diff --git a/src/transaction.js b/src/transaction.js
index fa1b18a..6e8fc6c 100644
--- a/src/transaction.js
+++ b/src/transaction.js
@@ -137,6 +137,22 @@ Transaction.prototype.addOutput = function (scriptPubKey, value) {
   }) - 1)
 }
 
+Transaction.prototype.byteLength = function () {
+  function scriptSize (script) {
+    var length = script.buffer.length
+
+    return bufferutils.varIntSize(length) + length
+  }
+
+  return (
+    8 +
+    bufferutils.varIntSize(this.ins.length) +
+    bufferutils.varIntSize(this.outs.length) +
+    this.ins.reduce(function (sum, input) { return sum + 40 + scriptSize(input.script) }, 0) +
+    this.outs.reduce(function (sum, output) { return sum + 8 + scriptSize(output.script) }, 0)
+  )
+}
+
 Transaction.prototype.clone = function () {
   var newTx = new Transaction()
   newTx.version = this.version
@@ -215,19 +231,7 @@ Transaction.prototype.getId = function () {
 }
 
 Transaction.prototype.toBuffer = function () {
-  function scriptSize (script) {
-    var length = script.buffer.length
-
-    return bufferutils.varIntSize(length) + length
-  }
-
-  var buffer = new Buffer(
-    8 +
-    bufferutils.varIntSize(this.ins.length) +
-    bufferutils.varIntSize(this.outs.length) +
-    this.ins.reduce(function (sum, input) { return sum + 40 + scriptSize(input.script) }, 0) +
-    this.outs.reduce(function (sum, output) { return sum + 8 + scriptSize(output.script) }, 0)
-  )
+  var buffer = new Buffer(this.byteLength())
 
   var offset = 0
   function writeSlice (slice) {