Stair Calculator
Lay out a straight stair and check it against IRC, NBC or UK Part K limits.
Stair
Tread = 620 mm − 2 × riser, but never less than the code minimum, and long enough for a pitch limit (42° in Approved Document K).
Width, headroom and stringer details optional
Options
| Risers | Riser | Tread | Run | Angle | 2R + T | Limits |
|---|
Checks
Setting-out table
| Step | Height of tread | Distance from first riser |
|---|
Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Stair Calculator
Plan a straight stair from the floor-to-floor height. The calculator divides the rise into equal risers, picks a comfortable tread with the 2R + T step rule (or uses your tread, or fits the run you have), and gives the total run, angle, stringer length, the stairwell opening needed for headroom and where a landing is required. Every option is checked against the limits you choose: the US IRC 2021, India’s NBC 2026 or NBC 2016 for one- and two-family dwellings, the English Approved Document K private stair, or your own figures.
It is an estimate to plan with — not a professional design. Codes are amended locally, and stairs also have rules for handrails, guards, winders and fire escape that are not covered here, so have the design checked by your architect, engineer or building authority.
How to use it
- Pick Metric or US and the code to check against.
- Measure the total rise from finished floor to finished floor (include floor finishes such as tiles and screed) and enter it — feet and inches such as
9' 1 1/4"work. - Choose how to set the tread: by the comfort rule, a tread you want, or the total run you have room for.
- Pick a row in the options table — the calculator suggests the first one that passes every limit — and read the riser and tread, run, angle and checks.
- Add the upper floor thickness to get the stairwell opening for headroom, the stair width and nosing to check them, and the stringer board depth and tread thickness for the cut-stringer layout. Copy or download the result.
Examples
15 risers of 7 5/16 in (7.283 in) and 14 treads of 10 in → run 11 ft 8 in, 36.1°, 2R + T = 24.57 in. Stringer 15 × √(7.283² + 10²) = 185.6 in; on a 2×12 (11¼ in) the throat is 5.36 in.
18 risers of 166.7 mm (the 175 mm limit needs at least 18) with 287 mm treads, 30.1°. 18 risers is more than 15 in one flight, so it needs a landing: two flights of 9, run 16 × 287 mm + 1.0 m landing = 5.59 m. Under a 150 mm thick upper floor the stairwell opening must reach back 4.76 m from the top riser — past the landing, which is only 1.5 m up — for 2.2 m of headroom.
16 risers of 187.5 mm with 250 mm treads (2R + T = 625 mm); NBC 2016 limits a flight to 12 risers, so two flights of 8 with a landing.
With the comfort rule’s 220 mm going, 12 risers (216.7 mm) and 13 risers (200 mm) would be 44.6° and 42.3°, steeper than the 42° limit, so the going is lengthened: 13 risers of 200 mm with 223 mm goings, 41.9°, 2R + G = 623 mm, run 2.68 m. 12 risers with 241 mm goings (41.96°, 2R + G = 674 mm) also meet Part K but are outside the 600–640 mm comfort band; 14 risers of 185.7 mm with 249 mm goings give a gentler 36.7°.
How it is worked out
Risers = total rise ÷ maximum riser, rounded up; riser = total rise ÷ risers, so every step is the same. A straight flight has one tread fewer than risers, because the upper floor is the last step: run = (risers − 1) × tread. With the comfort rule the tread is 620 mm − 2 × riser, raised to the code minimum and, where the code limits the pitch (42° in Approved Document K), lengthened until the stair is no steeper; it is capped at a maximum where there is one and rounded up to the next millimetre or 1/16 in.
Where the code needs landings, each landing takes the place of one tread: run = (risers − flights) × tread + landings × landing depth. Fit a run shares what is left of your run after the landings between the treads. The landing is 36 in deep under the IRC (R311.7.6, straight run) and as deep as the stair is wide under Approved Document K (para 1.20); NBC sets no landing size for dwellings, so the stair width, at least 1.0 m, is used.
Angle = atan(riser ÷ tread). A cut stringer’s top edge is risers × √(riser² + tread²) long, and the board left below the notches (the throat) is its depth − riser × tread ÷ √(riser² + tread²). Treads sitting on a stringer raise every step, so the bottom riser of the stringer is cut down by the tread thickness.
