diff --git a/core/src/main/java/com/google/bitcoin/core/Wallet.java b/core/src/main/java/com/google/bitcoin/core/Wallet.java
index b94937ea..a988b683 100644
--- a/core/src/main/java/com/google/bitcoin/core/Wallet.java
+++ b/core/src/main/java/com/google/bitcoin/core/Wallet.java
@@ -491,7 +491,7 @@ public class Wallet extends BaseTaggableObject implements Serializable, BlockCha
 
     /** Returns the address used for change outputs. Note: this will probably go away in future. */
     public Address getChangeAddress() {
-        return currentKey(KeyChain.KeyPurpose.CHANGE).toAddress(params);
+        return keychain.currentAddress(KeyChain.KeyPurpose.CHANGE);
     }
 
     /**
@@ -3895,14 +3895,17 @@ public class Wallet extends BaseTaggableObject implements Serializable, BlockCha
         int size = 0;
         for (TransactionOutput output : selection.gathered) {
             try {
-                if (output.getScriptPubKey().isSentToAddress()) {
-                    // Send-to-address spends usually take maximum pubkey.length (as it may be compressed or not) + 75 bytes
-                    final ECKey key = findKeyFromPubHash(output.getScriptPubKey().getPubKeyHash());
-                    size += checkNotNull(key, "Coin selection includes unspendable outputs").getPubKey().length + 75;
-                } else if (output.getScriptPubKey().isSentToRawPubKey())
-                    size += 74; // Send-to-pubkey spends usually take maximum 74 bytes to spend
-                else
-                    throw new IllegalStateException("Unknown output type returned in coin selection");
+                Script script = output.getScriptPubKey();
+                ECKey key = null;
+                Script redeemScript = null;
+                if (script.isSentToAddress()) {
+                    key = findKeyFromPubHash(script.getPubKeyHash());
+                    checkNotNull(key, "Coin selection includes unspendable outputs");
+                } else if (script.isPayToScriptHash()) {
+                    redeemScript = keychain.findRedeemScriptFromPubHash(script.getPubKeyHash());
+                    checkNotNull(redeemScript, "Coin selection includes unspendable outputs");
+                }
+                size += script.getNumberOfBytesRequiredToSpend(key, redeemScript);
             } catch (ScriptException e) {
                 // If this happens it means an output script in a wallet tx could not be understood. That should never
                 // happen, if it does it means the wallet has got into an inconsistent state.
diff --git a/core/src/main/java/com/google/bitcoin/script/Script.java b/core/src/main/java/com/google/bitcoin/script/Script.java
index 378d19d4..40dc1059 100644
--- a/core/src/main/java/com/google/bitcoin/script/Script.java
+++ b/core/src/main/java/com/google/bitcoin/script/Script.java
@@ -25,6 +25,7 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import org.spongycastle.crypto.digests.RIPEMD160Digest;
 
+import javax.annotation.Nullable;
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
@@ -54,6 +55,7 @@ import static com.google.common.base.Preconditions.checkState;
 public class Script {
     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;
 
     // The program is a set of chunks where each element is either [opcode] or [data, data, data ...]
     protected List<ScriptChunk> chunks;
@@ -446,6 +448,36 @@ public class Script {
         return 0;
     }
 
+    /**
+     * Returns number of bytes required to spend this script. It accepts optional ECKey and redeemScript that may
+     * be required for certain types of script to estimate target size.
+     */
+    public int getNumberOfBytesRequiredToSpend(@Nullable ECKey pubKey, @Nullable Script redeemScript) {
+        if (isPayToScriptHash()) {
+            // scriptSig: <sig> [sig] [sig...] <redeemscript>
+            checkArgument(redeemScript != null, "P2SH script requires redeemScript to be spent");
+            // for N of M CHECKMULTISIG redeem script we will need N signatures to spend
+            ScriptChunk nChunk = redeemScript.getChunks().get(0);
+            int n = Script.decodeFromOpN(nChunk.opcode);
+            return n * SIG_SIZE + getProgram().length;
+        } else if (isSentToMultiSig()) {
+            // scriptSig: OP_0 <sig> [sig] [sig...]
+            // for N of M CHECKMULTISIG script we will need N signatures to spend
+            ScriptChunk nChunk = chunks.get(0);
+            int n = Script.decodeFromOpN(nChunk.opcode);
+            return n * SIG_SIZE + 1;
+        } else if (isSentToRawPubKey()) {
+            // scriptSig: <sig>
+            return SIG_SIZE;
+        } else if (isSentToAddress()) {
+            // scriptSig: <sig> <pubkey>
+            int uncompressedPubKeySize = 65;
+            return SIG_SIZE + (pubKey != null ? pubKey.getPubKey().length : uncompressedPubKeySize);
+        } else {
+            throw new IllegalStateException("Unsupported script type");
+        }
+    }
+
     /**
      * <p>Whether or not this is a scriptPubKey representing a pay-to-script-hash output. In such outputs, the logic that
      * controls reclamation is not actually in the output at all. Instead there's just a hash, and it's up to the