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The rating of the assembly the boxes are being cut into. The box allowance stops at 2 hours.
The prescriptive membrane penetration allowance — 16 in² a box, 100 in² in any 100 ft², 24 in between opposing boxes — is granted to walls and partitions rated a maximum of 2 hours. Shaft enclosures, occupancy separations and area separation walls are commonly 3 or 4 hours, and there the allowance does not apply: a box in one of those needs a listed assembly tested at that rating, not this arithmetic.
The opening one box cuts through the board, not the volume of the box.
A standard 4 in square box presents 16 in² (0.0103 m²) — exactly the largest single face the code allows without a listed system. A single-gang device box is nearer 10 in² (0.0065 m²). Measure the hole through the membrane, since that is what the limit governs.
How many boxes are set into the area of wall you entered below.
Count every opening through the membrane, not just the outlets — data boxes, low-voltage rings, thermostats and speaker cans all cut the same board. The aggregate limit does not care what the box is for.
The area of rated wall the boxes above are distributed across.
The cap is written per 100 ft² (9.29 m²), and the default here is exactly that so the answer reads directly against the code's own reference. Take a genuinely representative patch — spreading a dense cluster of boxes across a whole floor of partition passes an arithmetic test the code did not intend.
The closest horizontal offset between a box on one side and a box on the other.
Measured horizontally, box to box, through the wall. Boxes set back to back in the same stud bay are the classic failure: the two openings face each other across a cavity and the assembly is no longer the one that was tested.
Whether a listed protection method is used in place of the separation distance.
The code allows the separation to be relaxed where boxes are protected by a listed material used strictly within its listing. That listing is specific about the box type, the pad and the assembly — a pad applied outside it protects nothing that anyone tested.
Aggregate box area against the cap
96 % of the cap
The layout is inside the code's limits for opening area, box size and separation. That is a check on the openings, not a verification of the wall's rating, which comes from the tested assembly it was built to.
- Aggregate box opening area
- 0.67 ft²
- Cap for the wall area entered
- 0.69 ft²
- Boxes this wall area can carry at that size
- 6 boxes
- Largest single box allowed without a listed system
- 0.11 ft²
- Separation required between opposing boxes
- 24 in
- Separation entered
- 24 in
They open the calculator with your figures already in it
Firestop Membrane Penetration Box Area Calculator: 96 % of the cap — 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)
- International Building Code, fire and smoke protection features — membrane penetrations of MAXIMUM 2-HOUR fire-resistance-rated walls and partitions by steel electrical boxes: individual face area not exceeding 16 in² (0.0103 m²), with the aggregate area of such openings not exceeding 100 in² (0.0645 m²) in any 100 ft² (9.29 m²) of wall area. The allowance does not extend to walls rated above 2 hours
- Same provision, boxes on opposite sides of a rated wall: separated by a horizontal distance of not less than 24 in (610 mm), unless separated by the insulation the code describes, by solid fireblocking, or protected by a listed putty pad or other listed material. Confirm against the edition your jurisdiction has adopted and any local amendment
- The wall's fire-resistance rating itself comes from a tested assembly under ASTM E119 or UL 263. This page checks the openings cut through the membrane against the code's limits; it does not establish or verify the rating
Inputs used
- Fire-Resistance Rating of the Wall
- 2 hours
- Face Area of a Single Box Opening
- 0.11 sq ft
- Number of Boxes in That Wall Area
- 6
- Wall Area Under Consideration
- 100 sq ft
- Horizontal Separation Between Boxes on Opposite Faces
- 24 in
- Listed Putty Pads or Equivalent Listed Protection
- No listed protection — the separation distance governs
Intermediate steps
- Aggregate box opening area
- 0.67 ft²
- Cap for the wall area entered
- 0.69 ft²
- Boxes this wall area can carry at that size
- 6 boxes
- Largest single box allowed without a listed system
- 0.11 ft²
- Separation required between opposing boxes
- 24 in
- Separation entered
- 24 in
Confidence note: The layout is inside the code's limits for opening area, box size and separation. That is a check on the openings, not a verification of the wall's rating, which comes from the tested assembly it was built to.
What this calculation does not cover
- Steel boxes only. Non-metallic boxes are permitted solely where they are part of a listed assembly, with their own conditions.
- The rating of the wall itself is not verified here; the assembly's own listing governs. Above 2 hours the prescriptive allowance does not apply at all, and this page says so rather than scoring the layout.
- Local amendments to the adopted code are common on this provision and are not applied.
- Larger openings, sleeves and through penetrations follow different provisions entirely.
Add the equipment this sizes
This result is a specification — 96 % of the cap — 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 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations3
- International Building Code, fire and smoke protection features — membrane penetrations of MAXIMUM 2-HOUR fire-resistance-rated walls and partitions by steel electrical boxes: individual face area not exceeding 16 in² (0.0103 m²), with the aggregate area of such openings not exceeding 100 in² (0.0645 m²) in any 100 ft² (9.29 m²) of wall area. The allowance does not extend to walls rated above 2 hours
- Same provision, boxes on opposite sides of a rated wall: separated by a horizontal distance of not less than 24 in (610 mm), unless separated by the insulation the code describes, by solid fireblocking, or protected by a listed putty pad or other listed material. Confirm against the edition your jurisdiction has adopted and any local amendment
- The wall's fire-resistance rating itself comes from a tested assembly under ASTM E119 or UL 263. This page checks the openings cut through the membrane against the code's limits; it does not establish or verify the rating
Which documents these citations point at
- International Building Code (United States)Buildings other than the dwellings the IRC covers — occupancy, egress, fire resistance and structural provisions.
Standards referenced: ASTM E119 (ASTM International, United States); UL 263 (UL Solutions, United States).
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.
Cite this page
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On aggregate area: 96% of the prescriptive cap, so this patch of wall is inside it at up to 6 boxes of the size entered. On separation: the boxes on opposite faces are at or beyond the 24 in (610 mm) the provision states.