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The fuel-burning appliance's total rated input, from its nameplate.
Use the combined input rating of all fuel-burning appliances sharing the confined space if more than one draws combustion air from it. The floor of 100 in² per opening applies below about 100,000 BTU/hr and governs most domestic water heaters. The code rule this applies is written per 1,000 BTU/hr of input; entering the rating in kW converts to that automatically.
Minimum free area per opening
100 in²
This is the NFPA 54/IFGC indoor air method for a confined space, requiring TWO openings of this minimum free area each (one high, one low). Outdoor air methods use different (typically smaller) sizing factors, and unconfined spaces may not require dedicated combustion air openings at all — confirm which method and space classification applies to your installation with the full code section before finalizing opening size, as this is a single-method screening calculation, not a complete combustion air analysis.
- Free area before the code's minimum is applied
- 100 in²
They open the calculator with your figures already in it
Appliance Combustion Air Free Area Calculator (Indoor Air Method): 100 in² — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Indoor air method per NFPA 54 (National Fuel Gas Code) / International Fuel Gas Code: each combustion air opening requires a minimum free area of 1 square inch per 1,000 BTU/hr of total appliance input but NOT LESS THAN 100 square inches (645 cm²) per opening, with two openings required (one within 12 in of the ceiling, one within 12 in of the floor) when using this method for a confined space.
Inputs used
- Total Appliance Input Rating
- 100000 BTU/hr
Intermediate steps
- Free area before the code's minimum is applied
- 100 in²
Confidence note: This is the NFPA 54/IFGC indoor air method for a confined space, requiring TWO openings of this minimum free area each (one high, one low). Outdoor air methods use different (typically smaller) sizing factors, and unconfined spaces may not require dedicated combustion air openings at all — confirm which method and space classification applies to your installation with the full code section before finalizing opening size, as this is a single-method screening calculation, not a complete combustion air analysis.
What this calculation does not cover
- Free area is not the size of the hole you cut. A grille or louver passes only its open fraction — roughly three-quarters for metal, as little as a quarter for wood — and an insect screen behind it takes more again, so a 100 in² (645 cm²) free-area requirement can need an opening of 130 to 400 in² (840 to 2,580 cm²) depending on what covers it. Size from the louver's published free area, or the appliance is starved through an opening that measures correctly.
- The openings only work if the space on the other side is large enough. This method assumes the adjoining room, plus everything freely communicating with it, holds at least 50 ft³ (1.4 m³) per 1,000 BTU/hr (0.3 kW) of combined appliance input; cutting two grilles into the partition of a small closet does not create combustion air, it shares one shortage between two rooms.
- Nothing here accounts for air being pulled out of the space. A kitchen hood, a clothes dryer or a bath fan can drop the room below atmospheric pressure and reverse an atmospheric flue no matter how generous the openings are, which is how a correctly sized opening still ends with a CO alarm — a house with substantial mechanical exhaust needs the appliance's air supply looked at as a pressure balance, not as an area.
Add the equipment this sizes
This result is a specification — 100 in² — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- Indoor air method per NFPA 54 (National Fuel Gas Code) / International Fuel Gas Code: each combustion air opening requires a minimum free area of 1 square inch per 1,000 BTU/hr of total appliance input but NOT LESS THAN 100 square inches (645 cm²) per opening, with two openings required (one within 12 in of the ceiling, one within 12 in of the floor) when using this method for a confined space.
Which documents these citations point at
- International Fuel Gas Code (United States)Fuel gas piping, appliances and venting. Its sizing provisions track NFPA 54.
- National Fuel Gas Code (NFPA 54) (United States)Fuel gas piping systems, appliance installation and venting, including the longest-length sizing method.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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