Combustion Air Sizing Calculator

Calculate combustion air requirements per NFPA 54 and mechanical codes

Combined input rating of all gas-fired appliances in the space (furnaces, water heaters, boilers). Found on the appliance nameplate.
BTU/hr
Elevation above sea level. Higher altitudes require more combustion air due to lower air density. Derating starts above 1000 ft.
ft
Indoor: air from within building. Outdoor: direct outdoor air openings. Combined: mix of both methods per code.
Volume of the mechanical room or space containing the combustion appliances. Required for indoor air method (50 cu ft per 1000 BTU/hr).
cu ft

Results

Required Combustion Air Openingsq in
Duct Size Needed
Direct Vent Openingsq in
Room Volume Check
References: NFPA 54/ANSI Z223.1, International Fuel Gas Code. Indoor air: 50 cu ft per 1000 BTU/hr input. Outdoor openings: 1 sq in per 4000 BTU/hr (direct) or 1 sq in per 2000 BTU/hr (indirect/ducted).