diff --git a/mempool/estimatefee.go b/mempool/estimatefee.go
index e3a46234f..ee7252401 100644
--- a/mempool/estimatefee.go
+++ b/mempool/estimatefee.go
@@ -208,7 +208,7 @@ func (ef *FeeEstimator) ObserveTransaction(t *TxDesc) {
 
 	hash := *t.Tx.Hash()
 	if _, ok := ef.observed[hash]; !ok {
-		size := uint32(t.Tx.MsgTx().SerializeSize())
+		size := uint32(GetTxVirtualSize(t.Tx))
 
 		ef.observed[hash] = &observedTransaction{
 			hash:     hash,