Staircase width would refer to the side-to-side distance if you were walking up or down the stairs. This distance must be at least 36 inches and does not include handrails.
In the India we use following terms and general guidelines:
(a) Tread: The horizontal top portion of a step where the footrests are known as the tread. The dimension ranges from 270 mm for residential buildings and factories to 300 mm for public buildings where a large number of persons use the staircase.
(b) Nosing: In some cases, the tread is projected outward to increase the space. This projection is designated as nosing.
(c) Riser: The vertical distance between two successive steps is termed as a riser. The dimension of the riser ranges from 150 mm for public buildings to 190 mm for residential buildings and factories.
(d) Waist: The thickness of the waist-slab on which steps are made is known as the waist. The depth (thickness) of the waist is the minimum thickness perpendicular to the soffit of the staircase (cl. 33.3 of IS 456). The steps of the staircase resting on waist-slab can be made of bricks or concrete.
(e) Going: Going is the horizontal projection between the first and the last riser of an inclined flight. The going should not be less than 250mm.
The respective dimensions of tread and riser for all the parallel steps should be the same in consecutive floor of a building.
The minimum vertical headroom above any step should be 2 m.
Generally, the number of risers in a flight should be restricted to twelve.
The minimum width of stair should be 850 mm, though it is desirable to have the width between 1.1 to 1.6 m. In public building, cinema halls etc., large widths of the stair should be provided.
Also angle of inclination is not more than 40 degree to horizontal.
Also find below attached general thumb rule:
Refer the codes for details on specific occupancy loads. Normally public spaces a minimum of 44” (112 cm) —providing sufficient space for one person and allowing the tight passage of two people. Comfortable two people, stair width, is between 49” (125 cm) to 60” (152 cm). For three users, a minimum of 74” (188 cm) is recommended. Handrails must be placed every 60” (152 cm).
For reduced use staircases such as accesses to lofts, attics and terraces, the minimum width shall be 600mm. The minimum tread shall be 250mm excluding nosing and the maximum riser will be 175mm for all buildings except for internal staircases of apartments, which can be permitted upto 190mm.