Stair Calculator

The Blondel formula (2R + T = 600–640 mm) was proposed by 17th-century French architect François Blondel to describe the ergonomic stride length on stairs. When this range is satisfied, climbing feels natural without overstretching or shuffling. This calculator uses 620 mm as the target and warns when the value falls outside 600–640 mm.

The tool rounds total rise ÷ preferred riser to the nearest integer to ensure every step is equal height. The actual riser (total rise ÷ rounded step count) will therefore differ slightly from your preferred value. Always verify the displayed 'actual riser' before drafting construction drawings.

Without a measured run, the Blondel formula T = 620 − 2R provides a recommended starting tread depth. This is useful in early design when only the floor-to-floor height is known. Enter the actual total run once the stair footprint is fixed to get precise tread dimensions.

This tool's “Shared” mode warns when the riser exceeds 180 mm or the tread is shallower than 260 mm, and “Residential” mode when the riser exceeds 200 mm or the tread is shallower than 240 mm. (1) Both sets of values come from Article 16(1) of Korea's Regulations on Housing Construction Standards, which apply to housing built under an approved housing construction project plan, such as apartment complexes: ‘stairs used in common’ need R ≤ 180 mm / T ≥ 260 mm, and ‘outdoor stairs of a building’ R ≤ 200 mm / T ≥ 240 mm (R = riser, T = tread). (2) Stairs in buildings over 200 m² total floor area follow Korea's Rules on Standards for Evacuation and Fire-Protection Structures of Buildings under Article 48(1) of the Enforcement Decree of the Building Act. Article 15(2) of those Rules sets riser and tread limits only for elementary schools (riser ≤ 160 mm, tread ≥ 260 mm) and middle and high schools (≤ 180 mm, ≥ 260 mm). (3) Neither regulation sets riser or tread limits for indoor stairs in detached houses or inside an apartment unit, so “Residential” uses the less strict of the two Article 16(1) sets of values, those for outdoor stairs, as a reference. Always verify current local authority requirements before permit submission.

This calculator does not compute stair width (the clear width between the two sides). The legal minimum depends on the building type and size. (1) Housing built under an approved housing construction project plan, such as apartment complexes: stairs used in common need a clear width of at least 120 cm, and outdoor stairs of a building at least 90 cm (Regulations on Housing Construction Standards, Article 16(1)). (2) Indoor stairs in buildings over 200 m² total floor area (the sum of all floor areas): under Article 15(2) of the Rules on Standards for Evacuation and Fire-Protection Structures of Buildings, at least 150 cm for elementary, middle and high schools, and at least 120 cm for performance halls, assembly halls, spectator venues, sales facilities and similar uses. In buildings other than the schools and performance, assembly, spectator and sales facilities above, stairs on an above-ground floor also need at least 120 cm if the habitable rooms (rooms used for living, work, assembly and the like) from the floor directly above that floor up to the top floor (or, if one of those upper floors is an evacuation floor, up to the floor below it) total 200 m² or more. Stairs on a basement floor whose habitable rooms total 100 m² or more also need at least 120 cm; all other stairs need at least 60 cm. Direct stairs that a quasi-high-rise building installs under the proviso to Article 34(4) of the Enforcement Decree of the Building Act are governed separately by Article 15(7) of the same Rules (at least 120 cm in apartment buildings, at least 150 cm in other buildings). (3) In buildings over 200 m² total floor area, railings (a wall or equivalent also counts) are required on both sides of stairs and landings (the flat resting areas between flights) more than 1 m high (same Rules, Article 15(1)2). (4) In housing complexes, railings must in principle be at least 120 cm above the finished floor, or at least 90 cm in lower-risk places such as stairs inside a building (Regulations on Housing Construction Standards, Article 18(2)1).

The geometry formulas apply equally to indoor and outdoor stairs, but outdoor stairs benefit from wider treads (to shed rainwater) and anti-slip finishes. In regions with winter icing, lowering risers to 150 mm or below is sometimes recommended.