diff --git a/cara/apps/calculator/static/js/form.js b/cara/apps/calculator/static/js/form.js
index 39625947..a15f403f 100644
--- a/cara/apps/calculator/static/js/form.js
+++ b/cara/apps/calculator/static/js/form.js
@@ -68,6 +68,12 @@ function require_fields(obj) {
     case "mask_off":
       require_mask(false);
       break;
+    case "hepa_yes":
+      require_hepa(true);
+      break;
+    case "hepa_no":
+      require_hepa(false);
+      break;
     default:
       break;
   }
@@ -135,6 +141,10 @@ function require_lunch(option) {
   $("#lunch_finish").prop('required', option);
 }
 
+function require_hepa(option) {
+  $("#hepa_amount").prop('required', option);
+}
+
 function setMaxInfectedPeople() {
   $("#infected_people").attr("max", $("#total_people").val());
 }
diff --git a/cara/apps/calculator/templates/calculator.form.html.j2 b/cara/apps/calculator/templates/calculator.form.html.j2
index b59b28c8..32e666fc 100644
--- a/cara/apps/calculator/templates/calculator.form.html.j2
+++ b/cara/apps/calculator/templates/calculator.form.html.j2
@@ -68,11 +68,11 @@ Beta v1.0.0 <span style="float:right; font-weight:bold">Please send feedback to
     </div>
     
     HEPA filtration:
-    <input type="radio" id="hepa_filter" name="hepa_option" value=1>
-    <label for="hepa_filter">Yes</label>
-    
-    <input type="radio" id="hepa_filter" name="hepa_option" value=0 checked="checked">
+    <input type="radio" id="hepa_no" name="hepa_option" value=0 onclick="require_fields(this)" checked="checked">
     <label for="hepa_filter">No</label>
+    <input type="radio" id="hepa_yes" name="hepa_option" value=1 onclick="require_fields(this)">
+    <label for="hepa_filter">Yes</label>
+    <input type="number" step=0.01 id="hepa_amount" name="hepa_amount" placeholder="(m³ / hour)">
     <hr width="80%">
 
     <b>Face masks:</b><br>