What is a grossing factor in space programming?
Short answer
A grossing factor is the multiplier that converts net assignable area, the space people and equipment actually occupy, into gross area, which adds circulation, walls, shafts, structure and building services. A factor of 1.35 means every 1,000 square feet of net needs 1,350 gross. Programmers often apply one factor per department and a second for the whole building.
In space programming, the first list of rooms and functions is measured in net area: the floor space that people, furniture and equipment need. A building cannot be built from net area alone, because it also needs corridors, wall thickness, stairs, elevators, shafts, mechanical rooms and structure. The grossing factor is the multiplier that adds all of that.
What is the formula for gross area from a grossing factor?
Gross area = Net area * Grossing factor
The inverse is the net-to-gross ratio, often called efficiency: Net-to-gross = Net / Gross = 1 / Grossing factor.
| Grossing factor | Net-to-gross ratio |
|---|---|
| 1.25 | 80.0 percent |
| 1.35 | 74.1 percent |
| 1.50 | 66.7 percent |
A higher factor means more of the building is not directly assignable to the program.
How do I apply a grossing factor in two steps?
Many programs, especially for hospitals, campuses and offices with large departments, gross up in two steps. The numbers below are example assumptions, not recommendations.
| Step | Calculation | Result (sq ft) |
|---|---|---|
| Net assignable area | sum of all rooms in the program | 18,400 |
| Department gross area | 18,400 × 1.25 | 23,000 |
| Building gross area | 23,000 × 1.15 | 26,450 |
The departmental factor of 1.25 covers circulation inside the department and the partitions between rooms. The building factor of 1.15 covers the shared parts: main corridors, stairs, elevators, shafts, mechanical and electrical rooms, and exterior walls. The combined factor is 26,450 / 18,400 = 1.4375, which is a net-to-gross ratio of about 69.6 percent.
In a spreadsheet, keep both factors as input cells and calculate =B2*B3*B4 (net area, departmental factor, building factor) rather than typing a single combined number, so you can see which assumption changes the answer.
Where do grossing factors for a building come from?
There is no universal grossing factor. The right value depends on the building type, structural grid, corridor layout, the depth of the floor plate, mechanical systems and the quality of the plan. Practitioners take factors from:
- Precedent projects of the same type, measured the same way.
- Design guides or standards for the building type, such as those published for hospitals, schools and research buildings.
- A test fit, where a quick sketch layout of the program shows how much area the circulation and services actually take.
An early program often uses a conservative factor, which is then replaced with measured areas as the design develops.
What do people confuse grossing factors with?
- Factor versus percentage. Some teams write 1.35, others write "35 percent grossing". The two mean the same thing, but state which one you use. Converting between them is simple: 35 percent added to net is a factor of 1.35.
- Net-to-gross versus grossing factor. They are reciprocals. A net-to-gross ratio of 74 percent is a grossing factor of 1 / 0.74, about 1.35, and not 1.26 (which is 1 + 0.26).
- Different area definitions. Gross area, rentable area and usable area are measured by different rules, and office leasing standards use a separate load factor for common areas. Say which standard your numbers follow before comparing projects.
How do I use a grossing factor in a space program?
Start with a list of rooms and their net areas, group them into departments, then apply the factors. The space program calculator sheet is built around this net-to-gross step, and the room and area schedule sheet is a place to keep the room-by-room areas that feed it. The blog post on room and area schedules for architects covers how to structure those lists.