diff --git a/cara/apps/calculator/static/js/form.js b/cara/apps/calculator/static/js/form.js
index bc0612a9..f26cfa16 100644
--- a/cara/apps/calculator/static/js/form.js
+++ b/cara/apps/calculator/static/js/form.js
@@ -1,25 +1,16 @@
-function on_ventilation_type_change() {
-  ventilation_types = $('input[type=radio][name=ventilation_type]');
-  ventilation_types.each(function (index) {
-    if (this.checked) {
-      getChildElement($(this)).show();
-      require_fields(this);
-    } else {
-      getChildElement($(this)).hide();
-      unrequire_fields(this);
-      // Clear the inputs for this newly hidden child element.
-      getChildElement($(this)).find('input').not('input[type=radio]').val('');
-      getChildElement($(this)).find('input[type=radio]').prop("checked", false);
-      getChildElement($(this)).find('input').prop("required", false);
-    }
-  });
-}
-
+/* ------- HTML structure ------- */
 function getChildElement(elem) {
   // Get the element named in the given element's data-enables attribute.
   return $("#" + elem.data("enables"));
 }
 
+function insertSpanAfter(referenceNode, text) {
+  var element = document.createElement("span");
+  element.classList.add("red_text");
+  element.innerHTML = "  " + text; 
+  referenceNode.parentNode.insertBefore(element, referenceNode.nextSibling);
+}
+
 /* -------Required fields------- */
 function require_fields(obj) {
   switch ($(obj).attr('id')) {
@@ -92,39 +83,47 @@ function unrequire_fields(obj) {
 }
 
 function require_room_volume(option) {
-  $("#room_volume").prop('required', option);
+  require_nonzero_field("room_volume", option);
 }
 
 function require_room_dimensions(option) {
-  $("#floor_area").prop('required', option);
-  $("#ceiling_height").prop('required', option);
+  require_nonzero_field("floor_area", option);
+  require_nonzero_field("ceiling_height", option);
 }
 
 function require_mechanical_ventilation(option) {
   $("#air_type_changes").prop('required', option);
   $("#air_type_supply").prop('required', option);
+  if (!option) {
+    removeInvalid("air_changes");
+    removeInvalid("air_supply");
+  }
 }
 
 function require_natural_ventilation(option) {
-  $("#windows_number").prop('required', option);
-  $("#window_height").prop('required', option);
-  $("#opening_distance").prop('required', option);
+  require_nonzero_field("windows_number", option);
+  require_nonzero_field("window_height", option);
+  require_nonzero_field("opening_distance", option);
   $("#always").prop('required', option);
   $("#interval").prop('required', option);
 }
 
 function require_air_changes(option) {
-  $("#air_changes").prop('required', option);
+  require_nonzero_field("air_changes", option);
 }
 
 function require_air_supply(option) {
-  $("#air_supply").prop('required', option);
+  require_nonzero_field("air_supply", option);
 }
 
 function require_single_event(option) {
-  $("#single_event_date").prop('required', option);
+  require_nonzero_field("single_event_date", option);
+}
+
+function require_nonzero_field(id, option) {
+  $("#"+id).prop('required', option);
   if (!option)
-    removeInvalidDate();
+    removeInvalid(id);
 }
 
 function require_recurrent_event(option) {
@@ -156,16 +155,14 @@ function require_mask(option) {
 }
 
 function require_hepa(option) {
-  $("#hepa_amount").prop('required', option);
+  require_nonzero_field("hepa_amount", option);
 }
 
 function setMaxInfectedPeople() {
-
   $("#training_limit_error").hide();
   var max = $("#total_people").val()
 
-  if ($("#activity_type").val() === "training")
-  {
+  if ($("#activity_type").val() === "training") {
     max = 1;
     $("#training_limit_error").show();
   }
@@ -173,6 +170,32 @@ function setMaxInfectedPeople() {
   $("#infected_people").attr("max", max);
 }
 
+function removeInvalid(id) {
+  var obj = document.getElementById(id)
+  if (obj.classList.contains("red_border")) {
+    obj.value = "";
+    $(obj).removeClass("red_border");
+    $(obj).next('span').remove();
+  }
+}
+
+function on_ventilation_type_change() {
+  ventilation_types = $('input[type=radio][name=ventilation_type]');
+  ventilation_types.each(function (index) {
+    if (this.checked) {
+      getChildElement($(this)).show();
+      require_fields(this);
+    } else {
+      getChildElement($(this)).hide();
+      unrequire_fields(this);
+      // Clear the inputs for this newly hidden child element.
+      getChildElement($(this)).find('input').not('input[type=radio]').val('');
+      getChildElement($(this)).find('input[type=radio]').prop("checked", false);
+      getChildElement($(this)).find('input').prop("required", false);
+    }
+  });
+}
+
 /* -------UI------- */
 $(function () {
   $(".datepicker").datepicker({
@@ -198,9 +221,14 @@ function show_disclaimer() {
 
 /* -------Form validation------- */
 function validate_form(form) {
-
   var submit = true;
 
