diff --git a/electrum b/electrum
index 28e305339..15fcb60dc 100755
--- a/electrum
+++ b/electrum
@@ -72,7 +72,7 @@ def arg_parser():
     parser.add_option("-a", "--all", action="store_true", dest="show_all", default=False, help="show all addresses")
     parser.add_option("-b", "--balance", action="store_true", dest="show_balance", default=False, help="show the balance of listed addresses")
     parser.add_option("-l", "--labels", action="store_true", dest="show_labels", default=False, help="show the labels of listed addresses")
-    parser.add_option("-f", "--fee", dest="tx_fee", default="0.005", help="set tx fee")
+    parser.add_option("-f", "--fee", dest="tx_fee", default=None, help="set tx fee")
     parser.add_option("-F", "--fromaddr", dest="from_addr", default=None, help="set source address for payto/mktx. if it isn't in the wallet, it will ask for the private key unless supplied in the format public_key:private_key. It's not saved in the wallet.")
     parser.add_option("-c", "--changeaddr", dest="change_addr", default=None, help="set the change address for payto/mktx. default is a spare address, or the source address if it's not in the wallet")
     parser.add_option("-s", "--server", dest="server", default=None, help="set server host:port:protocol, where protocol is t or h")
diff --git a/gui/gui_classic.py b/gui/gui_classic.py
index 5c48205d5..b44d0c80f 100644
--- a/gui/gui_classic.py
+++ b/gui/gui_classic.py
@@ -795,6 +795,10 @@ class ElectrumWindow(QMainWindow):
             self.show_message(str(e))
             return
 
+        if tx.requires_fee(self.wallet.verifier) and fee == 0:
+            QMessageBox.warning(self, _('Error'), _("This transaction requires a fee, or it will not be propagated by the network."), _('OK'))
+            return
+
         self.run_hook('send_tx', tx)
 
         if label: 
@@ -1928,8 +1932,8 @@ class ElectrumWindow(QMainWindow):
         fee_e = QLineEdit()
         fee_e.setText("%s"% str( Decimal( self.wallet.fee)/100000000 ) )
         grid_wallet.addWidget(fee_e, 0, 2)
-        msg = _('Fee per transaction input. Transactions involving multiple inputs tend to require a higher fee.') + ' ' \
-            + _('Recommended value') + ': 0.001'
+        msg = _('Fee per kilobyte of transaction.') + ' ' \
+            + _('Recommended value') + ': 0.0001'
         grid_wallet.addWidget(HelpButton(msg), 0, 3)
         fee_e.textChanged.connect(lambda: numbify(fee_e,False))
         if not self.config.is_modifiable('fee'):
diff --git a/gui/gui_gtk.py b/gui/gui_gtk.py
index 8568f7f5e..d3261ded2 100644
--- a/gui/gui_gtk.py
+++ b/gui/gui_gtk.py
@@ -198,7 +198,7 @@ def run_settings_dialog(wallet, parent):
     fee_entry.connect('changed', numbify, False)
     fee_entry.show()
     fee.pack_start(fee_entry,False,False, 10)
-    add_help_button(fee, 'Fee per transaction input. Transactions involving multiple inputs tend to have a higher fee. Recommended value:0.0005')
+    add_help_button(fee, 'Fee per kilobyte of transaction. Recommended value:0.0001')
     fee.show()
     vbox.pack_start(fee, False,False, 5)
             
@@ -843,6 +843,11 @@ class ElectrumWindow:
         except BaseException, e:
             self.show_message(str(e))
             return
+
+        if tx.requires_fee(self.wallet.verifier) and fee == 0:
+            self.show_message( "This transaction requires a fee, or it will not be propagated by the network." )
+            return
+
             
         if label: 
             self.wallet.labels[tx.hash()] = label
diff --git a/lib/bitcoin.py b/lib/bitcoin.py
index 6a8c48648..0b87b7845 100644
--- a/lib/bitcoin.py
+++ b/lib/bitcoin.py
@@ -876,6 +876,18 @@ class Transaction:
         return out
 
 
+    def requires_fee(self, verifier):
+        threshold = 57600000
+        size = len(self.raw)/2
+        sum = 0
+        for i in self.inputs:
+            age = verifier.get_confirmations(i["tx_hash"])[0]
+            sum += i["value"] * age
+        priority = sum / size
+        print_error(priority, threshold)
+        return priority < threshold
+
+
 
 
 def test_bip32():
diff --git a/lib/wallet.py b/lib/wallet.py
index fee23d328..46531ffbe 100644
--- a/lib/wallet.py
+++ b/lib/wallet.py
@@ -74,7 +74,7 @@ class Wallet:
         self.seed_version          = config.get('seed_version', SEED_VERSION)
         self.gap_limit             = config.get('gap_limit', 5)
         self.use_change            = config.get('use_change',True)
-        self.fee                   = int(config.get('fee',100000))
+        self.fee                   = int(config.get('fee',10000))
         self.num_zeros             = int(config.get('num_zeros',0))
         self.use_encryption        = config.get('use_encryption', False)
         self.seed                  = config.get('seed', '')               # encrypted
@@ -565,14 +565,20 @@ class Wallet:
             total += v
 
             inputs.append( item )
-            fee = self.fee*len(inputs) if fixed_fee is None else fixed_fee
+            if fixed_fee is None:
+                estimated_size =  len(inputs) * 180 + 80     # this assumes non-compressed keys
+                fee = self.fee * round(estimated_size/1024.)
+                if fee == 0: fee = self.fee
+            else:
+                fee = fixed_fee
             if total >= amount + fee: break
         else:
-            #print "not enough funds: %s %s"%(format_satoshis(total), format_satoshis(fee))
             inputs = []
 
         return inputs, total, fee
 
+
+
     def add_tx_change( self, outputs, amount, fee, total, change_addr=None ):
         change_amount = total - ( amount + fee )
         if change_amount != 0: