diff --git a/ihatemoney/forms.py b/ihatemoney/forms.py
index e8d437bf..8b7f6efd 100644
--- a/ihatemoney/forms.py
+++ b/ihatemoney/forms.py
@@ -8,6 +8,7 @@ from flask import request
 from werkzeug.security import generate_password_hash
 
 from datetime import datetime
+from re import match
 from jinja2 import Markup
 
 import email_validator
@@ -44,6 +45,28 @@ class CommaDecimalField(DecimalField):
         return super(CommaDecimalField, self).process_formdata(value)
 
 
+class CalculatorStringField(StringField):
+    """
+    A class to deal with math ops (+, -, *, /)
+    in StringField
+    """
+
+    def process_formdata(self, valuelist):
+        if valuelist:
+            error_msg = "Not a valid amount or expression"
+            value = str(valuelist[0]).replace(" ", "").replace(",", ".")
+
+            if not match(r'^[ 0-9\.\+\-\*/\(\)]{0,50}$', value) or "**" in value:
+                raise ValueError(error_msg)
+
+            try:
+                valuelist[0] = str(eval(value, {"__builtins__": None}, {}))
+            except (SyntaxError, NameError, TypeError, ZeroDivisionError):
+                raise ValueError(error_msg)
+
+        return super(CalculatorStringField, self).process_formdata(valuelist)
+
+
 class EditProjectForm(FlaskForm):
     name = StringField(_("Project name"), validators=[Required()])
     password = StringField(_("Private code"), validators=[Required()])
@@ -117,7 +140,7 @@ class BillForm(FlaskForm):
     date = DateField(_("Date"), validators=[Required()], default=datetime.now)
     what = StringField(_("What?"), validators=[Required()])
     payer = SelectField(_("Payer"), validators=[Required()], coerce=int)
-    amount = CommaDecimalField(_("Amount paid"), validators=[Required()])
+    amount = CalculatorStringField(_("Amount paid"), validators=[Required()])
     payed_for = SelectMultipleField(_("For whom?"),
                                     validators=[Required()], coerce=int)
     submit = SubmitField(_("Submit"))
diff --git a/ihatemoney/tests/tests.py b/ihatemoney/tests/tests.py
index 2f3d4ac2..d7af9d63 100644
--- a/ihatemoney/tests/tests.py
+++ b/ihatemoney/tests/tests.py
@@ -1349,6 +1349,69 @@ class APITestCase(IhatemoneyTestCase):
                               headers=self.get_auth("raclette"))
         self.assertStatus(404, req)
 
+    def test_bills_with_calculation(self):
+        # create a project
+        self.api_create("raclette")
+
+        # add members
+        self.api_add_member("raclette", "alexis")
+        self.api_add_member("raclette", "fred")
+
+        # add a bill
+        req = self.client.post("/api/projects/raclette/bills", data={
+            'date': '2011-08-10',
+            'what': 'fromage',
+            'payer': "1",
+            'payed_for': ["1", "2"],
+            'amount': '((100 + 200.25) * 2 - 100) / 2',
+        }, headers=self.get_auth("raclette"))
+
+        # should return the id
+        self.assertStatus(201, req)
+        self.assertEqual(req.data.decode('utf-8'), "1\n")
+
+        # get this bill details
+        req = self.client.get("/api/projects/raclette/bills/1",
+                              headers=self.get_auth("raclette"))
+
+        # compare with the added info
+        self.assertStatus(200, req)
+        expected = {
+            "what": "fromage",
+            "payer_id": 1,
+            "owers": [
+                {"activated": True, "id": 1, "name": "alexis", "weight": 1},
+                {"activated": True, "id": 2, "name": "fred", "weight": 1}],
+            "amount": 250.25,
+            "date": "2011-08-10",
+            "id": 1}
+
+        got = json.loads(req.data.decode('utf-8'))
+        self.assertEqual(
+            datetime.date.today(),
+            datetime.datetime.strptime(got["creation_date"], '%Y-%m-%d').date()
+        )
+        del got["creation_date"]
+        self.assertDictEqual(expected, got)
+
+        erroneous_amounts = [
+            "lambda ",  # letters
+            "(20 + 2",  # invalid expression
+            "20/0",  # invalid calc
+            "9999**99999999999999999",  # exponents
+            "2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2"  # greater than 50 chars
+        ]
+
+        for amount in erroneous_amounts:
+            req = self.client.post("/api/projects/raclette/bills", data={
+                'date': '2011-08-10',
+                'what': 'fromage',
+                'payer': "1",
+                'payed_for': ["1", "2"],
+                'amount': amount,
+            }, headers=self.get_auth("raclette"))
+            self.assertStatus(400, req)
+
     def test_statistics(self):
         # create a project
         self.api_create("raclette")