diff --git a/test/functional/test_framework/test_node.py b/test/functional/test_framework/test_node.py
--- a/test/functional/test_framework/test_node.py
+++ b/test/functional/test_framework/test_node.py
@@ -168,7 +168,7 @@
     def calculate_fee(self, tx):
         # Relay fee is in satoshis per KB.  Thus the 1000, and the COIN added
         # to get back to an amount of satoshis.
-        return int(self.relay_fee() / 1000 * len(ToHex(tx)) * COIN)
+        return int(self.relay_fee() / 1000 * tx.billable_size() * COIN)
 
     def calculate_fee_from_txid(self, txid):
         ctx = FromHex(CTransaction(), self.getrawtransaction(txid))