diff --git a/src/main/java/co/rsk/bitcoinj/script/Script.java b/src/main/java/co/rsk/bitcoinj/script/Script.java
index dffab60cc..ff7aa764b 100644
--- a/src/main/java/co/rsk/bitcoinj/script/Script.java
+++ b/src/main/java/co/rsk/bitcoinj/script/Script.java
@@ -96,7 +96,7 @@ public enum VerifyFlag {
 
     private static final Logger log = LoggerFactory.getLogger(Script.class);
     public static final long MAX_SCRIPT_ELEMENT_SIZE = 520;  // bytes
-    public static final int SIG_SIZE = 75;
+    public static final int SIG_SIZE = 73;
     /** Max number of sigops allowed in a standard p2sh redeem script */
     public static final int MAX_P2SH_SIGOPS = 15;
 
diff --git a/src/main/java/co/rsk/bitcoinj/wallet/SendRequest.java b/src/main/java/co/rsk/bitcoinj/wallet/SendRequest.java
index 400eaa89a..2ad9b07a6 100644
--- a/src/main/java/co/rsk/bitcoinj/wallet/SendRequest.java
+++ b/src/main/java/co/rsk/bitcoinj/wallet/SendRequest.java
@@ -60,6 +60,8 @@ public class SendRequest {
      */
     public BtcTransaction tx;
 
+    public boolean isSegwit = false;
+
     /**
      * When emptyWallet is set, all coins selected by the coin selector are sent to the first output in tx
      * (its value is ignored and set to {@link co.rsk.bitcoinj.wallet.Wallet#getBalance()} - the fees required
diff --git a/src/main/java/co/rsk/bitcoinj/wallet/Wallet.java b/src/main/java/co/rsk/bitcoinj/wallet/Wallet.java
index 9b439501b..6fc7f2afa 100644
--- a/src/main/java/co/rsk/bitcoinj/wallet/Wallet.java
+++ b/src/main/java/co/rsk/bitcoinj/wallet/Wallet.java
@@ -996,8 +996,22 @@ public FeeCalculation calculateFee(SendRequest req, Coin value, List<Transaction
                 checkState(input.getScriptBytes().length == 0);
             }
 
-            int size = tx.unsafeBitcoinSerialize().length;
-            size += estimateBytesForSigning(selection);
+            int size;
+            int totalSize;
+            int baseSize;
+
+            baseSize = calculateTxBaseSize(tx, req.isSegwit);
+            totalSize = baseSize + estimateBytesForSigning(selection);
+            totalSize += bytesToAdd(tx);
+
+            if (req.isSegwit) {
+                size = calculateWitnessTxVirtualSize(baseSize, totalSize);
+                log.info("The tx is segwit so the size is {} ", size);
+            }
+            else {
+                size = totalSize;
+                log.info("The tx is not segwit so the size is {} ", size);
+            }
 
             Coin feePerKb = req.feePerKb;
             if (needAtLeastReferenceFee && feePerKb.compareTo(BtcTransaction.REFERENCE_DEFAULT_MIN_TX_FEE) < 0) {
@@ -1017,6 +1031,61 @@ public FeeCalculation calculateFee(SendRequest req, Coin value, List<Transaction
 
     }
 
+    public static int calculateTxBaseSize(BtcTransaction spendTx, boolean isSegwit) {
+        int baseSize = 0;
+
+        int inputsQuantity = spendTx.getInputs().size();
+        for (int i = 0; i < inputsQuantity; i++) {
+            byte[] input = spendTx.getInput(i).bitcoinSerialize();
+            baseSize += input.length;
+            // at this time the scriptSig for every input is empty = 00. so we should count its bytes manually and remove the byte from the empty one.
+            baseSize += 36 - 1;
+        }
+
+        int outputsQuantity = spendTx.getOutputs().size();
+        for (int i = 0; i < outputsQuantity; i++) {
+            byte[] output = spendTx.getOutput(i).bitcoinSerialize();
+            baseSize += output.length;
+        }
+
+        baseSize += 4; // version size
+        baseSize += 4; // locktime size
+
+        if (isSegwit) {
+            baseSize += 1; // marker size
+            baseSize += 1; // flag size
+        }
+
+        return baseSize;
+    }
+
+    public static int calculateWitnessTxWeight(int txBaseSize, int txTotalSize) {
+
+        // As described in https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki#transaction-size-calculations
+        int txWeight = txTotalSize + (3 * txBaseSize);
+        return txWeight;
+    }
+
+    public static int calculateWitnessTxVirtualSize(int txBaseSize, int txTotalSize) {
+        double txWeight = calculateWitnessTxWeight(txBaseSize, txTotalSize);
+
+        // As described in https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki#transaction-size-calculations
+        int txVirtualSize = (int) Math.ceil(txWeight / 4);
+        return txVirtualSize;
+    }
+
+    public static int bytesToAdd(BtcTransaction tx) {
+        int bytes = 0;
+
+        bytes += 1; // inputs quantity bytes size
+        bytes += 1; // outputs quantity bytes size
+        bytes += 1; // empty vector before signatures bytes size
+        bytes += 1; // op_notif value bytes size
+        bytes += 4; // 3 or 4 bytes before the redeem are added. count them like 4
+
+        return bytes;
+    }
+
     private void addSuppliedInputs(BtcTransaction tx, List<TransactionInput> originalInputs) {
         for (TransactionInput input : originalInputs)
             tx.addInput(new TransactionInput(params, tx, input.bitcoinSerialize()));