Chimney Flue Size Calculator
Calculate the required chimney flue diameter and minimum height for wood stoves, open fireplaces, gas furnaces, and water heaters. Based on NFPA 211 and IRC chimney standards.
BTU/hr
Min. Flue Diameter
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Min. Chimney Height (ft) —
Flue Type Note —
Extended More scenarios, charts & detailed breakdown ▾
BTU/hr
ft
Recommended Flue Dia.
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Chimney Class —
Professional Full parameters & maximum detail ▾
BTU/hr
ft
Flue Sizing
Min. Flue Diameter —
Height Above Roof (min) —
Required Chimney Class —
Safety & Draft
Creosote Risk —
Liner Recommendation —
Draft & Cap Note —
How to Use This Calculator
- Select your appliance type (wood stove, fireplace, furnace, or water heater).
- Enter the BTU input from the appliance nameplate.
- Results show minimum flue diameter and height. Use Extended tabs for appliance-specific sizing. Professional tab adds chimney class, creosote risk, and liner recommendations.
Formula
Wood Stove: Flue dia. 6 in (≤42k BTU), 8 in (≤80k BTU), 10 in (>80k BTU)
Open Fireplace: Flue Area = Opening Area ÷ 10 → dia. = √(Flue Area ÷ π) × 2
Gas Appliance: Per NFPA 54 tables; 4–7 in typical range
Example
Example: Wood stove rated 65,000 BTU/hr → 8-inch round flue. Chimney 20 ft tall, medium-pitch roof → must terminate 3 ft above roof deck and 2 ft above any adjacent structure within 10 ft.
Frequently Asked Questions
- NFPA 211 and most stove manufacturers specify flue diameter by stove output. A stove up to 42,000 BTU/hr typically needs a 6-inch round flue; up to 80,000 BTU/hr needs 8 inches; larger stoves need 10 inches.
- Per NFPA 211, a chimney must extend at least 3 feet above the highest point where it passes through the roof AND at least 2 feet above any portion of a building within 10 horizontal feet.
- The flue cross-sectional area should be at least 1/10 of the fireplace opening area (1/12 for taller flues). For a 36 × 29 inch opening (1,044 sq in), the flue needs at least 104 sq in — roughly a 12-inch round flue.
- Class A chimneys are rated for all fuels including solid wood. Class B (Type B) vents are for gas appliances only — they cannot handle the higher temperatures and creosote from wood burning.
- Creosote is a combustible byproduct of wood burning that deposits on flue walls. It is the leading cause of chimney fires. Undersized flues and cool flue temperatures increase creosote accumulation. Annual chimney inspection and cleaning is recommended by CSIA.
Related Calculators
Sources & References (5) ▾
- NFPA 211 – Standard for Chimneys, Fireplaces, Vents, and Solid Fuel–Burning Appliances — National Fire Protection Association
- Wood Heat Organization – Chimney Sizing Guide — Wood Heat Organization
- EPA Wood Heater Emissions & Certification — U.S. Environmental Protection Agency
- Chimney Safety Institute of America – Installation Standards — Chimney Safety Institute of America
- NAHB – Residential Mechanical Systems Guide — National Association of Home Builders