Combustion & Makeup Air Sizing Calculator (IFGC / NFPA 54 Opening Sizing)

Size your combustion air openings instantly.

Use this calculator to check whether a gas appliance space is confined or unconfined, and size the combustion/makeup air openings it needs, per IFGC/NFPA 54.

🕒 Last updated: September 18, 2026

Inputs

Appliances in This Space

Add every gas appliance sharing this room/closet — furnace, water heater, boiler, etc.

Appliance 1

Appliance Name

Input Rating

BTU/hr

Appliance 2

Appliance Name

Input Rating

BTU/hr

Space Size

The room, closet, or alcove enclosing these appliances — used to check whether it's a "confined" space under IFGC.

Enter Space Size By

Room Length

Room Width

Room Height

Unusually Tight Construction?

Weatherstripped doors/windows, sealed sheathing, no fireplace/other natural draft — even an otherwise-unconfined space still needs dedicated outdoor air here.

Outdoor Opening Details

Used for the single-opening method and the louver/grille free-area adjustment.

Single-Opening Method Eligible?

Only valid when every appliance in the space has a draft hood (or is fan-assisted Category I) and meets its required clearances — check your specific appliance's installation manual.

Louver / Grille Type

▾

ℹ️Sets the free-area percentage used to convert required free area into the actual gross opening size you need to cut.

Confined Space — Combustion Air Required

30 sq in

each opening, outdoor two-opening method (direct/vertical duct) — see all methods below

160 cu ft actual space volume vs. 6,000 cu ft required (50 cu ft per 1,000 BTU/hr of 120,000 BTU/hr combined input) — this space is CONFINED.

Method	Free Area (Each)	Gross Area (Each)
Indoor, two openings (to adjacent indoor space, same story)	120 sq in	120 sq in
Outdoor, two openings — direct or vertical duct	30 sq in	30 sq in
Outdoor, two openings — horizontal duct	60 sq in	60 sq in
Outdoor, single opening	N/A — enable single-opening eligibility above

The 100 sq in floor above applies to the indoor method only. Every opening also needs a minimum 3 in cross-sectional dimension — this calculator computes area only, so confirm your actual width/height meets that separately.

Alternatively, a mechanical combustion air supply delivering at least 42 CFM from outdoors (interlocked so appliances can't fire without it running) satisfies all of the above.

Appliance Load

Appliances: 2

Combined input: 120,000 BTU/hr

Opening Sizing

Louver/grille type: Unlouvered Opening (No Grille/Louver) (100% free area)

Mechanical supply alternative: 42 CFM

Assumptions Used

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What Is a Combustion & Makeup Air Sizing Calculator?

A combustion and makeup air sizing calculator determines whether the room or closet enclosing your gas-fired appliances (furnace, boiler, water heater) is a "confined space" under the International Fuel Gas Code (IFGC) / NFPA 54, and if so, sizes the openings — or the mechanical air supply — needed to safely supply the air those appliances burn. Getting this wrong isn't a cosmetic error: an appliance starved of combustion air can backdraft, pulling combustion byproducts (including carbon monoxide) into the living space instead of venting them outdoors.

This calculator is built for homeowners, installers, and designers planning a mechanical room, furnace closet, or water heater alcove. It computes all four IFGC/NFPA 54 sizing methods side by side — indoor, outdoor two-opening, outdoor single-opening, and mechanical supply — since which one is actually usable is a site-specific, physical-constraint decision, not a single universally-correct formula. It does not replace your local jurisdiction's adopted code edition or a licensed mechanical contractor's review.

Why this calculator shows every method at once:

The Standard Method first checks whether your space even needs dedicated openings at all — 50 cu ft of room volume per 1,000 BTU/hr of combined appliance input
The indoor and outdoor two-opening methods use one high, one low opening — sized differently depending on whether the outdoor path is direct/vertical or runs through a horizontal duct
The outdoor single-opening method needs only one opening, but only when your appliances have draft hoods and meet clearance requirements
The mechanical supply method replaces passive openings with an interlocked fan, useful where wall access for two properly-placed openings isn't practical

Combustion Air Formula: How Is It Determined?