+  //Validate all non zero values
+  $("input[required].non_zero").each(function() {
+    if (!validateValue(this))
+      submit = false;
+  });
+
   //Validate all dates
   $("input[required].datepicker").each(function() {
     if (!validateDate(this))
@@ -216,28 +244,58 @@ function validate_form(form) {
   return submit;
 }
 
-function validateDate(obj) {
+function validateValue(obj) {
   $(obj).removeClass("red_border");
-  $(obj).next().hide();
+  $(obj).next('span').remove();
 
-  var fromDate = $(obj).val();
-  if (!isValidDateOrEmpty(fromDate)) {
+  if (!isNonZeroOrEmpty($(obj).val())) {
     $(obj).addClass("red_border");
-    $(obj).next().show();
+    insertSpanAfter(obj, "Value must be > 0");
     return false;
   }
   return true;
 }
 
+function isNonZeroOrEmpty(value) {
+  if (value === "") return true;
+  if (value == 0) 
+    return false;
+  return true;
+}
+
+function validateDate(obj) {
+  $(obj).removeClass("red_border");
+  $(obj).next('span').remove();
+
+  if (!isValidDateOrEmpty($(obj).val())) {
+    $(obj).addClass("red_border");
+    insertSpanAfter(obj, "Incorrect date format");
+    return false;
+  }
+  return true;
+}
+
+function isValidDateOrEmpty(date) {
+  if (date === "") return true;
+  var matches = /^(\d+)[-\/](\d+)[-\/](\d+)$/.exec(date);
+  if (matches == null) return false;
+  var d = matches[1];
+  var m = matches[2];
+  var y = matches[3];
+  if (y > 2100 || y < 1900) return false;
+  var composedDate = new Date(y + '/' + m + '/' + d);
+  return composedDate.getDate() == d && composedDate.getMonth() + 1 == m && composedDate.getFullYear() == y;
+}
+
 function validateFinishTime(obj) {
   $(obj).removeClass("red_border");
-  $(obj).next().hide();
+  $(obj).next('span').remove();
 
   var startTime = parseValToNumber($(obj).prev().val());
   var finishTime = parseValToNumber(obj.value);
   if (startTime > finishTime) {
     $(obj).addClass("red_border");
-    $(obj).next().show();
+    insertSpanAfter(obj, "Finish time must be after start");
     return false;
   }
   return true;
@@ -256,30 +314,8 @@ function validateStartTime() {
   }
 }
 
-function removeInvalidDate() {
-  var single_event_date = document.getElementById("single_event_date");
-  if (single_event_date.classList.contains("red_border"))
-  {
-    single_event_date.value = "";
-    $(single_event_date).next().hide();
-    $(single_event_date).removeClass("red_border");
-  }
-}
-
-function isValidDateOrEmpty(date) {
-  if (date === "") return true;
-  var matches = /^(\d+)[-\/](\d+)[-\/](\d+)$/.exec(date);
-  if (matches == null) return false;
-  var d = matches[1];
-  var m = matches[2];
-  var y = matches[3];
-  if (y > 2100 || y < 1900) return false;
-  var composedDate = new Date(y + '/' + m + '/' + d);
-  return composedDate.getDate() == d && composedDate.getMonth() + 1 == m && composedDate.getFullYear() == y;
-}
-
 function parseValToNumber(val) {
-    return parseInt(val.replace(':',''), 10);
+  return parseInt(val.replace(':',''), 10);
 }
 
 /* -------On Load------- */
@@ -303,6 +339,10 @@ $(document).ready(function () {
   $("#total_people").change(setMaxInfectedPeople);
   $("#activity_type").change(setMaxInfectedPeople);
 
+  //Validate all non zero values
+  $("input[required].non_zero").each(function() {validateValue(this)});
+  $(".non_zero").change(function() {validateValue(this)});
+
   //Validate all dates
   $("input[required].datepicker").each(function() {validateDate(this)});
   $(".datepicker").change(function() {validateDate(this)});
diff --git a/cara/apps/calculator/templates/calculator.form.html.j2 b/cara/apps/calculator/templates/calculator.form.html.j2
index a28fb5a8..91c90c4a 100644
--- a/cara/apps/calculator/templates/calculator.form.html.j2
+++ b/cara/apps/calculator/templates/calculator.form.html.j2
@@ -42,10 +42,10 @@
         <span class="tooltip_text">?</span>
     </div><br>
     <input type="radio" id="room_type_volume" name="volume_type" value="room_volume" onclick="require_fields(this)" required>
-    Room volume: &nbsp;&nbsp; <input type="number" step=0.01 id="room_volume" name="room_volume" placeholder="Room volume (m³)" min="0.01"><br>
+    Room volume: &nbsp;&nbsp; <input type="number" step="any" id="room_volume" class="non_zero" name="room_volume" placeholder="Room volume (m³)" min="0"><br>
     <input type="radio" id="room_type_dimensions" name="volume_type" value="room_dimensions" onclick="require_fields(this)" required>
-    Floor area: &nbsp;&nbsp; <input type="number" step=0.01 id="floor_area" name="floor_area" placeholder="Room floor area (m²)" min="0.01"><br>
-    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Ceiling height: &nbsp;&nbsp; <input type="number" step=0.01 id="ceiling_height" name="ceiling_height" placeholder="Room ceiling height (m²)" min="0.01"><br>
+    Floor area: &nbsp;&nbsp; <input type="number" step="any" id="floor_area" class="non_zero" name="floor_area" placeholder="Room floor area (m²)" min="0"><br>
+    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Ceiling height: &nbsp;&nbsp; <input type="number" step="any" id="ceiling_height" class="non_zero" name="ceiling_height" placeholder="Room ceiling height (m²)" min="0"><br>
     <hr width="80%">
 
