Chimney Flue Size & Height Calculator

How it works

Fireplace flue ≥ opening ÷ 12 (round), ÷ 10 (square or rectangular under 2:1), ÷ 8 (rectangular 2:1 or longer) — IRC R1003.15.1; or ÷ 8 under 15 ft and ÷ 10 at 15 ft+ · stove flue ≥ collar area and ≤ 3× it (2× for an exterior chimney) · 3-2-10 rule: top ≥ 3 ft above the penetration and ≥ 2 ft above anything within 10 ft, so height above roof ≥ max(3, roof rise within 10 ft + 2) · draft (in. w.c.) ≈ 7.63 × H × (1/T_out − 1/T_flue) × (P/P₀), T in °R

A chimney works by buoyancy: hot flue gas is lighter than the outside air, and the pressure difference that results — proportional to chimney height, to the difference in reciprocal absolute temperatures and to the air pressure, which falls with altitude — is the draft that pulls combustion air through the fire and smoke up the flue. The flue must be large enough to pass the gas volume but not so large that the gas cools and stalls. For an open masonry fireplace the IRC (R1003.15.1) sets the net flue area at 1/12 of the fireplace opening for a round flue, 1/10 for a square one or a rectangle under 2:1, and 1/8 for a longer rectangle, because round flues use their area best; an older rule of thumb scales it by height instead. For a stove, the flue should match the collar and be no more than three times its area — two times for an exterior chimney that runs cold — which is why a modern 6-inch stove vented into a big old masonry flue smokes back and grows creosote until a liner is installed. Clay tiles are sold by outside size, so the calculator uses their net inside areas from IRC Table R1003.14(2) when it suggests the smallest round liner or tile that works. Height rules protect against downdrafts from the roof and neighbouring structures: the top must be at least 3 ft above where it leaves the roof and 2 ft above anything within 10 ft, and 15 ft from the stove to the cap is the usual minimum for reliable draft.

Worked example

A 6″ stove (28 in²) vented into an existing 8 × 12″ clay flue (96 in², about 11″ equivalent) has 3.4 times the required area — more than the 3× limit for an interior chimney, so a 6″ insulated liner is the fix. With an 18 ft chimney the top must stand at least 5 ft above the roof at the penetration (3 ft of roof rise within 10 ft plus 2 ft) — on a 6/12 roof with the ridge 6 ft away the rise is 3 ft, the same answer. The theoretical draft at 350 °F flue gas and 30 °F outside is about 0.11 in. w.c. (28 Pa) — strong enough that a flue damper may be wanted. For a 36 × 28″ open fireplace (1,008 in²), the IRC asks for 84 in² with a round flue (a 12″ round liner) or about 101 in² with a square one (a 12 × 12″ tile at 102 in² net).

Sources: International Residential Code — R1003.9 and R1003.14–15 (as adopted in the California Residential Code) · Hearth.com — Chimney liner sizing (WABO Woodstove Installation Code Book) · NFPA 211 — Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances

Frequently asked questions