diff --git a/purchase.py b/purchase.py
index 145992f53c..9b9b9f1811 100644
--- a/purchase.py
+++ b/purchase.py
@@ -42,16 +42,17 @@ class Purchase(OSV):
             domain=[('type', '=', 'warehouse')], required=True, states=_STATES)
     currency = fields.Many2One('currency.currency', 'Currency', required=True,
         states=_STATES)
+    currency_digits = fields.Function('get_currency_digits', type='integer',
+            string='Currency Digits', on_change_with=['currency'])
     lines = fields.One2Many('purchase.line', 'purchase', 'Lines',
             states=_STATES)
     comment = fields.Text('Comment')
-    #TODO digits must depend of currency
     untaxed_amount = fields.Function('get_untaxed_amount', type='numeric',
-            digits=(16, 2), string='Untaxed')
+            digits="(16, currency_digits)", string='Untaxed')
     tax_amount = fields.Function('get_tax_amount', type='numeric',
-            digits=(16, 2), string='Tax')
+            digits="(16, currency_digits)", string='Tax')
     total_amount = fields.Function('get_total_amount', type='numeric',
-            digits=(16, 2), string='Total')
+            digits="(16, currency_digits)", string='Total')
     invoice_method = fields.Selection([
         ('manual', 'Manual'),
         ('order', 'From Order'),
@@ -160,6 +161,21 @@ class Purchase(OSV):
                     res['payment_term'], context=context)[0]
         return res
 
+    def on_change_with_currency_digits(self, cursor, user, ids, vals,
+            context=None):
+        currency_obj = self.pool.get('currency.currency')
+        if vals.get('currency'):
+            currency = currency_obj.browse(cursor, user, vals['currency'],
+                    context=context)
+            return currency.digits
+        return 2
+
+    def get_currency_digits(self, cursor, user, ids, name, arg, context=None):
+        res = {}
+        for purchase in self.browse(cursor, user, ids, context=context):
+            res[purchase.id] = purchase.currency.digits
+        return res
+
     def get_untaxed_amount(self, cursor, user, ids, name, arg, context=None):
         currency_obj = self.pool.get('currency.currency')
         res = {}
@@ -457,9 +473,11 @@ class PurchaseLine(OSV):
                 'required': "type == 'line'",
             })
     amount = fields.Function('get_amount', type='numeric', string='Amount',
+            digits="(16, parent.currency_digits)",
             states={
                 'invisible': "type not in ('line', 'subtotal')",
-            }, on_change_with=['type', 'quantity', 'unit_price'])
+            }, on_change_with=['type', 'quantity', 'unit_price',
+                'parent.currency'])
     description = fields.Char('Description', size=None, required=True)
     comment = fields.Text('Comment',
             states={
@@ -589,10 +607,16 @@ class PurchaseLine(OSV):
         return res
 
     def on_change_with_amount(self, cursor, user, ids, vals, context=None):
+        currency_obj = self.pool.get('currency.currency')
         if vals.get('type') == 'line':
-            #TODO add rounding
-            return Decimal(str(vals.get('quantity') or '0.0')) * \
-                    (vals.get('unit_price') or Decimal('0.0'))
+            if isinstance(vals.get('parent.currency'), (int, long)):
+                currency = currency_obj.browse(cursor, user,
+                        vals['parent.currency'], context=context)
+            else:
+                currency = vals['parent.currency']
+            return currency_obj.round(cursor, user, currency,
+                    Decimal(str(vals.get('quantity') or '0.0')) * \
+                    (vals.get('unit_price') or Decimal('0.0')))
         return Decimal('0.0')
 
     def get_amount(self, cursor, user, ids, name, arg, context=None):
diff --git a/purchase.xml b/purchase.xml
index f312d46f18..34507a7181 100644
--- a/purchase.xml
+++ b/purchase.xml
@@ -99,6 +99,7 @@
                             <field name="packings" colspan="4" widget="one2many"/>
                         </page>
                     </notebook>
+                    <field name="currency_digits" invisible="1"/>
                 </form>
                 ]]>
             </field>
@@ -120,6 +121,7 @@
                     <field name="invoice_state" select="2"/>
                     <field name="packing_state" select="2"/>
                     <field name="description" select="2"/>
+                    <field name="currency_digits" tree_invisible="1"/>
                 </tree>
                 ]]>
             </field>