Every method starts from the same combined Btu/hr input, then applies a different code-specified divisor and minimum.

Confined Space Check (Standard Method)

Required Volume (cu ft) = Combined Appliance Input (BTU/hr) ÷ 1,000 × 50

If your actual room/closet volume is less than this required volume, the space is "confined" and needs dedicated combustion air. At or above it, infiltration is normally adequate — unless the building is of unusually tight construction, which needs outdoor air regardless, sized using the same outdoor methods below (not a separate formula).

Two-Opening Methods (Indoor or Outdoor)

Indoor (to adjacent space, same story): 1 sq in per 1,000 BTU/hr, each opening

Outdoor, direct or vertical duct: 1 sq in per 4,000 BTU/hr, each opening

Outdoor, horizontal duct: 1 sq in per 2,000 BTU/hr, each opening

The indoor method's each opening also has its own 100 sq in floor — a small combined appliance load can compute to less than 100 sq in by the formula alone, but the code requires at least 100 sq in per opening regardless. This floor does NOT apply to either outdoor two-opening method. One opening goes within 12 in of the top of the enclosure, the other within 12 in of the bottom. A different, larger requirement applies to an indoor opening connecting a different story — not modeled here.

Outdoor Single-Opening Method

Free Area (sq in) = max(Combined Input ÷ 3,000, Total Vent Connector Area)

Only valid when every appliance has a draft hood (or is fan-assisted Category I) and meets its required clearances. The opening can never be smaller than the combined vent connector (draft-hood/flue-collar outlet) area of the appliances, even if the formula alone would allow it.

Gross (Actual Cut) Opening Size

Gross Area (sq in) = Free Area ÷ Louver/Grille Free Area %

Free area is the portion of an opening actually open to airflow — a louver or grille blocks some of it. Per IFGC 304.10's own default assumptions (when a specific product's free area isn't yet known): 100% for a bare unlouvered opening, 75% for a metal louver/grille, 25% for a wood louver.

Mechanical Combustion Air Supply (Alternative)

Required Supply (CFM) = Combined Appliance Input (BTU/hr) ÷ 1,000 × 0.35

Supplied from outdoors, with every served appliance interlocked to the supply fan so the main burner can't fire unless it's running. This replaces passive openings entirely where wall access is impractical.

Worked Example

This example walks through your current inputs above, using the same steps as the Formula section.

Input Values Used

Input	Value
Combined appliance input	120,000 BTU/hr (2 appliances)
Space volume	160 cu ft
Louver/grille type	Unlouvered Opening (No Grille/Louver) (100%)

Step-by-Step Calculation

Step	Calculation	Result
Required volume (confined-space check)	120000 ÷ 1,000 × 50	6,000 cu ft
Space status	160 vs. 6,000 cu ft	Confined
Outdoor two-opening, vertical (each)	120000 ÷ 4,000	30 sq in
Outdoor two-opening, horizontal (each)	120000 ÷ 2,000	60 sq in
Gross area, outdoor two-opening vertical (each)	30 ÷ 100%	30 sq in
Mechanical supply alternative	120000 ÷ 1,000 × 0.35	42 CFM

Therefore, 120,000 BTU/hr of combined appliance input in a 160 cu ft space is a confined space. Using the outdoor two-opening method with a direct or vertical duct, each opening needs at least 30 sq in of gross (actual cut) area — see the full method comparison above for the other options.

Combustion Air Reference Tables

Sizing divisors for each IFGC/NFPA 54 method, and free area percentages by louver/grille type.

Method	Sizing Basis
Indoor, two openings (same story)	1 sq in per 1,000 BTU/hr, each (100 sq in floor)
Outdoor, two openings — direct/vertical duct	1 sq in per 4,000 BTU/hr, each (no 100 sq in floor)
Outdoor, two openings — horizontal duct	1 sq in per 2,000 BTU/hr, each (no 100 sq in floor)
Outdoor, single opening	1 sq in per 3,000 BTU/hr (not less than vent connector area)
Tight construction, unconfined space	Same outdoor methods above (no separate formula)
Mechanical supply	0.35 CFM per 1,000 BTU/hr, interlocked
Every opening (all methods)	Minimum 3 in cross-sectional dimension

Louver / Grille Free Area (IFGC 304.10 Defaults)

Type	Free Area (When Manufacturer Data Unknown)
Unlouvered Opening (No Grille/Louver)	100%
Metal Louver / Grille	75%
Wood Louver	25%

When should you use this combustion air calculator?

