diff --git a/lib/platform.browser.js b/lib/platform.browser.js
index 0240fc2..c96d2f9 100644
--- a/lib/platform.browser.js
+++ b/lib/platform.browser.js
@@ -5,6 +5,8 @@
 "use strict";
 
 var hash = require("hash.js");
+var BN = require("bn.js");
+var assert = require("./util").assert;
 
 exports.sha512 = function(buf) {
   return new Buffer(hash.sha512().update(buf).digest());
@@ -23,3 +25,19 @@ exports.randomBytes = function(size) {
   window.crypto.getRandomValues(arr);
   return new Buffer(arr);
 };
+
+var B64 = new BN("18446744073709551616");
+
+exports.getTarget = function(opts) {
+  var length = new BN(opts.payloadLength);
+  length.iaddn(8);
+  length.iaddn(opts.payloadLengthExtraBytes);
+  var denominator = new BN(opts.ttl);
+  denominator.imul(length);
+  denominator.idivn(65536);
+  denominator.iadd(length);
+  denominator.imul(new BN(opts.nonceTrialsPerByte));
+  var target = parseInt(B64.div(denominator).toString(16), 16);
+  assert(target <= 9007199254740991, "Unsafe target");
+  return target;
+};
diff --git a/lib/platform.js b/lib/platform.js
index 2c7f0b1..4953c22 100644
--- a/lib/platform.js
+++ b/lib/platform.js
@@ -5,6 +5,9 @@
 "use strict";
 
 var crypto = require("crypto");
+var bignum = require("bignum");
+var assert = require("./util").assert;
+
 var createHash = crypto.createHash;
 
 exports.sha512 = function(buf) {
@@ -20,3 +23,25 @@ exports.ripemd160 = function(buf) {
 };
 
 exports.randomBytes = crypto.randomBytes;
+
+// 2^64.
+var B64 = bignum("18446744073709551616");
+
+// NOTE(Kagami): We can't calculate entire target in JavaScript but the
+// result can be represented in native number type without losing
+// precision (targets mainly much less than 2^53).
+exports.getTarget = function(opts) {
+  // Calculate it bottom-up, right-to-left.
+  var length = bignum(opts.payloadLength)
+    // To account for the nonce which we will append later.
+    .add(8)
+    .add(opts.payloadLengthExtraBytes);
+  var denominator = bignum(opts.ttl)
+    .mul(length)
+    .div(65536)
+    .add(length)
+    .mul(opts.nonceTrialsPerByte);
+  var target = B64.div(denominator).toNumber();
+  assert(target <= 9007199254740991, "Unsafe target");
+  return target;
+};
diff --git a/lib/pow.js b/lib/pow.js
index 795bfba..622c134 100644
--- a/lib/pow.js
+++ b/lib/pow.js
@@ -3,3 +3,34 @@
  * @see {@link https://bitmessage.org/wiki/Proof_of_work}
  * @module bitmessage/pow
  */
+
+"use strict";
+
+var platform = require("./platform");
+
+var DEFAULT_TRIALS_PER_BYTE = 1000;
+var DEFAULT_EXTRA_BYTES = 1000;
+
+/**
+ * Calculate target
+ * @param {{ttl: number, payloadLength: number}} opts - Target options
+ * @return {number} Target.
+ */
+// Just a wrapper around platform-specific implementation.
+exports.getTarget = function(opts) {
+  var nonceTrialsPerByte = opts.nonceTrialsPerByte;
+  // Automatically raise lower values per spec.
+  if (!nonceTrialsPerByte || nonceTrialsPerByte < DEFAULT_TRIALS_PER_BYTE) {
+    nonceTrialsPerByte = DEFAULT_TRIALS_PER_BYTE;
+  }
+  var payloadLengthExtraBytes = opts.payloadLengthExtraBytes;
+  if (!payloadLengthExtraBytes || payloadLengthExtraBytes < DEFAULT_EXTRA_BYTES) {
+    payloadLengthExtraBytes = DEFAULT_EXTRA_BYTES;
+  }
+  return platform.getTarget({
+    ttl: opts.ttl,
+    payloadLength: opts.payloadLength,
+    nonceTrialsPerByte: nonceTrialsPerByte,
+    payloadLengthExtraBytes: payloadLengthExtraBytes,
+  });
+};
diff --git a/package.json b/package.json
index 11784b4..e007065 100644
--- a/package.json
+++ b/package.json
@@ -7,7 +7,7 @@
     "./lib/platform.js": "./lib/platform.browser.js"
   },
   "scripts": {
-    "test": "ALL_TESTS=1 mocha && xvfb-run -a karma start && jshint .",
+    "test": "ALL_TESTS=1 mocha && ALL_TESTS=1 xvfb-run -a karma start && jshint .",
     "m": "mocha",
     "k": "xvfb-run -a karma start",
     "kc": "xvfb-run -a karma start --browsers Chromium",
@@ -46,10 +46,13 @@
     "mocha": "*"
   },
   "dependencies": {
+    "bignum": "^0.9.0",
+    "bn.js": "^1.0.0",
     "bs58": "^2.0.0",
     "buffer-equal": "~0.0.1",
     "eccrypto": "^0.1.2",
     "hash.js": "^1.0.2",
-    "object-assign": "^2.0.0"
+    "object-assign": "^2.0.0",
+    "sha.js": "^2.3.0"
   }
 }
diff --git a/test.js b/test.js
index 83a01be..7fd014a 100644
--- a/test.js
+++ b/test.js
@@ -15,6 +15,7 @@ var messageEncodings = structs.messageEncodings;
 var serviceFeatures = structs.serviceFeatures;
 var pubkeyFeatures = structs.pubkeyFeatures;
 var WIF = bitmessage.WIF;
+var POW = bitmessage.POW;
 var Address = bitmessage.Address;
 
 describe("Crypto", function() {
@@ -263,6 +264,13 @@ describe("WIF", function() {
   });
 });
 
+describe("POW", function() {
+  it("should calculate target", function() {
+    expect(POW.getTarget({ttl: 2418984, payloadLength: 628, nonceTrialsPerByte: 1000, payloadLengthExtraBytes: 1000})).to.equal(297422593171);
+    expect(POW.getTarget({ttl: 86400, payloadLength: 628})).to.equal(4864647698763);
+  });
+});
+
 describe("High-level classes", function() {
   // FIXME(Kagami): Add more fail tests.
   describe("Address", function() {
@@ -296,7 +304,9 @@ describe("High-level classes", function() {
       expect(addr2.ripe[0]).to.equal(0);
     });
 
-    if (allTests) {
+    // FIXME(Kagami): Don't run it in browser currently because it's
+    // very slow. This need to be fixed.
+    if (allTests && typeof window === "undefined") {
       it("should allow to generate shorter address", function() {
         this.timeout(60000);
         var addr = Address.fromRandom({ripelen: 18});