Work out chimney sizing instantly with clear inputs, formula shown and shareable results.
Natural draft comes from the density difference between hot flue gas and outside air: 0.52 x height x pressure x (1/Tambient - 1/Tflue) with absolute temperatures. Flue area is set by volume flow, taken as roughly 11.5 cubic feet of products per cubic foot of natural gas and expanded for temperature, sized at about 900 fpm.
Theoretical draft
D = 0.52 x H x P x (1/Ta - 1/Tg)
Flue area
Area in² = flue gas CFM / 900 x 144
Preliminary estimate only. Venting must be sized to the appliance manufacturer's tables and the applicable venting code, which account for connector rise, offsets and combined venting.
D = 0.52 x H x P x (1/Ta - 1/Tg). Natural draft comes from the density difference between hot flue gas and outside air: 0.52 x height x pressure x (1/Tambient - 1/Tflue) with absolute temperatures.
Draft depends on the flue being hotter than outside air. Until the masonry warms up the density difference is small, so a cold external chimney may spill products at start-up.
This calculator takes 4 inputs: Chimney height, Flue gas temperature, Ambient temperature, Appliance input. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.