Chimney Flue Size Calculator
Size a chimney flue to NFPA 211 from the appliance collar or the fireplace opening, including the maximum area that is allowed and the 3-2-10 height rule.
How to use this calculator
- 1Take the collar diameter off the appliance itself, or from its manual. The manual always wins over any general rule.
- 2For a masonry fireplace, measure the opening and pick the flue shape you actually have.
- 3Be honest about whether the chimney has an exterior wall below the roofline - it halves the allowable oversizing.
- 4Check the height rule as well as the area. A correctly sized flue that terminates too low still backdrafts.
How the calculation works
Flue area >= appliance collar area
Flue area <= 3 x collar (masonry, no exterior walls below the roofline)
Flue area <= 2 x collar (masonry, one or more exterior walls)
Fireplace: flue area >= opening / 10 round, opening / 8 rectangular
Height above roof = max(3 ft, rise of the roof within 10 ft + 2 ft)- Collar area
- Cross-sectional area of the appliance outlet - pi times the radius squared for a round collar
- 3x and 2x
- The NFPA 211 maximums for masonry chimneys. Exterior chimneys cool flue gas faster, so they get half the allowance
- 1/10 and 1/8
- Fireplace flue area as a fraction of the opening. Rectangular flues need more because their corners sit outside the rotating gas column
- 3-2-10
- At least 3 ft above the roof penetration, and at least 2 ft above anything within 10 ft horizontally
The maximum is the unfamiliar half of the rule and the one that matters when a modern stove is connected to an old chimney. Draft depends on the flue gas staying hot, and an oversized flue is a very effective heat exchanger in exactly the wrong direction.
The 3-2-10 rule is about wind rather than about smoke rising. A chimney terminating below the pressure shadow of its own roof can have wind driving down it, which pushes combustion products into the house.
Round outperforms rectangular of the same area, which is why liner sizing is quoted in diameters and why relining a square clay flue with a round liner often improves draft even though it reduces the area.
Worked example
A 6 in wood stove on an interior masonry chimney
- 1.A 6 in collar has an area of pi x 3^2 = 28.3 square inches, which is the minimum.
- 2.The chimney is masonry with no exterior walls below the roofline, so NFPA 211 allows up to three times that: 84.8 square inches.
- 3.That is a range of 6 in to 10.4 in of round liner, so a 6, 7 or 8 in liner all qualify - and 6 in, matching the collar, is the one that keeps the gas hottest and fastest.
- 4.The ridge is 6 ft away horizontally on a 6/12 roof, so it stands 3 ft above the penetration; the chimney must clear it by 2 ft, giving 5 ft above the roof.
Result: 6 in liner, standing 5 ft above the roof
The same stove on an exterior chimney
- 1.Same 28.3 square inch minimum from the collar.
- 2.But the chimney has a wall exposed to outdoor air, so the maximum falls from three times to twice: 56.5 square inches.
- 3.The 8 in liner at 50.3 sq in still fits under the lower cap, but a 10 in at 78.5 sq in is now firmly out.
- 4.Exterior chimneys also start every fire cold, which is why they backdraft on lighting far more than interior ones and why priming the flue first is worth doing.
Result: The allowance halves - and the practical answer is still 6 in
A 36 by 29 in masonry fireplace
- 1.The opening is 36 x 29 = 1,044 square inches.
- 2.For a round flue NFPA 211 wants at least a tenth of that: 104.4 square inches, which is a 11.5 in diameter - so a 12 in liner.
- 3.A rectangular flue would need an eighth instead, 130.5 square inches, because the corners of a square flue do not participate in the rising gas column.
- 4.The ridge is 14 ft away, so the 10 ft limit governs: at 8/12 the roof rises 6.7 ft over 10 ft, and the chimney must stand 2 ft above that - 8.7 ft above the penetration.
Result: 12 in round liner, and a chimney standing 8.7 ft above the roof
Draft is a temperature problem
A chimney works because the column of gas inside it is hotter, and therefore lighter, than the column of outside air of the same height. The difference in weight is what pulls combustion products up and out, and it is measured in hundredths of an inch of water column - a genuinely small force doing an important job.
Two things make that force larger: height, and temperature difference. That is the whole of it. A tall chimney drafts better than a short one, and a hot flue drafts better than a cool one, which is why the same stove behaves so differently on a cold still night and a mild damp evening.
Everything about flue sizing follows from the temperature half. Flue gas leaving an appliance has a fixed amount of heat in it, and how hot the column stays depends on how much surface it has to lose heat to and how fast it is travelling. A flue matched to the appliance keeps the gas fast and hot. An oversized one slows it, spreads it over more cold masonry, and cools it - and once the gas cools the draft weakens, which slows it further.
Why an oversized flue is a fire risk, not just a nuisance
Wood smoke carries unburned volatiles. While the flue gas is hot they stay in suspension and leave with it. As the gas cools below about 250 degrees Fahrenheit they condense onto whatever surface is nearest, which is the inside of the chimney, and what they form is creosote.
