis = source.get_axis_from_description(axis, allow_int=False)
+    dim = source.dim_tags[axis]
     if add_time_axis:
-      shape = (1,)
-      time_dim_axis = 1
-    else:
-      shape = ()
-      time_dim_axis = None
-    return Data(
-      name="%s_length" % name,
-      shape=shape,
-      batch_dim_axis=0,
-      time_dim_axis=time_dim_axis,
-      dtype=dtype,
-      sparse=sparse, dim=None if sparse else NotSpecified)
+      assert dim.dyn_size_ext and dim.dyn_size_ext.have_batch_axis() and dim.dyn_size_ext.batch_ndim == 1  # [B]
+      return Data(
+        name="%s_length" % name,
+        shape=[1], batch_dim_axis=0, time_dim_axis=1,
+        dtype=dtype, sparse=sparse, dim=None if sparse else NotSpecified)
+    if not dim.dyn_size_ext:  # yet undefined
+      return Data(
+        name="%s_length" % name,
+        shape=(), batch_dim_axis=0, time_dim_axis=None,
+        dtype=dtype, sparse=sparse, dim=None if sparse else NotSpecified)
+    return dim.dyn_size_ext
 
 
 class SoftmaxOverSpatialLayer(_ConcatInputLayer):