Limits used
- IRC 2021 §R311.7: riser ≤ 7¾ in (196 mm); tread ≥ 10 in (254 mm); headroom ≥ 6 ft 8 in (2,032 mm) measured from the sloped line of the nosings; width ≥ 36 in; one flight rises ≤ 12 ft 7 in (151 in) between floors or landings; nosing ¾–1¼ in unless the tread is 11 in or more; landing as wide as the stair and ≥ 36 in deep on a straight run; handrail 34–38 in. Text read in the 2021 Seattle Residential Code, chapter 3, which adopts the IRC 2021.
- NBC 2026 (SP 7:2026, Part F 4.4.2.4.3) — one or two family dwellings: riser ≤ 175 mm, tread without nosing ≥ 250 mm, 12 risers per flight recommended and not more than 15, width ≥ 1.00 m (0.75 m for two-storey row houses), headroom under landings and stairs ≥ 2.2 m. Other buildings: riser ≤ 150 mm and tread ≥ 300 mm. SP 7:2026 is listed on bis.gov.in; the clauses were read from the copy at archive.org/details/nbc-2026.
- NBC 2016 (Part 3 cl. 12.18, Part 4 cl. 4.4.2.4.3) — one or two family dwellings: riser ≤ 190 mm, tread ≥ 250 mm, 12 risers per flight, width ≥ 1.00 m, headroom ≥ 2.2 m (archive.org/details/nationalbuilding01).
- Approved Document K 2013, Table 1.1 — private stair: rise 150–220 mm, going 220–300 mm, pitch ≤ 42°, 2R + G between 550 and 700 mm; headroom 2.0 m, or 1.9 m at the centre and 1.8 m at the side for loft conversions (gov.uk).
- Comfort: Blondel’s step rule (Cours d’architecture, 1675–1683) links two risers and a tread to one walking pace; 600–640 mm is used as the comfort band. It is a guide, not a code limit.
Headroom and the stairwell opening
Headroom is measured straight up from the line through the tread nosings (or from a landing) to the underside of whatever is above. The stairwell opening must reach back from the top riser until the walking surface below is at least the code headroom + the floor thickness below the upper floor. For one straight flight that gives opening length = (headroom + floor thickness) × tread ÷ riser + nosing. With a landing part-way up, the landing stays high for its whole depth, so the opening usually has to extend over the landing as well — the calculator follows the real flights and landings. Enter the opening you have to see the headroom at its edge.
Limitations
- Straight stairs only: winders, spiral stairs and turning landings follow other rules (IRC R311.7.4 and R311.7.10, for example).
- Handrails, guards and balusters, open-riser gaps, fire-escape stairs, ramps and accessibility rules are not checked.
- Codes are amended by states, cities and municipal bye-laws — your local rules decide.
- Concrete waist slabs, steel stringers and timber sizes need structural design; the stringer figures are for setting out only.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
How many steps do I need for a 10 ft rise?
120 in ÷ 7¾ in = 15.5, so at least 16 risers of 7½ in under the IRC. With 10 in treads that is 15 treads and a 12 ft 6 in run. The comfort rule would also be satisfied: 2 × 7½ + 10 = 25 in (635 mm).
What is the maximum riser height in India?
For one or two family dwellings, NBC 2026 (SP 7:2026) allows risers up to 175 mm with treads of at least 250 mm; NBC 2016 allowed 190 mm. Other buildings are limited to 150 mm risers and 300 mm treads. Check your municipal bye-laws, which may differ.
Why does the calculator add a landing?
NBC 2026 allows at most 15 risers in a flight (12 recommended) and NBC 2016 at most 12; the IRC limits one flight to 12 ft 7 in of rise. Above that the stair is split into flights with a landing between them, and the run grows by the landing.
Is the total rise measured to the floor slab or the finished floor?
To the finished floor at both ends — tiles, screed and flooring included. If the floor finishes are not laid yet, add their thickness, or the first and last steps will come out different from the rest.
What is the 2R + T rule?
Two risers plus one tread should be about one walking pace. Blondel stated it in the 17th century; 600–640 mm (about 24–25 in) feels natural to most people. Approved Document K turns it into a requirement of 550–700 mm for every stair.
How long should the stringer board be?
At least risers × √(riser² + tread²) along its top edge — for 15 risers of 7.28 in and 10 in treads that is 185.6 in (15 ft 5.6 in) — so buy the next standard length up and keep enough throat below the notches for the span.