Chimney Draft Calculator
See how height and the hot-to-cold temperature spread build the pull that carries smoke up — a labeled theoretical figure, not a safety verdict.
1 Enter your numbers
Draft is what pulls smoke up — it grows with chimney HEIGHT and with the temperature difference between the hot flue gas and the cold outside air. This theoretical figure is about 0.022 in w.c., LABELED indicative — real draft is measured with a manometer by a professional and depends on the whole system.
Draft is the quiet pull that makes a chimney work — the pressure difference that lifts smoke out instead of into the room. It comes from two things: height and the temperature difference between the hot flue gas inside and the cold air outside. Taller chimney, bigger spread, stronger draft. That is the stack effect, and this calculator puts a theoretical number on it.
Enter the local pressure, the chimney height and the two temperatures, and you get a figure in inches of water column. Read it as a labeled indicative value — it shows how the levers move, not whether a specific system is safe. Real draft is measured on site with a manometer and depends on flue size, connector runs, wind, house pressure and how airtight the building is.
Formula
The theoretical stack draft:
ΔP ≈ 0.0342 × B × H × (1÷Tout − 1÷Tin)B= barometric pressure (in Hg),H= height (ft)- Temperatures in Rankine:
°R = °F + 460
The 0.0342 coefficient rolls up the gas constants into these units; the result is in inches of water column. It is a theoretical maximum — friction and leakage lower the real number.
Worked example
A tall exterior chimney on a cold night: B = 30.15 in Hg, H = 32 ft, outside 15 °F (475 °R), flue gas 400 °F (860 °R). Then ΔP ≈ 0.0342 × 30.15 × 32 × (1÷475 − 1÷860) = 0.031 in w.c. Let that flue gas drop to 250 °F and the draft falls with it — which is exactly why an oversized or uninsulated flue that cools the gases drafts so poorly. Height helps, but keeping the gases hot is half the battle.
Read it as indicative, measure the real thing
Treat this as a direction, not a diagnosis. The formula shows how height and heat drive draft; it does not model your actual chimney. Keep in mind:
- Real draft is measured with a manometer — a pro does this, and appliances have a target range.
- Too much draft is a problem too: it burns fuel fast and can over-fire a stove. A key or barometric damper tames it.
- Wind, a nearby ridge (see the 3-2-10 rule) and a tight, depressurized house all shift the real number.
Reference table
How the theoretical figure grows with height, holding a 20 °F outside and 350 °F flue gas — taller chimneys pull harder. Indicative only; a pro measures the real draft with a manometer.
| Chimney height | Theoretical draft |
|---|---|
| 10 ft | 0.009 in w.c. |
| 15 ft | 0.013 in w.c. |
| 20 ft | 0.017 in w.c. |
| 25 ft | 0.022 in w.c. |
| 30 ft | 0.026 in w.c. |
| 40 ft | 0.035 in w.c. |
Frequently asked questions
What is a good chimney draft?
Most wood appliances want roughly 0.04–0.08 in w.c. at the appliance, but the exact target is in the manufacturer’s manual. This tool gives a theoretical figure to show the trend; a pro measures the real draft with a manometer.
Why does a taller chimney draft better?
Draft scales with height — a taller column of hot gas creates a bigger pressure difference against the cold outside air. That is why stub chimneys and short runs so often draw weakly.
Does cold weather help or hurt draft?
Cold outside air increases the temperature difference, so it generally strengthens draft — provided the flue keeps the gases hot. A cold, oversized flue can cool the exhaust enough to cancel the benefit.
Can a chimney draft too hard?
Yes. Excess draft pulls heat and fuel up the flue too fast and can over-fire an appliance. It is controlled with a key damper or a barometric damper, sized by a professional.
Is this figure a safety guarantee?
No. It is a labeled indicative calculation of theoretical stack draft. Venting safety depends on the whole system and is verified on site — follow NFPA 211, the appliance manual and a certified professional.