Planning a new furnace closet, water heater alcove, or mechanical room, before finalizing its size or opening placement.
Checking whether an existing appliance space actually meets code — a common finding during a home inspection or renovation.
Comparing the four sizing methods to pick whichever is physically practical for a specific installation (wall access, duct routing, appliance type).
Diagnosing a suspected backdrafting issue by checking whether the existing combustion air openings are actually adequately sized.
Sizing a mechanical combustion air supply fan as an alternative when passive openings aren't practical.
Comparing two scenarios side by side (via Compare mode) — e.g. before and after adding a second appliance to a shared space.

Quick Sizing Tips

Always use each appliance's Btu/hr INPUT rating (nameplate), not its output/AFUE-adjusted capacity — combustion air requirements are based on fuel input, not delivered heat.
Don't count a sealed-combustion (direct-vent) furnace or water heater — it draws air from outdoors through its own intake, not from the room.
Compare all four methods before committing to one — the practical choice usually comes down to wall access and duct routing, not which formula gives the smallest number.
If your space is borderline confined, double-check the room volume calculation before assuming the larger unconfined-space outcome — obstructions like ductwork or shelving reduce usable volume.
Round up to the next real, buildable opening size — never down — since undersizing directly risks backdrafting and carbon monoxide exposure.
If choosing the mechanical supply alternative, confirm the appliance interlock is included in the installation plan — a fan without it doesn't meet the code provision it's based on.

Common Mistakes

Using an appliance's output BTU rating instead of its input rating — input is always the correct figure for combustion air sizing.
Forgetting to include a secondary appliance (a water heater alongside a furnace) sharing the same enclosed space.
Applying the indoor method's 100 sq in floor to an outdoor opening — that floor is an indoor-only provision and does not apply to either outdoor two-opening method.
Using the vertical-duct divisor (4,000) for an opening that actually runs through a horizontal duct, which needs double the free area (2,000 divisor).
Applying the single-opening method without actually verifying every appliance has a draft hood and meets its clearance requirements, or leaving the vent connector area unchecked.
Installing a mechanical combustion air fan without the required burner interlock, leaving appliances able to fire with no air supply running.
Assuming a large-looking mechanical room is automatically unconfined without actually running the volume-per-Btu/hr check.
Forgetting the separate 3 in minimum cross-sectional dimension requirement — a very elongated opening can meet the required area yet still be code-invalid if narrower than 3 in.
Assuming an indoor opening to a space on a different story uses the same 1,000 divisor as a same-story connection — a different, larger requirement applies and is not modeled by this calculator.

Limitations

This is a planning estimate based on cross-verified IFGC/NFPA 54 provisions, not a substitute for your local jurisdiction's adopted code edition (which can include local amendments) or a licensed mechanical contractor's design and review.
Does not model the Known Air Infiltration Rate (KAIR) method, an alternative, more complex engineering calculation some jurisdictions allow — not modeled here, matching most other combustion air sizing tools.
The indoor two-opening method models only a SAME-STORY connection to an additional indoor space. A different, larger free-area requirement applies to an opening connecting spaces on different stories — not modeled here; do not use this calculator's indoor-method figure for a different-story connection.
Does not verify appliance-specific clearance requirements for the single-opening method — this is modeled as a single eligibility toggle, not per-appliance geometry.
Computes required area only, not opening width/height. Every combustion air opening also needs a minimum 3 in cross-sectional dimension regardless of area — confirm your actual planned dimensions meet this separately.
Does not account for depressurization from exhaust fans, clothes dryers, or other appliances operating simultaneously, which can cause backdrafting even with correctly-sized combustion air openings.
Does not size the appliance itself, its venting/flue, or its gas supply piping — use this site's other HVAC and plumbing calculators for those separate calculations.
Louver/grille free area percentages are IFGC 304.10's own default assumptions — a specific product's manufacturer-published free area should always be used once a model is selected.
Does not replace a carbon monoxide alarm, combustion analyzer testing, or a qualified inspector's field verification.

Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.

FAQ

A space is 'confined' under the IFGC/NFPA 54 Standard Method if its volume is less than 50 cubic feet per 1,000 BTU/hr of the COMBINED input rating of every atmospheric-combustion gas appliance in it. Below that volume, normal air infiltration through the building envelope can't be relied on to supply enough combustion air, so a dedicated opening (or mechanical supply) is required. Above it, infiltration is normally considered adequate — unless the building itself is of unusually tight, energy-efficient construction, in which case outdoor air is still required regardless of the room's volume.

Horizontal ducts introduce meaningfully more airflow resistance to the natural convective circulation the two-opening method relies on, so IFGC/NFPA 54 sizes them at 1 sq in of free area per 2,000 BTU/hr, versus 1 sq in per 4,000 BTU/hr for a duct that runs vertically (or an opening that communicates with outdoors directly, with no duct at all). At the same combined appliance input, that's exactly double the free area.

Even a very small combined appliance load can compute to a formula-based free area well under 100 sq in — but the code sets a 100 sq in floor on each individual opening regardless, since a smaller opening isn't considered a reliable, unobstructable combustion air path in practice. This floor applies to EACH of the two openings separately, not their combined total. It's an indoor-method-specific provision — it does NOT apply to either outdoor two-opening method (direct/vertical or horizontal duct), which can size smaller than 100 sq in for a modest combined appliance load.

It's an alternative to the two-opening methods that uses just one opening, sized at 1 sq in of free area per 3,000 BTU/hr of combined input (not less than the combined vent connector/draft-hood outlet area of the appliances), located within 12 in of the enclosure's top. It's only valid when every appliance in the space has a draft hood (or is fan-assisted Category I) and meets its required installation clearances — this calculator models that as a single eligibility toggle, since verifying the underlying appliance conditions is a real installation-specific check, not something a calculator can determine for you.

Instead of sizing a passive opening, IFGC/NFPA 54 allows supplying all combustion air mechanically (via a dedicated fan) at a rate of at least 0.35 CFM per 1,000 BTU/hr of combined appliance input, drawn from outdoors. Critically, every served appliance must be interlocked with that mechanical air supply system so its main burner can't fire unless the fan is actually running — without that interlock, this isn't a code-compliant substitute.

No. A sealed-combustion (direct-vent) appliance draws its combustion air directly from outdoors through its own dedicated intake pipe, not from the room it's installed in — it doesn't add to the room's combustion air demand the way an atmospheric or fan-assisted (non-direct-vent) appliance does. Only include appliances that actually draw combustion air from the enclosed space itself.

If you already know a specific product's manufacturer-published free area, use the Custom option and enter that figure directly — it always supersedes a generic assumption. If you don't yet have a specific product, this calculator defaults to IFGC 304.10's own stated assumptions: 100% for a bare, unlouvered opening, 75% for a metal louver or grille, and 25% for a wood louver — these are the code's own fallback figures when a manufacturer's data isn't yet known, not a universal constant.

If the building itself is of unusually tight, energy-efficient construction (weatherstripped doors/windows, sealed sheathing, no fireplace or other natural draft path), normal air infiltration can no longer be assumed adequate — even in a room whose volume alone would otherwise pass the confined-space check. The code directs this case to obtain that air using the same outdoor sizing methods (two-opening, single-opening, or mechanical supply) already covered above — not a separate formula.

No. This is a planning-stage estimate built from cross-verified IFGC/NFPA 54 provisions, intended to help you understand and prepare for a real combustion air sizing requirement — not a substitute for your local jurisdiction's adopted code edition (which can include local amendments), a licensed mechanical contractor's design, or your building department's plan review and inspection.

Combined input is 120,000 BTU/hr. Using the outdoor two-opening method with a direct or vertical duct, that's 120,000 ÷ 4,000 = 30 sq in of free area per opening (the indoor method's 100 sq in floor doesn't apply here). With an unlouvered opening, that's also the gross (actual cut) size; with a 75% free-area metal louver, the gross size grows to about 40 sq in per opening.

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