Gross Floor Area Calculator
The gross floor area a brief’s net room areas will need once circulation, walls and structure, plant and cores are added — with the net-to-gross efficiency that results.
A schedule of accommodation lists the rooms; the building is bigger.
How the gross floor area calculator works
A schedule of accommodation lists the rooms; the building is bigger. Corridors, lobbies and stairs add 15–35% depending on the building type, walls and columns 5–10%, plant rooms and risers 3–8%, and lifts and cores in multi-storey buildings more. Each is a percentage of the net area; add them for the gross, and the net divided by the gross is the efficiency — 65–80% for most buildings, lower for hospitals and hotels.
Formula: gross = net × (1 + circulation + walls + plant + cores); efficiency = net ÷ gross
Worked examples
| Inputs | Gross floor area | Note |
|---|---|---|
| 2,400 m² office | 3,144 m² | 3,144 m² gross, 76% efficient |
| 2,400 m² hospital | 3,744 m² | 3,744 m², 64% |
| Custom 20% circulation | 1,360 m² | 1,360 m² |
FAQFrequently asked questions
What is a good efficiency?
Offices 75–85% net lettable to gross, apartments 75–82% saleable to gross, schools 65–75%, hotels 60–70%, hospitals 50–60%. Deep plans and simple shapes score higher; long thin buildings and many small rooms lower.
Which net area is this?
The usable rooms from the brief — the schedule of accommodation. Different measurement standards (net lettable, net internal, usable) draw the line in different places; keep the same definition throughout a project.
Why does building type set the circulation?
Because the plan type does: an open office is one big room with little corridor, a hospital is all corridors and waiting areas, a hotel has a double-loaded corridor on every floor. The presets are typical mid-range figures.
How does this affect cost?
Cost is paid on the gross area, income earned on the net. A 5-point gain in efficiency on a 10,000 m² building is 500 m² you build but cannot let — early plan choices decide it.
Where these figures come from
- Sabine (1922) — Collected Papers on Acoustics — the reverberation-time equation
- CIBSE Guide A — Environmental design — daylight, comfort and ventilation criteria
- Neufert — Architects' Data — the standard reference for space and dimension
- International Code Council — International Building Code — occupant load and egress factors
- International Code Council — publisher of the model codes most US jurisdictions adopt
Last checked: September 2026. Formulas are the standard building-science and code relationships; default thresholds are common international values and should be replaced with the ones in your code.