What size flue liner do I need for a wood burning stove?

For a wood burning stove up to 20kW output, you need a minimum 150mm (6 inch) diameter flue liner per UK Building Regulations Approved Document J. For stoves over 20kW, a 200mm (8 inch) liner is required. Some stove manufacturers allow a 125mm (5 inch) liner for small stoves under 5kW, but always check the stove installation manual as the manufacturer's specification takes priority.

How long should my flue liner be?

Your flue liner needs to run from the stove connection at the base of the chimney to the top of the chimney pot, plus extra length for any bends. Add approximately 0.5m for each 45-degree bend and 1.0m for each 90-degree bend. The total flue height from stove to terminal should be at least 4.5m for adequate draught. Buy liner in one continuous length where possible.

Do I need to insulate a flue liner?

Yes, insulating a flue liner is strongly recommended and often required by Building Control. Insulation (usually vermiculite granules poured around the liner in the chimney void) keeps flue gases hot, improving draught and reducing condensation and tar build-up. One 100-litre bag of vermiculite typically covers about 2m of liner in a standard 225mm chimney.

What is a register plate and do I need one?

A register plate is a metal plate that seals across the fireplace opening just above the stove, through which the flue liner passes. It is essential for all stove installations — it prevents soot, debris and cold air from falling into the room, and ensures all combustion gases go up through the liner. It must be made from galvanised steel or stainless steel, minimum 1.5mm thick.

How high must a flue terminate above the roof?

Per Approved Document J (Diagram 25), a flue must terminate at least 600mm above the ridge if it passes through the roof within 2.3m horizontal distance of the ridge. If more than 2.3m from the ridge, it must be at least 1m above the highest point of the roof within 2.3m. The flue terminal (cowl) must also be at least 1m above any openable window or ventilation opening within 2.3m.

Flue Liner Calculator — What Size Flue Liner Do I Need?

Calculate the correct flue liner diameter, length, and insulation for wood burners, multi-fuel stoves and gas fires per UK Building Regs Approved Document J.

Chimney Height

From stove connection to chimney top

Stove Output

Nominal heat output — check stove spec sheet

Fuel Type

Determines minimum liner diameter

Existing Chimney Internal Diameter

Internal width of chimney void. Standard: 225mm

Bends in Flue Route

45-degree Bends

Each adds ~0.5m to liner length

90-degree Bends

Each adds ~1.0m to liner length. Avoid if possible.

Cowl Type

Rain cap is standard; anti-downdraught for exposed locations

Include vermiculite insulation

Recommended — improves draught and reduces condensation

Price Per Metre of Liner (optional)

Enter price per metre for a liner cost estimate

How We Calculate This

This calculator determines the correct flue liner diameter based on your stove output and fuel type, then calculates the total liner length needed (including allowance for bends), and estimates vermiculite insulation bags.

Flue liner sizing (Approved Document J)

Wood/multi-fuel up to 20kW: minimum 150mm (6") diameter
Gas fire/stove: minimum 125mm (5") diameter
Over 20kW: minimum 200mm (8") diameter

Liner length calculation

Total liner length = Chimney height + Bend allowances

Each 45-degree bend adds approximately 0.5m
Each 90-degree bend adds approximately 1.0m
Maximum 4 bends allowed per Approved Document J

Vermiculite insulation

Vermiculite is poured into the void between the flue liner and the chimney wall. The volume is calculated from the chimney internal diameter, liner external diameter, and chimney height. One 100-litre bag covers approximately 2m of liner in a standard 225mm internal chimney.

Chimney height requirements

Minimum 4.5m total flue height for adequate draught. The flue must terminate at least 600mm above the ridge if within 2.3m, or 1m above any roof surface within 2.3m (Approved Document J, Diagram 25).

Standards

UK Building Regulations Approved Document J (Combustion appliances and fuel storage systems). Flue liners per BS EN 1856-2 (metal liners). Flue system design per BS EN 15287.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.