General Definition
- Stair width is the clear horizontal distance between the inner finished surfaces of stairway walls, handrails, or guardrails, measured perpendicular to the direction of travel. It determines the capacity of a stairway for occupant movement and is a critical factor in egress safety, accessibility, and code compliance.
- IBC (USA): Section 1011.2, establishes stair width based on occupant load.
- BSL (Japan): Article 119, specifies fixed minimum stair widths for different building types.
Minimum Required Stair Width
| Code | Standard Stair Width Requirement | Large Occupancy (>50 occupants) |
| IBC (Section 1011.2 – USA) | 36 inches (914 mm) if occupant load ≤50 | 44 inches (1118 mm) if occupant load >50 |
| BSL (Article 119 – Japan) | 90 cm (35.4 inches) minimum | 120 cm (47.2 inches) for large assembly buildings |
- BSL requires a wider minimum stair width for large assemblies (120 cm vs. IBC’s 44 inches / 1118 mm).
- IBC bases stair width on occupant load, whereas BSL enforces a fixed standard for large buildings.
- IBC requires a minimum of 36 inches (914 mm) for small occupant loads, whereas BSL mandates a slightly narrower 90 cm (35.4 inches).
Adjustments Based on Occupant Load
| Code | Occupant Load Impact | Scaling for Large Buildings |
| IBC (1011.2) | – Stair width increases as occupant load grows. – Requires 0.3 inches (7.62 mm) per occupant beyond the first 50 occupants. | – Large auditoriums, theaters, and stadiums require wider stairs depending on capacity. |
| BSL (Article 119) | – Fixed minimum widths apply regardless of occupant load. | – Automatically requires 120 cm (47.2 inches) for large assembly buildings without scaling per occupant. |
- IBC dynamically scales stair width based on occupancy, while BSL sets a fixed minimum for large buildings.
- IBC’s approach allows stair widths to grow with increasing occupant loads, whereas BSL’s 120 cm standard ensures a wide starting point for large buildings.
Special Considerations for Accessibility & Fire Safety
| Code | Fire Safety Adjustments | Accessibility Compliance |
| IBC (1011.2) | – Requires increased stair width for fire exits based on occupant calculations. | – Explicit ADA compliance required (minimum 48 inches if handrails reduce clearance width). |
| BSL (Article 119) | – Fire safety evaluations determine additional stair width beyond 120 cm if needed. | – General accessibility standards apply, but no direct ADA-equivalent compliance. |
- IBC strictly enforces ADA compliance, while BSL follows a different accessibility framework that does not directly align with ADA.
- Both codes require stair width adjustments for fire safety, but IBC uses occupant-based calculations, whereas BSL follows building type classifications.
Key Differences & Considerations
| Aspect | IBC (1011.2 – USA) | BSL (Article 119 – Japan) |
| Standard Stair Width | 36 inches (914 mm) for ≤50 occupants | 90 cm (35.4 inches) minimum |
| Large Occupancy Stair Width | 44 inches (1118 mm) for >50 occupants | 120 cm (47.2 inches) for large assembly buildings |
| Occupant Load Impact | Stair width increases based on occupant count. | Fixed width standards apply, no scaling per occupant. |
| Fire Safety Considerations | Requires additional width for emergency exits. | Additional width may be required based on fire evaluations. |
| Accessibility Compliance | Strict ADA compliance (minimum 48 inches if reduced by handrails). | General accessibility compliance, no direct ADA equivalent. |
Recommendations for Improvement
| Issue | IBC Recommendation | BSL Recommendation |
| Minimum Stair Width for Large Assembly Buildings | Increase minimum stair width to 48 inches (1220 mm) for large A-1 occupancies to align with international best practices. | Introduce occupant-based stair width scaling beyond 120 cm for high-density buildings. |
| Stair Width Scaling in High-Occupancy Spaces | Establish fixed width minimums for simplicity in theaters and stadiums instead of occupant-based calculations. | Provide greater stair width flexibility based on specific assembly occupancy needs. |
| Accessibility Standardization | Ensure all stair designs consider universal access beyond ADA minimums. | Strengthen clear width requirements for handrails and wheelchair-accessible stair alternatives. |
Summary
- IBC requires stair width to increase with occupant load, while BSL enforces a fixed minimum of 120 cm (47.2 inches) for large buildings.
- BSL mandates a wider standard stair width for assembly occupancies (120 cm vs. IBC’s 44 inches).
- IBC dynamically adjusts stair width based on the number of occupants, while BSL applies a set standard for all large buildings.
- IBC enforces strict ADA compliance for accessibility, while BSL follows separate accessibility guidelines.
- Both codes require stair width adjustments for fire safety, but IBC uses occupant-based scaling while BSL requires additional evaluations.