Fireplace & Chimney Calculator

This calculator sizes a chimney flue using the classic 10:1 fireplace-to-flue area ratio (tightened to 12:1 for round flues since round liners draft more efficiently per unit of area) that traces back to NFPA 211, the Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances, and it also produces an estimated draft pressure. Fireplace Opening Width and Fireplace Opening Height contribute to the fireplace opening area in exactly the same way, since that area is nothing more than width multiplied by height, so an equal percentage change in either dimension moves Required Flue Area and Flue Size by the same amount. Flue Type swaps the ratio the required area is divided by -- 12:1 for a round liner versus 10:1 for rectangular -- so a round flue can come out smaller for an identical fireplace opening.

Chimney Height sits outside that whole calculation: it has no bearing on Required Flue Area or Flue Size, which come entirely from the opening's dimensions and the flue type chosen. What Chimney Height does drive is Min Chimney Height, simply the larger of what you entered and a fixed 15-foot floor needed for reliable draft, and from there it feeds Estimated Draft Pressure, a simplified physics estimate built from the temperature gap between outside air and flue gas. Treat this as a sizing aid based on a simplified rule of thumb, and always confirm against local building code and the manufacturer's flue liner specifications before installation.