Creosote in its early stage is a light flaky soot that a brush removes. Left to build, and particularly where it is repeatedly re-heated and re-condensed, it turns into a hard shiny glaze that is very difficult to remove and burns fiercely. A chimney fire in a glazed flue reaches temperatures that crack clay tiles and can ignite framing through a chimney wall.
So an oversized flue is not merely inefficient. It cools gas that would otherwise have carried its volatiles away, and it does so continuously, every fire, all season. This is the single strongest argument for lining an old fireplace chimney down to size when a stove is installed in it, and it is why "the chimney is already there, we will just connect to it" is such a costly assumption.
- Matched to the collar — Hot, fast gas. Best draft, least deposit. Almost always the right answer.
- Up to 2 or 3 times — Permitted by NFPA 211 depending on exposure, and progressively worse as it grows.
- An unlined fireplace flue — Often eight or ten times a stove collar. Slow, cold, and a creosote factory.
- Too small — Also a fault - the appliance cannot exhaust, and it spills into the room. The manual is the authority.
The 3-2-10 rule and what it is really about
A chimney must terminate at least three feet above the point where it passes through the roof, and at least two feet above anything within ten feet of it horizontally. It is one of the most quoted rules in the trade and one of the least understood.
It is not about giving smoke room to rise. It is about wind. Air flowing over a roof separates and forms a region of turbulence and locally raised pressure on the downwind side of the ridge and in the lee of any obstruction. A chimney terminating inside that region can find itself with higher pressure at its top than at its bottom, and the flow reverses - which puts combustion products, including carbon monoxide, into the house.
The ten feet is a horizontal radius and it includes anything: the ridge, a dormer, an adjacent wing of the building, a neighbouring wall. On a steep roof with the chimney low on the slope the rule can demand a startlingly tall stack, and that is the correct answer rather than an over-reading of the code. Where a taller chimney is genuinely impractical, the answer is a listed wind-directional cap, not a shorter chimney.
What this assumes, and where it stops
Assumptions
- Figures follow NFPA 211 for solid fuel appliances. Manufacturer instructions for a listed appliance take precedence over any general rule.
- Flue areas are net free area - the actual opening, after any liner thickness.
- The maximum multiples assume masonry construction. Factory-built metal chimneys are sized by their own listing.
- The height calculation treats the roof as a plane rising uniformly from the chimney towards the ridge.
Limitations
- It sizes a flue. It says nothing about clearance to combustibles, connector pipe routing, hearth extension, or the condition of an existing chimney - all of which matter as much and several of which matter more.
- Gas and oil venting follows entirely different tables by appliance category, and condensing appliances must not vent into masonry at all. Use the appliance instructions and the fuel gas code.
- It does not model draft. Actual draft depends on height, temperature, altitude, house pressure and wind, and a house tight enough to starve the chimney of make-up air will backdraft a perfectly sized flue.
- Multiple appliances on one flue is a separate and mostly prohibited case, and where it is permitted it has its own sizing rules.
- A chimney that has had a fire in it needs a level 2 inspection with a camera before reuse, regardless of what any sizing calculation says.
Common questions
Can a chimney flue be too big?
Yes, and it is the failure most people do not anticipate. NFPA 211 caps a masonry flue at three times the appliance collar area, or twice where the chimney has an exterior wall. An oversized flue lets the gas slow and cool, which weakens the draft and condenses creosote inside the chimney. Connecting a modern stove to an unlined fireplace chimney is the classic case, and the fix is a liner sized to the appliance.
What size liner does a 6 inch wood stove need?
Six inches, in nearly every case. The collar area is the minimum and matching it keeps the flue gas hottest and fastest. Larger sizes are permitted up to the code multiple, but there is no benefit in using them and there is a real cost in draft and creosote. The one exception is where the appliance manual specifies otherwise, and the manual always wins.
How tall does my chimney have to be?
At least three feet above where it passes through the roof, and at least two feet above anything within ten feet of it horizontally - the 3-2-10 rule. On a steep roof with the chimney low on the slope that can be a tall stack. Separately, most wood stove manufacturers want at least fifteen feet from the appliance outlet to the top for adequate draft.
Why does my fireplace smoke into the room?
Most commonly the flue is too small for the opening - it wants at least a tenth of the opening area for a round flue, an eighth for rectangular. After that: a cold exterior chimney that has not been primed, a house too tight to supply make-up air, a chimney terminating too low so wind pushes down it, or a damper or smoke shelf obstruction. Reducing the opening height with a smoke guard fixes a surprising number of them.
Does a round flue really work better than a square one?
Yes. Flue gas rotates as it rises, and in a square or rectangular flue the corners sit outside that rotating column - they carry little flow, they cool the gas, and they collect deposits. That is why NFPA 211 asks for a larger fraction of the fireplace opening for rectangular flues, and why relining a square clay flue with a round liner often improves draft despite reducing the cross-sectional area.
Sources
- NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances — National Fire Protection Association
- Chimney and venting safety guidance — Chimney Safety Institute of America
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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