Stair calculator

Type in the height from the ground to the top of the deck or floor, and how deep you want each tread. The calculator splits the height into equal steps up, each no more than 190 mm, the most the building code allows. Then it checks each rule and works out the run and the stringer.

The deck or floor is the top step, so you build one tread fewer than the steps up. Want a gentler stair? Type in more steps.

The National Construction Code 2022 sets these sizes for stairs in a house. Queensland still uses the 2022 code: the 2025 code starts here on 1 May 2027.

NCC 2022 Housing Provisions, Table 11.2.2a and clauses 11.2.2 and 11.2.5, checked 27 October 2026. Spiral stairs have their own limits.

Every step in a flight must be the same. Steps next to each other can differ by no more than 5 mm, and the biggest and smallest in a flight by no more than 10 mm.

The stringer is the board that runs up the slope and holds the treads. Its length is the long side of a triangle, with the height one way and the run the other. Square both, add them, and take the square root.

Take a deck 1,000 mm up. That needs 6 steps of about 167 mm. With 250 mm treads, the 5 treads run 1,250 mm. 1,000 squared plus 1,250 squared, square-rooted, is about 1,601 mm. Buy the stringer longer, so there is timber to cut at the top and bottom.

A stair needs a handrail along at least one side for the full flight, unless it climbs less than 1 m. The top of the handrail sits at least 865 mm above the front edge of the treads. Where you could fall 1 m or more off the side, it also needs a balustrade at least 865 mm high, with no gap a 125 mm ball fits through. That is NCC 2022 Part 11.3.

The balustrade calculator counts the balusters for you.