diff --git a/lib/bcoin/tx.js b/lib/bcoin/tx.js
index 881eac3b..a1523c80 100644
--- a/lib/bcoin/tx.js
+++ b/lib/bcoin/tx.js
@@ -41,6 +41,12 @@ function TX(data, block) {
 
   // ps = Pending Since
   this.ps = this.ts === 0 ? +new Date() / 1000 : 0;
+
+  this.m = data.m || 1;
+  this.n = data.n || 1;
+  this.change = data.change || null;
+  this.fee = data.fee || 10000;
+  this.dust = 5460;
 }
 module.exports = TX;
 
@@ -57,6 +63,11 @@ TX.prototype.render = function render() {
   return bcoin.protocol.framer.tx(this);
 };
 
+TX.prototype.input = function input(i, index) {
+  this._input(i, index);
+  return this;
+};
+
 TX.prototype._input = function _input(i, index) {
   if (i instanceof TX)
     i = { tx: i, index: index };
@@ -88,10 +99,10 @@ TX.prototype._input = function _input(i, index) {
   var index = this._inputIndex(hash, index);
   if (index !== -1) {
     var ex = this.inputs[index];
-
-    ex.out.tx = input.out.tx || ex.out.tx;
-    ex.seq = input.seq || ex.seq;
-    ex.script = input.script.length ? input.script : ex.script;
+    input.out.tx = input.out.tx || ex.out.tx;
+    input.seq = input.seq || ex.seq;
+    input.script = input.script.length ? input.script : ex.script;
+    this.inputs[index] = input;
   } else {
     this.inputs.push(input);
     index = this.inputs.length - 1;
@@ -224,14 +235,10 @@ TX.prototype.scriptSig = function(input, key, pub, type, nsigs) {
   return input.script;
 };
 
-TX.prototype.input = function input(i, index) {
-  this._input(i, index);
-  return this;
-};
-
-TX.prototype.out = function out(output, value) {
+TX.prototype.output = function output(output, value) {
   if (output instanceof bcoin.wallet)
     output = output.getAddress();
+
   if (typeof output === 'string') {
     output = {
       address: output,
@@ -239,16 +246,26 @@ TX.prototype.out = function out(output, value) {
     };
   }
 
-  var script = output.script ? output.script.slice() : [];
+  this.outputs.push({
+    value: new bn(output.value),
+    script: this.scriptOutput(output)
+  });
+};
 
-  if (Array.isArray(output.keys || output.address)) {
+// compat
+TX.prototype.out = TX.prototype.output;
+
+TX.prototype.scriptOutput = function(options) {
+  var script = [];
+
+  if (Array.isArray(options.keys || options.address)) {
     // Raw multisig transaction
     // https://github.com/bitcoin/bips/blob/master/bip-0010.mediawiki
     // https://github.com/bitcoin/bips/blob/master/bip-0011.mediawiki
     // https://github.com/bitcoin/bips/blob/master/bip-0019.mediawiki
     // [required-sigs] [pubkey-hash1] [pubkey-hash2] ... [number-of-keys] checkmultisig
-    var keys = output.keys || output.address;
-    if (keys === output.address) {
+    var keys = options.keys || options.address;
+    if (keys === options.address) {
       keys = keys.map(function(address) {
         return bcoin.wallet.addr2hash(address, 'normal');
       });
@@ -260,7 +277,7 @@ TX.prototype.out = function out(output, value) {
       return key;
     });
     script = [
-      [ output.minSignatures || keys.length ]
+      [ options.minSignatures || keys.length ]
     ].concat(
       keys,
       [ [ keys.length ], 'checkmultisig' ]
@@ -268,33 +285,28 @@ TX.prototype.out = function out(output, value) {
     // outputs: [ [ 2 ], 'key1', 'key2', [ 2 ], 'checkmultisig' ]
     // in reality:
     // outputs: [ [ 2 ], [0,1,...], [2,3,...], [ 2 ], 'checkmultisig' ]
-  } else if (bcoin.wallet.validateAddress(output.address, 'p2sh')) {
+  } else if (bcoin.wallet.validateAddress(options.address, 'p2sh')) {
     // p2sh transaction
     // https://github.com/bitcoin/bips/blob/master/bip-0016.mediawiki
     // hash160 [20-byte-redeemscript-hash] equal
     script = [
       'hash160',
-      bcoin.wallet.addr2hash(output.address, 'p2sh'),
+      bcoin.wallet.addr2hash(options.address, 'p2sh'),
       'eq'
     ];
-  } else if (output.address) {
+  } else if (options.address) {
     // p2pkh transaction
     // dup hash160 [pubkey-hash] equalverify checksig
     script = [
       'dup',
       'hash160',
-      bcoin.wallet.addr2hash(output.address, 'normal'),
+      bcoin.wallet.addr2hash(options.address, 'normal'),
       'eqverify',
       'checksig'
     ];
   }
 
