Stair calculator with riser, tread and headroom checks
This stair calculator lays out a straight flight from one number: the total rise from finished floor to finished floor. It divides the rise into equal risers of a comfortable height, works out the treads, the total run, the stringer length and the pitch angle, draws a side elevation to scale and checks the result against the 2R + T comfort rule and the riser, tread, pitch and headroom limits of the Indian NBC, the US IRC or the UK Building Regulations.
Stairs are unforgiving: every riser in a flight must be the same height (codes allow only 5–10 mm variation), one step too few makes the flight steep and one too many eats into the room below. Nudging the riser count up or down and seeing the angle, run and headroom change immediately makes planning on paper worthwhile before cutting a stringer or pouring concrete.
Dimensions are in millimetres or inches, with long lengths shown in metres or feet and inches. It is a planning aid — confirm the final design with your local building rules and, for concrete stairs, a structural engineer.
How to use the Stair Calculator
- Choose millimetres or inches and the rules to check against (NBC India, IRC, UK Part K or comfort rule only).
- Enter the total rise — floor to floor, including the thickness of the upper floor and both floor finishes.
- Set the ideal riser height (170–180 mm / 7 in is comfortable) and the tread depth (going). If the space is limited, enter the available total run instead and the tread is calculated to fit.
- Use the − and + buttons to try one riser fewer or more; the riser height, angle and checks update at once.
- For headroom, enter the upper floor thickness and the length of the stairwell opening measured back from the top riser. Read the checks, the diagram and the table, then copy or share the layout.
Stair Calculator features
Automatic riser count
Total rise ÷ ideal riser, rounded to a whole number so every riser is identical; override with the − / + buttons.
Comfort and code checks
Blondel’s 2R + T = 600–660 mm, maximum riser, minimum tread, maximum pitch and headroom for NBC India, IRC and UK Part K.
Stringer and angle
Stringer length along the nosing line and the pitch in degrees for cutting or ordering stringers.
Headroom from the opening
Enter the stairwell opening length and floor thickness to get the clear headroom at the edge of the opening.
Scaled SVG diagram
Side view with every riser and tread, nosings, the stringer line, the upper floor and the headroom line.
Fit to an available run
Give the space you have and the tread depth is solved for you.
How the stair is calculated
- Number of risers = total rise ÷ ideal riser, rounded: 2,700 ÷ 180 = 15 risers.
- Riser height = total rise ÷ risers = 180 mm exactly. Treads = risers − 1 = 14 (the upper floor is the last “tread”).
- Total run = tread depth × treads = 280 × 14 = 3,920 mm. If you entered an available run instead, tread = run ÷ treads.
- Stringer length = √(rise² + run²) = √(2,700² + 3,920²) = 4,760 mm; angle = atan(2,700 ÷ 3,920) = 34.6°.
- Headroom at the edge of the stairwell opening = opening length × (riser ÷ tread) − floor thickness. With a 4 m opening: 4,000 × 0.643 − 250 = 2,321 mm.
Rules of thumb and code limits
| Rule | Limit | Notes |
|---|---|---|
| Blondel comfort formula | 2R + T = 600–660 mm (23.6–26 in) | The classic stride rule; also R + T ≈ 430–460 mm and R × T ≈ 450 cm² |
| NBC India 2016, residential | riser ≤ 190 mm, tread ≥ 250 mm, headroom ≥ 2.2 m | Minimum width 1.0 m for residential stairs |
| IRC R311 (US residential) | riser ≤ 7¾ in, tread ≥ 10 in, headroom ≥ 6 ft 8 in | Risers in a flight may not differ by more than ⅜ in |
| UK Part K, private stair | riser 150–220 mm, going 220–300 mm, pitch ≤ 42°, headroom ≥ 2.0 m | 2R + going must be 550–700 mm |
| Comfortable pitch | 30–38° | Over 42° feels like a ladder; under 25° wastes floor space |
Planning tips
- Measure the total rise from finished floor to finished floor, not from slab to slab — a 20 mm tile on one level changes the riser height of the whole flight.
- Cut stringers to the riser height minus the tread thickness at the bottom step, so the first and last risers end up equal once the treads are on.
- A landing counts as a tread in the run but not as a riser; split long flights with a landing after 12–16 risers.
- Concrete stairs: the concrete calculator has a solid-steps option that uses the same riser and tread figures. Deck steps pair with the deck board calculator.
Stair Calculator FAQ
How many steps do I need for a 9 ft (2.7 m) rise?
With a 7 in (180 mm) ideal riser, 108 in ÷ 7 = 15.4 → 15 risers of 7.2 in (180 mm) and 14 treads. The stair calculator shows the run, stringer length and the code checks for that choice.
What is a comfortable riser height and tread depth?
170–180 mm (6¾–7 in) risers with 270–300 mm (10½–11¾ in) treads suit most people. Check with 2R + T: 2 × 180 + 280 = 640 mm sits in the comfortable 600–660 mm band.
How do I calculate stringer length?
It is the hypotenuse of the rise and run: √(rise² + run²). For 2,700 mm rise and 3,920 mm run the stringer along the nosing line is 4,760 mm; buy material a little longer for the cuts at each end.
How do I check headroom?
Headroom is the vertical distance from the nosing line to the underside of the floor above at the edge of the stairwell. The calculator uses opening length × riser ÷ tread − floor thickness; most codes require 2.0–2.2 m (6 ft 8 in in the US).
Why must all risers be the same height?
People learn the rhythm of a stair in the first two steps; an odd riser of even 10 mm more is a trip hazard. Codes limit the variation in a flight to about 5–10 mm, which is why the calculator divides the total rise exactly.
Does the calculator design spiral or winder stairs?
No — it covers straight flights (and straight flights between landings). Winders and spirals need the going measured along the walking line and have their own rules.