Wood Burner Air Vent Calculator: What Size Vent Do You Need?

A wood burner needs enough fresh air to breathe, and Building Regulations set exactly how much. This calculator works out the free air area your installation needs and turns it into a practical vent size. The catch most people miss: the well known "nothing needed under 5kW" rule only applies to older, draughtier homes. In an airtight modern house even a small stove needs a vent, so the rating on the stove is only half the answer.

The figure above is the minimum free air area, meaning the actual open area for air to pass through, not the outside size of the grille. Vents and air bricks are sold with a stated free area, so pick one that meets or beats the number here. The vent must be permanently open and non-closable, sited so it will not be blocked by furniture, and ideally drawing from outside directly into the room with the stove.

These figures follow Approved Document J for England and Wales and cover a closed appliance (a stove), not an open fire, which is sized on its throat opening instead. Scotland and Northern Ireland have their own rules. Always confirm the final specification with your HETAS installer, as chimney draught, extractor fans and other combustion appliances in the same room can all increase what you need.