Fireplace Flue Size Calculator
Measure the opening, run the 1/10 rule, and get the round or rectangular flue your fireplace has to have — rounded up to a real liner size.
1 Enter your numbers
The honest answer is the 1/10 rule: a round flue must be at least 1/12 of the fireplace opening, a rectangular one at least 1/10. Your 30 × 29 opening is 870 in², so the round flue needs about 72.5 in² (a 9.6" hole) — round UP to a 10" round liner or the nearest clay tile so the fireplace drafts. Always round up, never down.
Here is the honest version most quotes skip: a fireplace flue is sized off the opening, not off whatever tile happens to be in the chimney. Multiply the opening width by its height to get the area in square inches, then a round flue needs to be at least one-twelfth of that and a rectangular flue at least one-tenth. That is the whole trade rule. An undersized flue smokes into the room; a wildly oversized one runs cool and cakes with creosote.
Punch in your two measurements and the tool returns the minimum round area, the exact bore diameter, the rectangular alternative, and — the part you actually order — the next standard round liner up. Always up, never down: rounding a 9.6″ result down to an 8″ liner is how a good fireplace gets a smoke problem.
Formula
Two measurements, one ratio each:
- Opening area
Ao = width × height(in²) - Round flue area
Around ≥ Ao ÷ 12 - Round bore
d = 2 × √(Around ÷ π), rounded up to a standard liner - Rectangular flue area
Arect ≥ Ao ÷ 10
The 1/12 and 1/10 divisors are the long-standing masonry flue rule; π ties a round area back to a buyable diameter.
Worked example
Take a slightly tall opening: 34″ wide × 27.5″ high. That is 34 × 27.5 = 935 in² of opening. A round flue must clear 935 ÷ 12 = 77.9 in², which is a bore of 2√(77.9 ÷ π) = 9.96″ — so you order a 10″ round liner, not the 8″ that was probably there. If you go rectangular instead, you need 935 ÷ 10 = 93.5 in², i.e. a nominal 12×12 clay tile. Same opening, two legal answers.
Measure the opening, size up, never down
Measure the clear opening, not the surround. The number that matters is the firebox mouth the flames see — brick to brick, hearth to lintel — not the decorative stone face around it. Common misses from the field:
- Using the existing tile size as the target. That is backwards; the opening sets the flue.
- Forgetting a set of glass doors changes the effective opening — size to the open condition.
- Rounding the diameter down “to save on liner.” Round up or the fireplace will spill smoke.
Reference table
The signature reference: required flue area and the nearest standard liner or clay tile for common fireplace openings, straight from the 1/10–1/12 rule. Labeled planning values — confirm your own width and height and round up.
| Opening (W×H) | Opening area | Round flue ≥ | Round dia. | Rect. flue ≥ | Nearest standard flue |
|---|---|---|---|---|---|
| 24 × 24 in | 576 in² | 48.0 in² | 8″ | 57.6 in² | 8" round / 8x12 tile |
| 30 × 29 in | 870 in² | 72.5 in² | 10″ | 87.0 in² | 10" round / 12x12 tile |
| 36 × 30 in | 1,080 in² | 90.0 in² | 12″ | 108.0 in² | 12" round / 12x16 tile |
| 42 × 32 in | 1,344 in² | 112.0 in² | 12″ | 134.4 in² | 12" round / 16x16 tile |
| 48 × 36 in | 1,728 in² | 144.0 in² | 14″ | 172.8 in² | 14–15" round / 16x20 tile |
Frequently asked questions
What size flue do I need for a 30 x 29 fireplace?
A 30″ × 29″ opening is 870 in². Round: 870 ÷ 12 = 72.5 in², a 9.6″ bore, so a 10″ round liner. Rectangular: 870 ÷ 10 = 87 in², a nominal 12×12 clay tile.
Why is the round rule 1/12 but the rectangular rule 1/10?
A round flue moves gas more efficiently than a rectangular one of the same area — less wall friction and no dead corners — so it can be a touch smaller. Rectangular tiles need the extra area to do the same work.
Can a flue be too big for a fireplace?
Yes. An oversized flue lets the gases cool before they exit, which weakens the draft and lays down creosote fast. Match the rule; do not just install the largest tile that fits.
Does this work for a prefab or gas fireplace?
No — a factory-built or gas unit is sized to the manufacturer’s listed vent, not the 1/10 rule. This tool is for a masonry wood-burning fireplace. For an appliance, use the flue-liner-size tool and the listing.
The result lands between two liner sizes. Which do I buy?
The larger one. The rule gives a minimum; rounding up keeps the draft honest. Rounding down is the single most common cause of a smoky fireplace.