Sizing & Draft

The deterministic sizing & draft cluster — the highest-demand, least-defended chimney queries: the SIGNATURE fireplace-flue-size calculator (opening → required round/rectangular flue area & diameter by the 1/10 rule), the NFPA 211 3-2-10 chimney-height rule, flue-liner size by appliance outlet, a round↔rectangular flue-area converter, wood-stove flue size, a theoretical chimney-draft calculator, and chimney-cap size. Confirm your exact flue and opening dimensions and follow NFPA 211; round flue area up to the nearest standard liner or tile, never down.

Typical planning values. These come from timeless flue geometry. Your real flue requirements, draft and clearances vary by appliance, fuel, chimney and roof geometry — confirm your exact flue and opening dimensions, follow NFPA 211, the appliance manufacturer’s instructions and local code. Size flue area up to the nearest standard liner or tile, never down.
Fireplace Flue SizeFind the flue your fireplace needs from the opening: the 1/10 rule sizes a round or rectangular flue, gives the diameter, and rounds up to a standard liner.Chimney Height (3-2-10 Rule)Size a chimney by the NFPA 211 3-2-10 rule: 3 ft above the roof penetration and 2 ft above any ridge within 10 ft. Enter two heights, read the required top.Flue-Liner SizeMatch a flue liner to your appliance outlet, not the old masonry flue. Enter the outlet diameter and get the cross-section and the next standard liner size.Round to Rectangular Flue AreaConvert a round flue to its rectangular equivalent by AREA, not width. Enter a diameter to get the cross-section in square inches and the equal square side.Wood-Stove Flue SizeSize a wood-stove flue to the collar and flag an oversized masonry flue that runs cool. Enter the outlet and existing flue area for the right insulated liner.Chimney DraftEstimate theoretical chimney draft from height and the flue-gas-to-outside temperature difference. A labeled indicative figure in inches of water column.Chimney-Cap SizeSize a chimney cap so its screen never chokes the flue: net free vent area must meet or beat the flue cross-section. Enter the flue diameter to get the minimum.

Every Sizing & Draft tool is calculated server-side — visible immediately — with its formula, a worked example and a reference table, and works on the dimensions you measure and the prices you enter from your own quotes. Background reading is in the guides.