diff --git a/src/block.js b/src/block.js
index 5181265..e6d2365 100644
--- a/src/block.js
+++ b/src/block.js
@@ -116,15 +116,20 @@ Block.prototype.toHex = function (headersOnly) {
   return this.toBuffer(headersOnly).toString('hex')
 }
 
-var ZEROS = '0000000000000000000000000000000000000000000000000000000000000000'
-
 Block.calculateTarget = function (bits) {
   var exponent = ((bits & 0xff000000) >> 24) - 3
-  var mantissa = bits & 0x00ffffff
-  var target = mantissa * Math.pow(2, 8 * exponent)
-  var targetHex = Math.floor(target).toString(16)
+  var mantissa = bits & 0x007fffff
+  var i = 31 - exponent
+
+  var target = new Buffer(32)
+  target.fill(0)
+
+  target[i] = mantissa & 0xff
+  target[i - 1] = mantissa >> 8
+  target[i - 2] = mantissa >> 16
+  target[i - 3] = mantissa >> 24
 
-  return new Buffer(ZEROS.slice(0, 64 - targetHex.length) + targetHex, 'hex')
+  return target
 }
 
 Block.prototype.verifyPow = function () {
diff --git a/test/fixtures/block.json b/test/fixtures/block.json
index abd7fbc..a18fca5 100644
--- a/test/fixtures/block.json
+++ b/test/fixtures/block.json
@@ -6,7 +6,7 @@
     },
     {
       "bits": "1b80ffff",
-      "expected": "000000000080ffff000000000000000000000000000000000000000000000000"
+      "expected": "000000000000ffff000000000000000000000000000000000000000000000000"
     },
     {
       "bits": "1b0404cb",
@@ -15,6 +15,10 @@
     {
       "bits": "1814dd04",
       "expected": "000000000000000014dd04000000000000000000000000000000000000000000"
+    },
+    {
+      "bits": "cffca00",
+      "expected": "00000000000000000000000000000000000000007fca00000000000000000000"
     }
   ],
   "valid": [