Flue Size Calculator
Choose the appliance, enter its output in kW or the fireplace opening, the stove's outlet size and the flue height. The calculator gives the minimum flue diameter and checks the height.
- Flue diameter
- Flue height
- Liner
The flue must never be smaller than the stove's outlet.
Most wood stoves up to 20 kW need a 150 mm (6 in) flue. An 8 kW wood burning stove needs at least a 150 mm flue, at least 4.5 m tall. The same stove on smokeless fuel only can use 125 mm, unless its outlet is bigger.
Flue size chart
Simplified from Approved Document J. Search for an appliance.
| Appliance | Minimum flue |
|---|---|
| Stove up to 20 kW, smokeless fuel | 125 mm |
| Stove up to 20 kW, wood or any fuel | 150 mm |
| Stove 20 to 30 kW | 150 mm |
| Stove 30 to 50 kW | 175 mm |
| Open fire up to 500 × 550 mm | 200 mm round or 175 mm square |
| Larger open fire | 15% of the opening area |
mm.
Stove output and flue
Wood stove. Slide the output.
The flue is drawn above the stove.
How to size a flue
Appliance to flue. Step through it.
Appliance
Type.
Sweep the flue at least once a year when burning wood.
Twin wall, liner or masonry
Flue types. Compare them.
New flue.
Old chimney.
Original.
An insulated flue stays warm and draws better.
Before you install
Flue checks. Tick each item.
Tick each item as you go.
Fit a carbon monoxide alarm in the room.
What you get
Flue sizing. Each tile explains one result.
mm and in.
Checked.
Type.
Equivalent.
Advice.
A flue that is too large cools the smoke and builds up tar.
Frequently Asked Questions
No question matches that search.
What size flue does a wood burner need?
At least one hundred and fifty millimetres for most stoves up to twenty kilowatts.
Can I use a five inch flue?
Only for stoves burning smokeless fuel, if the maker allows it.
How tall should a flue be?
At least four and a half metres from the appliance.
Can the flue be smaller than the stove outlet?
No. It must be at least the outlet size.
What size flue for an open fire?
About two hundred millimetres for openings up to five hundred by five hundred and fifty.
Do I need a flue liner?
Usually, if the old chimney is cracked or too large.
Who can fit a flue?
A HETAS or other certified installer in the UK.
A flue that draws
Size the flue to your appliance and smoke goes up cleanly every time you light it.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.