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Natural Light — IBC §1204 (2021)

Minimum net glazed area (8 % of the floor area), borrowed light through an adjoining room, the artificial-light alternative and an optional natural-ventilation check.

Room with exterior glazing

Floor area ft²

Net glazed area (clear glass) ft²

Openable area to outdoors (optional) ft²

Artificial light, average at 30 in (optional) fc

Interior room lit through this room (optional)

Include an interior room (§1204.2.1) · Opening into a sunroom / patio cover (exception)

Interior room floor area ft²

Common wall area ft²

Opening in the common wall ft²

Results

Required net glazed area—

§1204 lighting—

Natural ventilation (≥ 4 %)—

§	Item	Limit	OK

IBC 2021§1204.1–1204.3 as reproduced in the California Building Code and quoted by the Florida Building Commission (older numbering §1205). Natural light OR artificial light (10 fc) — local amendments may require both. The IRC uses 6 fc; it is a separate code.

VerifiedHand calculations: 200 ft² → 16 ft²; room plus interior room 300 + 150 ft² → 36 ft²; borrowed-light opening ≥ half the common wall and ≥ max(1/10 of the interior room, 25 ft²).

Preliminary calculation tool. Final design requires the full text of the referenced standard (see the sources on this page) and review by a licensed PE.