     <!-- Ventilation Options -->
@@ -60,15 +60,15 @@
 
     <div id="DIVmechanical_ventilation" style="display:none">
         <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=0.01 id="air_supply" name="air_supply" min="0" placeholder="(m³ / hour)"><br>
+        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=0.01 id="air_changes" 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"><br>
     </div>
 
     <div id="DIVnatural_ventilation" style="display:none">
-        Number of windows: <input type="number" id="windows_number" name="windows_number" min="1"><br>
-        Height of window: <input type="number" step=0.01 id="window_height" name="window_height" placeholder="meters" min="0"><br>
-        Opening distance: <input type="number" step=0.01 id="opening_distance"  name="opening_distance" placeholder="meters" min="0"><br>
+        Number of windows: <input type="number" id="windows_number" class="non_zero" name="windows_number" min="1"><br>
+        Height of window: <input type="number" step="any" id="window_height" class="non_zero" name="window_height" placeholder="meters" min="0"><br>
+        Opening distance: <input type="number" step="any" id="opening_distance" class="non_zero" name="opening_distance" placeholder="meters" min="0"><br>
         Windows open: <input type="radio" id="always" name="windows_open" value="always">
                       <label for="always">Always</label>
                       <input type="radio" id="interval" name="windows_open" value="interval">
@@ -81,7 +81,7 @@
     <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)" min="0">
+    <input type="number" step="any" id="hepa_amount" class="non_zero" name="hepa_amount" placeholder="(m³ / hour)" min="0">
     <hr width="80%">
 
     <b>Face masks:</b>
@@ -106,7 +106,7 @@
     </div><br>
     Total number of occupants: <input type="number" id="total_people" name="total_people" min=1 required><br>
     Number of infected people: <input type="number" id="infected_people" name="infected_people" min=1 required><br>
-    <span id="training_limit_error" class="red_text" hidden>Training activities limited to 1 infected</span><br>
+    <span id="training_limit_error" class="red_text" hidden>Training activities limited to 1 infected<br></span>
     <hr width="80%">
 
     Activity type: <select id="activity_type" name="activity_type">
@@ -119,12 +119,10 @@
                     <option value="gym">Gym</option>
                   </select><br>
     Start: <input type="time" id="activity_start" class="start_time" name="activity_start" value="09:00" required> &nbsp;&nbsp;
-    Finish: <input type="time" id="activity_finish" class="finish_time" name="activity_finish" value="18:00" required>
-    <span id="activity_time_error" class="red_text" hidden>Finish time must be after start</span><br>
+    Finish: <input type="time" id="activity_finish" class="finish_time" name="activity_finish" value="18:00" required><br>
     Infected person(s) presence: <br>
     Start: <input type="time" id="infected_start" class="start_time" name="infected_start" value="09:00" required> &nbsp;&nbsp;
-    Finish: <input type="time" id="infected_finish" class="finish_time" name="infected_finish" value="18:00" required>
-    <span id="infected_time_error" class="red_text" hidden>Finish time must be after start</span><br>
+    Finish: <input type="time" id="infected_finish" class="finish_time" name="infected_finish" value="18:00" required><br>
     <hr width="80%">
 
     When is the event?
@@ -135,8 +133,7 @@
 
     <label for="event_type_single">Single event</label> &nbsp;&nbsp;
     <label for="single_event_date">Date: </label>
-    <input type="text" id="single_event_date" class="datepicker" name="single_event_date" placeholder="dd/mm/yyyy">
-    <span id="event_time_error" class="red_text" hidden> Incorrect date format</span><br>
+    <input type="text" id="single_event_date" class="datepicker" name="single_event_date" placeholder="dd/mm/yyyy"><br>
     <input type="radio" id="event_type_recurrent" name="event_type" value="recurrent_event" onclick="require_fields(this)" required>
     <label for="event_type_recurrent">Recurrent usage</label>
     <select id="recurrent_event_month" name="recurrent_event_month">
@@ -165,8 +162,7 @@
     
     <div id="DIVlunch_break">
         Start: <input type="time" id="lunch_start" class="start_time" name="lunch_start" value="12:30" required> &nbsp;&nbsp;
-        Finish: <input type="time" id="lunch_finish" class="finish_time" name="lunch_finish" value="13:30" required>
-        <span id="lunch_time_error" class="red_text" hidden>Finish time must be after start</span><br>
+        Finish: <input type="time" id="lunch_finish" class="finish_time" name="lunch_finish" value="13:30" required><br>
     </div>
 
     <!-- Coffee Options -->