diff --git a/test/functional/p2p_segwit.py b/test/functional/p2p_segwit.py
index f34908e134d..74eda6620f3 100755
--- a/test/functional/p2p_segwit.py
+++ b/test/functional/p2p_segwit.py
@@ -450,8 +450,7 @@ class SegWitTest(BitcoinTestFramework):
             rpc_details = self.nodes[0].getblock(block.hash, True)
             assert_equal(rpc_details["size"], len(block.serialize()))
             assert_equal(rpc_details["strippedsize"], len(block.serialize(False)))
-            weight = 3 * len(block.serialize(False)) + len(block.serialize())
-            assert_equal(rpc_details["weight"], weight)
+            assert_equal(rpc_details["weight"], block.get_weight())
 
             # Upgraded node should not ask for blocks from unupgraded
             block4 = self.build_next_block(version=4)
diff --git a/test/functional/test_framework/messages.py b/test/functional/test_framework/messages.py
index 8d0bd9f69af..1abe604b289 100755
--- a/test/functional/test_framework/messages.py
+++ b/test/functional/test_framework/messages.py
@@ -746,6 +746,13 @@ class CBlock(CBlockHeader):
             self.nNonce += 1
             self.rehash()
 
+    # Calculate the block weight using witness and non-witness
+    # serialization size (does NOT use sigops).
+    def get_weight(self):
+        with_witness_size = len(self.serialize(with_witness=True))
+        without_witness_size = len(self.serialize(with_witness=False))
+        return (WITNESS_SCALE_FACTOR - 1) * without_witness_size + with_witness_size
+
     def __repr__(self):
         return "CBlock(nVersion=%i hashPrevBlock=%064x hashMerkleRoot=%064x nTime=%s nBits=%08x nNonce=%08x vtx=%s)" \
             % (self.nVersion, self.hashPrevBlock, self.hashMerkleRoot,