diff --git a/sdks/python/apache_beam/coders/coder_impl.py b/sdks/python/apache_beam/coders/coder_impl.py
index 3e0b5218b166..2a3ce2cc8828 100644
--- a/sdks/python/apache_beam/coders/coder_impl.py
+++ b/sdks/python/apache_beam/coders/coder_impl.py
@@ -316,6 +316,9 @@ def decode(self, encoded):
     proto_message.ParseFromString(encoded)  # This is in effect "ParsePartial".
     return proto_message
 
+  def estimate_size(self, value, nested=False):
+    return self._get_nested_size(value.ByteSize(), nested)
+
 
 class DeterministicProtoCoderImpl(ProtoCoderImpl):
   """For internal use only; no backwards-compatibility guarantees."""
@@ -335,6 +338,9 @@ def encode(self, value):
   def decode(self, value):
     return self.proto_plus_type.deserialize(value)
 
+  def estimate_size(self, value, nested=False):
+    return self._get_nested_size(type(value).pb(value).ByteSize(), nested)
+
 
 UNKNOWN_TYPE = 0xFF
 NONE_TYPE = 0