diff --git a/basicswap/interface/base.py b/basicswap/interface/base.py
index dc708ef..9a8e0f2 100644
--- a/basicswap/interface/base.py
+++ b/basicswap/interface/base.py
@@ -9,6 +9,7 @@
 import threading
 
 from enum import IntEnum
+from typing import List
 
 from basicswap.chainparams import (
     chainparams,
@@ -180,13 +181,16 @@ class CoinInterface:
 
 
 class AdaptorSigInterface:
-    def getScriptLockTxDummyWitness(self, script: bytes):
+    def getP2WPKHDummyWitness(self) -> List[bytes]:
+        return [bytes(72), bytes(33)]
+
+    def getScriptLockTxDummyWitness(self, script: bytes) -> List[bytes]:
         return [b"", bytes(72), bytes(72), bytes(len(script))]
 
-    def getScriptLockRefundSpendTxDummyWitness(self, script: bytes):
+    def getScriptLockRefundSpendTxDummyWitness(self, script: bytes) -> List[bytes]:
         return [b"", bytes(72), bytes(72), bytes((1,)), bytes(len(script))]
 
-    def getScriptLockRefundSwipeTxDummyWitness(self, script: bytes):
+    def getScriptLockRefundSwipeTxDummyWitness(self, script: bytes) -> List[bytes]:
         return [bytes(72), b"", bytes(len(script))]
 
 
diff --git a/basicswap/interface/btc.py b/basicswap/interface/btc.py
index 029613c..8fba86c 100644
--- a/basicswap/interface/btc.py
+++ b/basicswap/interface/btc.py
@@ -2,7 +2,7 @@
 # -*- coding: utf-8 -*-
 
 # Copyright (c) 2020-2024 tecnovert
-# Copyright (c) 2024-2025 The Basicswap developers
+# Copyright (c) 2024-2026 The Basicswap developers
 # Distributed under the MIT software license, see the accompanying
 # file LICENSE or http://www.opensource.org/licenses/mit-license.php.
 
@@ -828,7 +828,7 @@ class BTCInterface(Secp256k1Interface):
 
     def getScriptDummyWitness(self, script: bytes) -> List[bytes]:
         if self.isScriptP2WPKH(script):
-            return [bytes(72), bytes(33)]
+            return self.getP2WPKHDummyWitness()
         raise ValueError("Unknown script type")
 
     def createSCLockRefundTx(
@@ -1943,9 +1943,16 @@ class BTCInterface(Secp256k1Interface):
         raise ValueError("Unimplemented")
 
     def getWitnessStackSerialisedLength(self, witness_stack):
-        length = getCompactSizeLen(len(witness_stack))
-        for e in witness_stack:
-            length += getWitnessElementLen(len(e))
+        length: int = 0
+        if len(witness_stack) > 0 and isinstance(witness_stack[0], list):
+            for input_stack in witness_stack:
+                length += getCompactSizeLen(len(input_stack))
+                for e in input_stack:
+                    length += getWitnessElementLen(len(e))
+        else:
+            length += getCompactSizeLen(len(witness_stack))
+            for e in witness_stack:
+                length += getWitnessElementLen(len(e))
 
         # See core SerializeTransaction
         length += 1  # vinDummy
diff --git a/basicswap/interface/part.py b/basicswap/interface/part.py
index 646f895..a40a99b 100644
--- a/basicswap/interface/part.py
+++ b/basicswap/interface/part.py
@@ -2,7 +2,7 @@
 # -*- coding: utf-8 -*-
 
 # Copyright (c) 2020-2024 tecnovert
-# Copyright (c) 2024-2025 The Basicswap developers
+# Copyright (c) 2024-2026 The Basicswap developers
 # Distributed under the MIT software license, see the accompanying
 # file LICENSE or http://www.opensource.org/licenses/mit-license.php.
 
@@ -137,7 +137,7 @@ class PARTInterface(BTCInterface):
 
     def getScriptDummyWitness(self, script: bytes) -> List[bytes]:
         if self.isScriptP2WPKH(script) or self.isScriptP2PKH(script):
-            return [bytes(72), bytes(33)]
+            return self.getP2WPKHDummyWitness()
         raise ValueError("Unknown script type")
 
     def formatStealthAddress(self, scan_pubkey, spend_pubkey) -> str:
@@ -146,9 +146,16 @@ class PARTInterface(BTCInterface):
         return encodeStealthAddress(prefix_byte, scan_pubkey, spend_pubkey)
 
     def getWitnessStackSerialisedLength(self, witness_stack) -> int:
-        length: int = getCompactSizeLen(len(witness_stack))
-        for e in witness_stack:
-            length += getWitnessElementLen(len(e))
+        length: int = 0
+        if len(witness_stack) > 0 and isinstance(witness_stack[0], list):
+            for input_stack in witness_stack:
+                length += getCompactSizeLen(len(input_stack))
+                for e in input_stack:
+                    length += getWitnessElementLen(len(e))
+        else:
+            length += getCompactSizeLen(len(witness_stack))
+            for e in witness_stack:
+                length += getWitnessElementLen(len(e))
         return length
 
     def getWalletRestoreHeight(self) -> int:
diff --git a/tests/basicswap/test_btc_xmr.py b/tests/basicswap/test_btc_xmr.py
index 53a1e8e..18329da 100644
--- a/tests/basicswap/test_btc_xmr.py
+++ b/tests/basicswap/test_btc_xmr.py
@@ -1509,6 +1509,23 @@ class BasicSwapTest(TestFunctions):
         vsize = tx_decoded["vsize"]
         expect_fee_int = round(self.test_fee_rate * vsize / 1000)
 
+        tx_obj = ci.loadTx(lock_tx)
+        vsize_from_ci = ci.getTxVSize(tx_obj)
+        assert vsize == vsize_from_ci
+        tx_no_witness = tx_obj.serialize_without_witness()
+
+        dummy_witness_stack = []
+        for txi in tx_obj.vin:
+            dummy_witness_stack.append(ci.getP2WPKHDummyWitness())
+        witness_bytes_len_est: int = ci.getWitnessStackSerialisedLength(
+            dummy_witness_stack
+        )
+        tx_obj_no_witness = ci.loadTx(tx_no_witness)
+        vsize_estimated = ci.getTxVSize(
+            tx_obj_no_witness, add_witness_bytes=witness_bytes_len_est
+        )
+        assert vsize <= vsize_estimated and vsize_estimated - vsize < 4
+
         out_value: int = 0
         for txo in tx_decoded["vout"]:
             if "value" in txo:
@@ -1556,7 +1573,7 @@ class BasicSwapTest(TestFunctions):
 
         expect_vsize: int = ci.xmr_swap_a_lock_spend_tx_vsize()
         assert expect_vsize >= vsize_actual
-        assert expect_vsize - vsize_actual < 10
+        assert expect_vsize - vsize_actual <= 10
 
         # Test chain b (no-script) lock tx size
         v = ci.getNewRandomKey()
@@ -1577,7 +1594,7 @@ class BasicSwapTest(TestFunctions):
 
         expect_vsize: int = ci.xmr_swap_b_lock_spend_tx_vsize()
         assert expect_vsize >= lock_tx_b_spend_decoded["vsize"]
-        assert expect_vsize - lock_tx_b_spend_decoded["vsize"] < 10
+        assert expect_vsize - lock_tx_b_spend_decoded["vsize"] <= 10
 
     def test_011_p2sh(self):
         # Not used in bsx for native-segwit coins