The Safelincs Door Gap Gauge is the perfect tool designed with clearly marked dark blue “Recommended Door Gap” area and red “Door Gap Not Recommended” area can quickly tell at a glance if the gaps around the fire door are within acceptable fire spread preventing range. The gaps between your fire door and its frame need to be 2 to 4mm in width to ensure that both the door can close smoothly and that fire cannot spread through the gap of the fire door as well as smoke. Lightweight and durable plastic body Angular construction ensures no-fuss, intuitive use regardless of architrave Simple insert-and-glance design with clearly marked "Recommended Door Gap" area Compact design for easy storage and transport – fits in your pocket! Dimensions: 70 x 40 x 2-9 mm