Stair Calculator

Plan a straight stair: risers, treads, total run, stringer length and angle from the floor-to-floor rise, checked against US, UK or Australian rules.

A straight stair is worked out from one measurement: the total rise from finished floor to finished floor. Divide it by the tallest riser allowed, round up to get the number of risers, and every riser is then the total rise divided by that number. There is one fewer tread than risers, because the top floor is the last step. For a 108 in rise with the IRC's 7¾ in limit, that's 14 risers of 7.71 in and 13 treads; at 10½ in per tread the stair runs 136½ in, the stringer is about 14½ ft long and the stair climbs at 36°. Choose US, England or Australia and the calculator checks the riser, going, 2R + G, pitch and flight limits for houses, and draws the stringer.

Units
Building code
in
in
in
in
in
in
Risers
14 × 7.714 in

13 treads of 10.5 in

Total run
136.5 in

11.37 ft

Stringer length
14.5 ft

buy 16 ft boards × 4

Angle
36.3°

2R + G 25.93 in

Checked against International Residential Code (US)

  • Passes: Riser at most 7.75 in
  • Passes: Going (tread) at least 10 in
total run 136.5 inrise 108 in
The stringer cut from a 11.25 in board: 14 risers, 13 treads. Not to scale for very long flights.
How it was worked out
  1. 108 in total rise ÷ 7.75 in maximum riser = 13.94, rounded up to 14 risers of 7.714 in
  2. 14 risers need 13 treads (the top floor is the last step): 13 × 10.5 in = 136.5 in total run
  3. Stringer line length = √(rise² + run²) = 14.5 ft (174.1 in); buy at least 16 ft to allow for the top and bottom cuts
  4. Angle = tan⁻¹(7.714 in ÷ 10.5 in) = 36.3°; 2 × riser + going = 25.93 in
  5. Each notch is 6.217 in deep (square to the edge), leaving a throat of 5.033 in in a 11.25 in board
  6. Stringers: 36 in wide at 16 in spacing = 4 stringers

The checks cover the main rise and going limits for stairs in a house; headroom (6 ft 8 in), handrails, guards, winders and landings have rules too, and local codes can differ. Measure the total rise from finished floor to finished floor.

Common use cases

  • Plan stairs for a deck, porch, basement or loft
  • Check an existing stair against the rules before a renovation
  • Work out how much floor space a new stair will take
  • Size and buy stringers

How to use

  1. Choose your units and the building code: US (IRC), England (Part K) or Australia (NCC).
  2. Enter the total rise, measured from finished floor to finished floor.
  3. Keep the maximum riser the code allows or lower it for a gentler stair, and enter the going (tread depth).
  4. Read the risers, treads, run, stringer length and angle, and the code checks.

FAQ

What are the stair rules for houses?

In the US, the International Residential Code allows risers up to 7¾ in and treads of at least 10 in, with 6 ft 8 in of headroom. In England, Approved Document K allows private stairs a rise of 150 to 220 mm and a going of 220 to 300 mm, with 2R + G between 550 and 700 mm and a pitch no steeper than 42°. Australia's NCC Housing Provisions allow risers of 115 to 190 mm and goings of 240 to 355 mm. Local codes can differ, so check before you build.

How many steps do I need?

Divide the total rise by the tallest riser you want and round up. A 108 in (2.74 m) rise at no more than 7¾ in per riser needs 14 risers and 13 treads.

What is 2R + G?

Twice the riser plus the going. It's a long-standing comfort rule, used in the UK and Australian rules: between 550 and 700 mm (about 22 to 27½ in) feels natural to climb. A 180 mm riser with a 250 mm going gives 610 mm.

Related tools

Results are estimates for planning and buying materials, worked out from the figures you enter and typical values (weights, coverage, sizes) that vary by product and supplier. They are not engineering, building or professional advice. Check your local building code, the product maker's instructions and your supplier's figures before you buy or build, and use a qualified professional for structural, electrical and gas work. The code checks cover the main dimensions only; the rules where you live, and your building inspector, have the final say.