How Occupant Load Is Calculated Under the IBC

A practical walkthrough of IBC Section 1004 — the formula, the table, and worked examples you can apply to your next plan set.

What is occupant load?

Occupant load is the number of people a building or space is designed to accommodate. It drives egress requirements — exit widths, number of exits, plumbing fixture counts, and ventilation rates. Under the International Building Code (IBC), occupant load is defined in Section 1004.

The formula

For any given space the calculation is:

Floor area is the gross or net area of the space (IBC defines when each applies). The occupant load factor comes from IBC Table 1004.5 and varies by use — for example, business areas use 150 sq ft/person (gross) while assembly with unconcentrated seating uses 15 sq ft/person (net).

Common occupant load factors (IBC Table 1004.5)

Worked example

A 3,000 sq ft open-plan office (Business occupancy):

Those 20 occupants set the baseline for exit width (IBC 1005), number of exits (IBC 1006), and downstream calculations like plumbing fixture counts (IPC).

Mixed-use spaces

When a floor contains multiple uses, calculate the occupant load for each area separately and sum them. Do not average the factors — each space is calculated independently per its designated use, then the totals are combined for the floor or building.

Why it matters at plan review

Reviewers check occupant load early because nearly every life-safety requirement cascades from it. An incorrect factor or area measurement can under-count occupants, leading to undersized exits, too few fixtures, and a correction letter. InstaPreView flags occupant-load discrepancies automatically so you can fix them before submittal.