-  this.outputs.push({
-    value: new bn(output.value),
-    script: script
-  });
-
-  return this;
+  return script;
 };
 
 TX.prototype.getSubscript = function getSubscript(index) {
@@ -347,6 +359,7 @@ TX.prototype.verify = function verify(index, force) {
     if (!res)
       return false;
 
+    // XXX sighash_all here?
     return stack.length > 0 && utils.isEqual(stack.pop(), [ 1 ]);
   }, this);
 };
@@ -355,10 +368,7 @@ TX.prototype.isCoinbase = function isCoinbase() {
   return this.inputs.length === 1 && +this.inputs[0].out.hash === 0;
 };
 
-TX.prototype.maxSize = function maxSize(m, n) {
-  m = m || 1;
-  n = n || 1;
-
+TX.prototype.maxSize = function maxSize() {
   // Create copy with 0-script inputs
   var copy = this.clone();
   copy.inputs.forEach(function(input) {
@@ -369,7 +379,11 @@ TX.prototype.maxSize = function maxSize(m, n) {
 
   // Add size for signatures and public keys
   copy.inputs.forEach(function(input) {
-    var s = input.out.tx.outputs[input.out.index].script;
+    // Get the previous output's script
+    // var s = input.out.tx.outputs[input.out.index].script;
+
+    // Get the previous output's subscript
+    var s = input.out.tx.getSubscript(input.out.index);
 
     if (bcoin.script.isPubkeyhash(s) || bcoin.script.isSimplePubkeyhash(s)) {
       // Signature + len
@@ -384,18 +398,25 @@ TX.prototype.maxSize = function maxSize(m, n) {
       // Empty byte
       size += 1;
       // Signature + len
+      var m = s[0][0] || this.m;
+      assert(m >= 1 && m <= 7);
       size += 74 * m;
       return;
     }
 
-    // 1 empty byte
-    // 1 mcode byte
-    // loop:
-    //   1 byte key length
-    //   65? byte key
-    // 1 ncode byte
-    // 1 checkmultisig byte
     if (bcoin.script.isScripthash(s)) {
+      var script = this.inputs[i].script;
+      var redeem, m, n;
+      if (script) {
+        redeem = script[script.length - 1];
+        m = redeem[0] - constants.opcodes['1'] + 1;
+        n = redeem[redeem.length - 2] - constants.opcodes['1'] + 1;
+      } else {
+        m = this.m;
+        n = this.n;
+      }
+      assert(m >= 1 && m <= n);
+      assert(n >= 1 && n <= 7);
       // Multisig
       // Empty byte
       size += 1;
@@ -412,11 +433,113 @@ TX.prototype.maxSize = function maxSize(m, n) {
       size += 1;
       return;
     }
-  });
+  }, this);
 
   return size;
 };
 
