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Sheet LS-141
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Plumbing Fixture Countconcept

Plumbing fixture count: occupant load to required water closets, lavatories, and drinking fountains

One-line orientation

Plumbing fixture count uses occupancy and occupant load to find the minimum number of water closets, lavatories, and drinking fountains. It answers how many fixtures, not how large the pipes.

Key points

  • The five-step sequence:
    1. Occupancy classification — identify the use (office, residential, educational, assembly).
    2. Occupant load — area ÷ the occupant-load factor (office/Business = 1 per 150 sq ft gross, IBC 2021 Table 1004.5).
    3. Fixture ratios — read required fixtures per occupant from IBC §2902.1 or IPC §403.1.
    4. Gender split — assume ~50/50 unless the program states otherwise.
    5. Round up + accessibility — round each result up to whole fixtures; add accessible fixtures per ADA and the required drinking fountains.
  • Worked example (office, 10,000 sq ft, IBC/IPC 2021):
    • Occupant load = 10,000 ÷ 150 ≈ 67 occupants → divide in half: 33.5 per sex (IPC §403.1.1).
    • Water closets: ratio 1 per 25 for the first 50 → 33.5 ÷ 25 = 1.34 → round up = 2 per sex (4 total); men may substitute a urinal for one WC (up to 50% in Business, IPC §424.2).
    • Lavatories: ratio 1 per 40 for the first 80 → 33.5 ÷ 40 = 0.84 → round up = 1 per sex (2 total).
    • Plus: drinking fountain 1 per 100 → 1, and 1 service sink.
  • It is a minimum, not a target — provide at least the required count; more is allowed.
  • Special uses change the ratios — assembly areas, schools, and similar carry their own (often much tighter) numbers, so always confirm the occupancy row in the table.

Required plumbing fixtures: occupant load drives the count

The code workflow for the required fixture count — each step notes the section it comes from.

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Plumbing fixture count as an occupant-load-driven calculation workflow A left-to-right calculation workflow, each step noting its code basis. Start with a 10,000 square foot office (the given program); divide by the 150 square-foot-per-occupant Business factor from IBC Table 1004.5 (highlighted) to get 67 occupants; read required fixture ratios per sex from the code ratio table (IBC Table 2902.1 or IPC 403.1); split the load roughly 50/50 per sex, round each count up per IBC 2902.1.1, and add accessible fixtures. The worked result per sex (times two): two water closets and one lavatory, plus a urinal substitution up to 50 percent in Business, one drinking fountain, and one service sink. The point: it is a required count driven by the occupant load — not pipe sizing, which is a separate Systems calculation. 1 · OCCUPANCYoffice10,000 SFthe given program2 · OCCUPANT LOAD÷ 150 SF/occ≈ 67 occupantsIBC Table 1004.53 · FIXTURE RATIOScode ratio tableWC 1:25 · lav 1:40 per sexIBC Table 2902.1 · IPC 403.14 · SPLIT, ROUND UPsplit ~50/50 per sexround up each + access.IBC 2902.1.1Per sex (×2): WC 2 · lav 1 + urinal substitution ≤ 50% (Business), 1 drinking fountain, 1 service sink

A required count (sanitation minimum) from the occupant load — not pipe sizing (a separate Systems calc).

Confusions / comparison

Code fixture count (this card)Systems plumbing sizing
Question answeredHow many fixtures are requiredHow big the pipes / what demand
Driven byOccupant load + code ratio tableFixture units, flow, pressure
SourceIBC Ch. 29 (2902) / IPC Ch. 4 (403)Plumbing supply/DWV design
OutputCounts of WC, lavatories, fountainsPipe diameters, pump/pressure
Tested asShort occupant-load → count calcHydraulic/sizing reasoning

→ Occupancy groups overview (this module): the occupancy classification that selects the fixture row · Means of egress parts (this module): occupant load also drives egress, a parallel use of the same number · Water pressure & supply, Piping materials (Systems): the sizing side this card is deliberately distinguished from.

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