diff --git a/lung_cancer_screening/questions/forms/metric_weight_form.py b/lung_cancer_screening/questions/forms/metric_weight_form.py
index 8b4a8199..dd7d2fea 100644
--- a/lung_cancer_screening/questions/forms/metric_weight_form.py
+++ b/lung_cancer_screening/questions/forms/metric_weight_form.py
@@ -11,14 +11,17 @@ def __init__(self, *args, **kwargs):
         self.instance.participant = self.participant
 
         self.fields["weight_metric"] = DecimalField(
+            decimal_places=1,
             label="Kilograms",
             classes="nhsuk-input--width-4",
             required=True,
             error_messages={
                 'required': 'Enter your weight',
+                'max_decimal_places': 'Kilograms must be to 1 decimal place, for example 90.2kgs',
             },
             suffix="kg"
         )
+
     def clean_weight_metric(self):
         return self.cleaned_data['weight_metric'] * 10
 
diff --git a/lung_cancer_screening/questions/tests/unit/forms/test_metric_weight_form.py b/lung_cancer_screening/questions/tests/unit/forms/test_metric_weight_form.py
index d4d89a7e..49b1cdfc 100644
--- a/lung_cancer_screening/questions/tests/unit/forms/test_metric_weight_form.py
+++ b/lung_cancer_screening/questions/tests/unit/forms/test_metric_weight_form.py
@@ -100,3 +100,17 @@ def test_accepts_maximum_valid_weight(self):
             }
         )
         self.assertTrue(form.is_valid())
+
+    def test_is_invalid_with_multiple_decimal_places(self):
+        form = MetricWeightForm(
+            participant=self.participant,
+            instance=self.response_set,
+            data={
+                "weight_metric": "100.01"  # too many decimal places
+            }
+        )
+        self.assertFalse(form.is_valid())
+        self.assertEqual(
+            form.errors["weight_metric"],
+            ["Kilograms must be to 1 decimal place, for example 90.2kgs"]
+        )