+// Building a TX:
+// 1. Add outputs:
+//   - this.output({ address: ..., value: ... });
+//   - this.output({ address: ..., value: ... });
+// 2. Add inputs with utxos and change output:
+//   - this.fillUnspent(unspentItems, [changeAddr]);
+// 3. Fill input scripts (for each input):
+//   - this.scriptInput(input, pub, [nsigs])
+//   - this.signInput(input, key, [sigHashType])
+TX.prototype.utxos = function utxos(unspent) {
+  // NOTE: tx should be prefilled with all outputs
+  var cost = this.funds('out');
+
+  // Use initial fee for starters
+  var fee = 1;
+
+  // total = cost + fee
+  var total = cost.add(new bn(this.fee));
+
+  var inputs = this.inputs.slice();
+  var utxos = [];
+
+  var lastAdded = 0;
+  function addInput(unspent, i) {
+    // Add new inputs until TX will have enough funds to cover both
+    // minimum post cost and fee
+    var index = this._input(unspent);
+    utxos.push(this.inputs[index]);
+    lastAdded++;
+    return this.funds('in').cmp(total) < 0;
+  }
+
+  // Transfer `total` funds maximum
+  // var unspent = wallet.unspent();
+  unspent.every(addInput, this);
+
+  // Add dummy output (for `left`) to calculate maximum TX size
+  this.output({ address: null, value: new bn(0) });
+
+  // Change fee value if it is more than 1024 bytes
+  // (10000 satoshi for every 1024 bytes)
+  do {
+    // Calculate maximum possible size after signing
+    var byteSize = this.maxSize();
+
+    var addFee = Math.ceil(byteSize / 1024) - fee;
+    total.iadd(new bn(addFee * this.fee));
+    fee += addFee;
+
+    // Failed to get enough funds, add more inputs
+    if (this.funds('in').cmp(total) < 0)
+      unspent.slice(lastAdded).every(addInput, this);
+  } while (this.funds('in').cmp(total) < 0 && lastAdded < unspent.length);
+
+  // Still failing to get enough funds
+  if (this.funds('in').cmp(total) < 0) {
+    this.inputs = inputs;
+    this.outputs.pop();
+    this.cost = total;
+    return null;
+  }
+
+  // How much money is left after sending outputs
+  var left = this.funds('in').sub(total);
+
+  // Clear the tx of everything we added.
+  this.inputs = inputs;
+  this.outputs.pop();
+  this.cost = total;
+
+  // Return necessary utxos and change.
+  return {
+    utxos: utxos,
+    change: left,
+    cost: total
+  };
+};
+
+TX.prototype.fillUnspent = function fillUnspent(unspent, change) {
+  var result = this.utxos(unspent);
+
+  if (!result)
+    return result;
+
+  result.utxos.forEach(function(utxo) {
+    this.input(utxo, null);
+  }, this);
+
+  // Not enough money, transfer everything to owner
+  if (result.change.cmpn(this.dust) < 0) {
+    // NOTE: that this output is either `postCost` or one of the `dust` values
+    this.outputs[this.outputs.length - 1].value.iadd(result.change);
+  } else {
+    this.output({
+      address: change || this.change,
+      value: result.change
+    });
+  }
+
+  return result;
+};
+
 TX.prototype.inputAddrs = function inputAddrs() {
   return this.inputs.filter(function(input) {
     return bcoin.script.isPubkeyhashInput(input.script);
diff --git a/lib/bcoin/wallet.js b/lib/bcoin/wallet.js
index db20d8d9..2e03b69d 100644
--- a/lib/bcoin/wallet.js
+++ b/lib/bcoin/wallet.js
@@ -450,6 +450,14 @@ Wallet.prototype.fill = function fill(tx, cb) {
   return tx;
 };
 
+Wallet.prototype.fill = function fill(tx, cb) {
+  cb = utils.asyncify(cb);
+  tx.fillUnspent(this.unspent(), this.getAddress());
+  this.sign(tx);
+  cb(null, tx);
+  return tx;
+};
+
 // P2SH Multisig redeem script
 Wallet.prototype.getRedemption = function() {
   var sharedKeys = this.sharedKeys.slice().map(utils.toKeyArray);