diff --git a/cara/apps/calculator/templates/calculator.form.html.j2 b/cara/apps/calculator/templates/calculator.form.html.j2
index 91c90c4a..1ed5ff3a 100644
--- a/cara/apps/calculator/templates/calculator.form.html.j2
+++ b/cara/apps/calculator/templates/calculator.form.html.j2
@@ -62,7 +62,7 @@
         <input type="radio" id="air_type_supply" name="mechanical_ventilation_type" value="air_supply" onclick="require_fields(this)">
         Air supply flow rate &nbsp;&nbsp; <input type="number" step="any" id="air_supply" class="non_zero" name="air_supply" min="0" placeholder="(m³ / hour)"><br>
         <input type="radio" id="air_type_changes" name="mechanical_ventilation_type" value="air_changes" onclick="require_fields(this)">
-        Air changes per hour &nbsp;&nbsp; <input type="number" step="any" id="air_changes" class="non_zero" name="air_changes" min="0"><br>
+        Air changes per hour &nbsp;&nbsp; <input type="number" step="any" id="air_changes" class="non_zero" name="air_changes" min="0" placeholder="(h⁻¹) only fresh air"><br>
     </div>
 
     <div id="DIVnatural_ventilation" style="display:none">