Blondel's Rule (2 × Riser + Tread = 63 cm) is the golden standard in stair design, developed by French architect Nicolas-François Blondel in the 17th century. The constant 63 cm represents the average human step length. When 2R+T falls between 60–66 cm, the staircase feels natural and comfortable to climb. Values close to 63 cm are considered optimal for residential and public stairs alike.

For residential stairs, the ideal riser height is between 15–20 cm (approx. 6–7.9 inches), with 17–18 cm being the sweet spot for most homes. Risers below 15 cm can feel too shallow and cause tripping, while risers above 20 cm become steep and tiring. Building codes in many countries cap residential risers at 19–20 cm maximum. Always check your local building regulations.

Divide the total rise (floor-to-floor height) by your desired riser height (typically 17–18 cm). Round to the nearest whole number. For example, a 280 cm total rise ÷ 17.5 cm = 16 steps. The actual riser height is then 280 ÷ 16 = 17.5 cm. All risers in a staircase must be uniform—uneven risers are a major tripping hazard and violate most building codes.

The minimum tread depth (going) for residential stairs is typically 25 cm (approx. 10 inches) in most international building codes. However, 28–30 cm is recommended for optimal comfort and safety. Treads shallower than 25 cm make it difficult for an adult foot to rest securely, increasing fall risk. For commercial or public buildings, minimum tread depths are often larger—check your local regulations.

A stair angle between 30° and 38° is generally considered safe and comfortable for residential use. Angles below 30° result in very shallow, ramp-like stairs that take up excessive space. Angles above 38° feel steep and become difficult for children, elderly, or anyone carrying items. The stair angle is calculated as arctan(Riser Height ÷ Tread Depth). For example, R=17.5 cm and T=28 cm gives approximately 32°, which is ideal.

Total Run is the horizontal projection of the entire staircase. For a staircase with N steps (risers), there are N-1 treads (the top step is flush with the landing). So: Total Run = Tread Depth × (N - 1). For example, 16 steps with 28 cm treads give a total run of 28 × 15 = 420 cm. If you have limited horizontal space, you may need to reduce tread depth or consider a different stair configuration like an L-shape or spiral staircase.

Uniform riser height is critical for safety. The human brain quickly memorizes the step rhythm when climbing or descending stairs. If even one riser differs by more than 3–5 mm from the others, it disrupts this rhythm and significantly increases the risk of tripping and falling. Most building codes require all risers in a flight to be within 3 mm (1/8 inch) of each other. This is non-negotiable for safety.

Residential stairs typically allow steeper angles (up to ~38°) with risers up to 20 cm and treads as small as 25 cm. Commercial and public stairs must be more gradual: risers are usually capped at 15–18 cm, and treads must be at least 28–30 cm to accommodate high foot traffic, accessibility requirements, and emergency egress. Commercial stairs also require handrails on both sides above certain widths, and landings at specific intervals. Always consult your local building code (IBC, IRC, Eurocode, etc.) for exact requirements.