What flue diameter does my stove need?

Approved Document J specifies minimum flue diameters: up to 5 kW = 125mm, 5-8 kW = 150mm, 8-15 kW = 175mm, over 15 kW = 200mm. Some stove manufacturers may specify a larger flue — always check the stove installation manual. Never reduce the flue below the stove outlet size.

How high does a stove flue need to be?

Approved Document J requires a minimum effective flue height of 4.5 metres (measured from the top of the stove to the top of the flue terminal). The flue must terminate at least 1 metre above the highest point of the roof within 2.3 metres horizontally. Taller flues provide better draw.

Do I need a vent for a wood burning stove?

Stoves up to 5 kW rated output generally don’t need additional room ventilation if the room has normal background ventilation. Above 5 kW, Approved Document J requires 550 mm² of permanent free ventilation area per kW above 5 kW. A direct air supply to the stove may be used instead.

Twin wall flue or flexible liner — which should I choose?

Flexible liner (316 or 316L stainless steel) is used when an existing chimney is available but needs relining. Twin wall insulated flue is used when there’s no existing chimney. Twin wall provides better insulation (better draft) but costs more. Both must comply with Document J.

Do I need a carbon monoxide alarm?

Building Regulations require a carbon monoxide (CO) alarm in any room where a solid fuel burning appliance is installed. The alarm must be ceiling-mounted (or high on a wall) and comply with BS EN 50291. This is mandatory for all new installations and strongly recommended for existing ones.

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Stove Flue Calculator — Approved Document J

Calculate flue diameter from stove kW, height for adequate draw, twin wall vs liner selection, and ventilation air requirements per Approved Document J.

Stove Output

Stove rated heat output

Stove Type

Fuel type

Flue Type

Flue system type

Flue Height

Effective flue height (stove top to terminal)

Room Volume

Room volume where stove is installed

Direct Air Supply

Stove connected to external air supply?

How We Calculate This

This calculator sizes stove flues in accordance with Approved Document J (Combustion Appliances and Fuel Storage Systems). It determines the minimum flue diameter, height for adequate draw, and ventilation requirements.

Document J rules

Flue diameter: Minimum based on kW output
Flue height: Minimum 4.5m, 1m above ridge
Draft: ~3 Pa per metre of effective height
Ventilation: 550 mm² per kW above 5 kW
CO alarm: Mandatory for all solid fuel appliances

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.