diff --git a/cara/apps/calculator/templates/calculator.form.html.j2 b/cara/apps/calculator/templates/calculator.form.html.j2
index 0e865235..24fcc42d 100644
--- a/cara/apps/calculator/templates/calculator.form.html.j2
+++ b/cara/apps/calculator/templates/calculator.form.html.j2
@@ -75,7 +75,7 @@
     <label for="hepa_no">No</label>
     <input type="radio" id="hepa_yes" name="hepa_option" value=1 onclick="require_fields(this)">
     <label for="hepa_yes">Yes</label>
-    <input type="number" step=0.01 id="hepa_amount" name="hepa_amount" placeholder="(m³ / hour)">
+    <input type="number" step=0.01 id="hepa_amount" name="hepa_amount" placeholder="(m³ / hour)" min="0">
     <hr width="80%">
 
     <b>Face masks:</b><br>