diff --git a/CHANGELOG b/CHANGELOG
index 1e843ecdb7..5ee6de1d83 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,4 @@
+* Change probability into conversion probability
 * Manage readonly state on Opportunity Line
 
 Version 4.0.0 - 2016-05-02
diff --git a/opportunity.py b/opportunity.py
index beb2b855ba..b577073c03 100644
--- a/opportunity.py
+++ b/opportunity.py
@@ -8,7 +8,7 @@ from sql.aggregate import Max, Count, Sum
 from sql.conditionals import Case
 from sql.functions import Extract
 
-from trytond.model import ModelView, ModelSQL, Workflow, fields, Check
+from trytond.model import ModelView, ModelSQL, Workflow, fields
 from trytond import backend
 from trytond.pyson import Eval, In, If, Get, Bool
 from trytond.transaction import Transaction
@@ -85,7 +85,12 @@ class SaleOpportunity(Workflow, ModelSQL, ModelView):
         states=_STATES_STOP, depends=_DEPENDS_STOP)
     state = fields.Selection(STATES, 'State', required=True, select=True,
             sort=False, readonly=True)
-    probability = fields.Integer('Conversion Probability', required=True,
+    conversion_probability = fields.Float('Conversion Probability',
+        digits=(1, 4), required=True,
+        domain=[
+            ('conversion_probability', '>=', 0),
+            ('conversion_probability', '<=', 1),
+            ],
         states={
             'readonly': ~Eval('state').in_(
                 ['opportunity', 'lead', 'converted']),
@@ -138,16 +143,18 @@ class SaleOpportunity(Workflow, ModelSQL, ModelView):
                         where=sale.id == sale_id))
             table.drop_column('sale', exception=True)
 
+        # Migration from 4.0: change probability into conversion probability
+        if table.column_exist('probability'):
+            cursor.execute(*sql_table.update(
+                    [sql_table.conversion_probability],
+                    [sql_table.probability / 100.0]))
+            table.drop_column('probability')
+            table.drop_constraint('check_percentage')
+
     @classmethod
     def __setup__(cls):
         super(SaleOpportunity, cls).__setup__()
         cls._order.insert(0, ('start_date', 'DESC'))
-        t = cls.__table__()
-        cls._sql_constraints += [
-            ('check_percentage',
-                Check(t, (t.probability >= 0) & (t.probability <= 100)),
-                'Probability must be between 0 and 100.')
-            ]
         cls._error_messages.update({
                 'delete_cancel': ('Sale Opportunity "%s" must be cancelled '
                     'before deletion.'),
@@ -199,8 +206,8 @@ class SaleOpportunity(Workflow, ModelSQL, ModelView):
         return Date.today()
 
     @staticmethod
-    def default_probability():
-        return 50
+    def default_conversion_probability():
+        return 0.5
 
     @staticmethod
     def default_company():
diff --git a/view/opportunity_form.xml b/view/opportunity_form.xml
index dab6448e4f..f3e82e3b44 100644
--- a/view/opportunity_form.xml
+++ b/view/opportunity_form.xml
@@ -17,8 +17,12 @@ this repository contains the full copyright notices and license terms. -->
         <field name="amount"/>
         <field name="currency"/>
     </group>
-    <label name="probability"/>
-    <field name="probability"/>
+    <label name="conversion_probability"/>
+    <group id="conversion_probability">
+        <field name="conversion_probability" factor="100" xexpand="0"/>
+        <label name="conversion_probability" string="%"
+            xalign="0.0" xexpand="1"/>
+    </group>
     <notebook colspan="4">
         <page string="Lead/Opportunity" id="opportunity">
             <label name="company"/>
diff --git a/view/opportunity_tree.xml b/view/opportunity_tree.xml
index 14f57f3db0..900f6d9b14 100644
--- a/view/opportunity_tree.xml
+++ b/view/opportunity_tree.xml
@@ -11,5 +11,7 @@ this repository contains the full copyright notices and license terms. -->
     <field name="amount"/>
     <field name="currency"/>
     <field name="state"/>
-    <field name="probability"/>
+    <field name="conversion_probability" factor="100">
+        <suffix name="conversion_probability" string="%"/>
+    </field>
 </tree>