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

Per Approved Document J Table 2: a closed appliance (stove) burning any fuel up to 30 kW requires a minimum 150 mm (6") diameter flue, above 30 kW and up to 50 kW requires 175 mm (7"), and appliances above 50 kW must be sized by design to BS EN 13384-1 (AD J does not tabulate them). Gas fires/stoves need 125 mm (5"). A 125 mm (5") flue is allowed only for a Clean Air Act / DEFRA-exempt appliance up to 20 kW burning smokeless or low-volatiles fuel. In every case the flue must never be smaller than the appliance outlet collar, and the manufacturer's installation manual takes priority. Use our Stove Flue Calculator for complete flue system sizing.

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.

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. There is no fixed coverage figure — the number of bags depends on the chimney cross-section, its shape, and the liner size, so always work it out from the geometry rather than a flat rule of thumb. This calculator does that from the annular volume (chimney void area minus liner area, times height, plus a settling allowance). For a 150mm liner the coverage of a 100-litre bag ranges from roughly 4m per bag in a narrow 225mm round clay-lined flue down to only about 2m per bag in a wider 250mm square brick flue — square brick flues hold noticeably more vermiculite than round ones of the same width, so they need more bags per metre.

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. Approved Document J does not specify a plate thickness; as a practical rule it is made from galvanised or stainless steel, typically 1–2mm thick, and must be robust enough to support the weight of any vermiculite poured above it.

Per Approved Document J (Diagram 17, solid-fuel appliances), a flue must terminate at least 600mm above the ridge if it passes through the roof at or within 600mm of the ridge. Elsewhere on the roof it must be at least 2,300mm (2.3m) horizontally from the nearest point on the roof, and either at least 1,000mm above the highest point where the chimney meets the roof or at least as high as the ridge. The outlet must also be at least 1,000mm above the top of any openable rooflight, dormer window or other opening within 2.3m, and at least 600mm above any adjacent building within 2.3m.