Stair Calculator: Rise, Run and Risers

Guide

How the calculator works

Take a 2.7 m floor-to-floor height and a target riser of 180 mm. 2,700 ÷ 180 = 15, so 15 risers of exactly 180 mm. A flight with 15 risers has 14 treads. At a 250 mm going the run is 14 × 250 = 3.5 m. The stringer is √(2.7² + 3.5²) = 4.42 m and the angle is 35.8°. Twice the riser plus the going is 2 × 180 + 250 = 610 mm, inside 550 to 700 mm.

You can type your own number of risers instead. With 14 risers each is 192.9 mm, over the 190 mm limit, so the calculator marks it as outside the limits.

The rules it checks, and where they come from

Table 11.2.2a of the NCC 2022 Housing Provisions gives a riser of 115 to 190 mm, a going of 240 to 355 mm, and 2R + G of 550 to 700 mm. A flight has 2 to 18 risers. Risers and goings count as constant if adjacent ones differ by no more than 5 mm and the largest and smallest in a flight differ by no more than 10 mm. A riser may not have an opening that lets a 125 mm sphere through. Headroom has its own rule in the NCC. Source: the ABCB page linked below.

These limits are for houses (Class 1 and 10 buildings). Other building classes and spiral stairs have different limits. Check your state variations and local rules.

A tight space

If space is short, a shorter going is tempting. At 230 mm the run falls to 3.22 m, but 230 mm is under the 240 mm minimum and the calculator flags it. Put your space in the optional box and it tells you whether the run fits and by how much.

What it does not check

• Headroom above the stairs, handrail and guard heights, landings, winders and tapered treads, which all have their own rules.
• Structure: stringer size and spacing, fixings and the floor above. Ask a builder or engineer.
• Local amendments. Councils and states can be stricter than the national or model code, and a permit or inspection may be needed.
• Existing stairs. Old houses are sometimes allowed to differ; ask your building authority.

Common questions