Public Building Staircase Width Calculator (Units of Passage)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The minimum width of a staircase in a French public building is calculated in units of passage (1 UP = 0.60 m): number of UP = round up (occupancy ÷ 100), increased by one unit if the total occupancy is below 501 people. For an occupancy of 100 people, that gives 2 UP, a minimum width of 1.40 m (a regulatory exception at 2 UP).
Explanation
In a French public building ("établissement recevant du public", ERP), the width of staircases and other escape routes isn't set freely: it follows directly from the occupancy the staircase must evacuate, under article CO 38 of the French fire safety code. The basic unit of this calculation is the "unit of passage" (UP), a standard width of 0.60 m corresponding to the space taken up by one person walking abreast; the number of UP needed is calculated at one unit per 100 people or fraction thereof, this occupancy figure combining, depending on the levels served, the people on the level concerned with those on levels above (upper floors) or below (basement). The article's text provides for an extra unit as a safety margin for any building whose total occupancy stays below 501 people, which is why even a very small public building (a few dozen people) must already provide a 2-UP staircase. Two notable regulatory exceptions then adjust the theoretical width (UP × 0.60 m): at exactly 1 UP, the width is raised to 0.80 m rather than 0.60 m, and at exactly 2 UP, it is raised to 1.40 m rather than 1.20 m — two thresholds that, combined with the safety margin, explain why the minimum regulatory width most often cited for a small public building is precisely 1.40 m. This calculator applies this method to give a useful order of magnitude at the sketch-design stage, but never replaces a calculation note validated by a fire safety design office or a certified control body, the only ones authorized to apply the full set of rules specific to the category and type of the building concerned — an approach of the same nature as the one already recommended for the step count and tread depth of a residential staircase, another dimension that mixes a simple formula with a professional validation step.
Example: a staircase serving an occupancy of 100 people
Inputs
Total occupancy to evacuate through this staircase: 100 people.
Calculation
Number of UP = round up (100 ÷ 100) = 1, increased by one unit (100 < 501) = 2 UP. Minimum width = exception at 2 UP = 1.40 m (rather than 2 × 0.60 = 1.20 m).
Result
This staircase must be at least 1.40 m wide.
Frequently asked questions
Why doesn't the width simply follow UP × 0.60 m in every case?
Because article CO 38 explicitly provides for two regulatory thresholds at 1 and 2 units of passage: at 1 UP, the theoretical 0.60 m width is deemed insufficient for a comfortable evacuation and is raised to 0.80 m; at 2 UP, the theoretical 1.20 m width is likewise raised to 1.40 m. Beyond 2 UP, the simple UP × 0.60 m rule applies directly, with no further threshold.
Where does the extra unit for buildings under 501 people come from?
This extra unit is a safety margin built directly into the text of article CO 38: it ensures that a small or medium-sized building always has an escape-route width slightly above the strict proportional calculation, deemed necessary for effective evacuation in these more common configurations. It disappears from 501 people onward, the threshold beyond which the simple proportional calculation applies without any increase.
Is this calculator enough to validate a building permit for a public building?
No, this calculator only gives an order of magnitude for the minimum regulatory width of a single staircase, from an occupancy figure already determined. A building permit file for a public building requires a full study by a fire safety design office, which accounts for the building's category and type, the actual combined occupancy across levels, the total number of required escape routes, and many other rules from the French fire safety decree outside the scope of this simple calculation — much like the regulatory ceiling height calculator, another building dimension governed by its own dedicated regulatory text.