Stair Calculator

How to use this calculator

1. Measure total rise from finished floor to finished floor (include flooring you will add).
2. Enter the maximum riser your code allows — the calculator spreads the rise evenly.
3. Enter tread depth (going) excluding nosing.
4. Choose a code to check against; warnings appear if a limit is exceeded.

Method & assumptions

Risers = Total rise ÷ Max riser, rounded up
Riser height = Total rise ÷ Risers
Treads = Risers − 1; Total run = Treads × Tread depth
Stringer ≈ √(Rise² + Run²); Pitch = atan(Riser ÷ Tread)

Going

UK term for tread depth (horizontal)

2R + G

comfort rule: twice the riser plus the going, ideally 550–700 mm

Assumptions & limits

• Straight flight with the top tread being the upper floor.
• Code limits shown are common figures for private dwellings and are informational only.
• Headroom, landings, handrails and guards are not checked.

Rounding: purchase quantities are always rounded up to whole bags, boxes, sheets or bundles. See our methodology.

Worked examples

In depth: stairs planning notes

The rise-and-run rules

A comfortable stair balances riser height and tread depth. A widely used rule of thumb is that twice the riser plus the tread should be about 24–25 in (600–630 mm); in the UK and Ireland it is written as 2R + G between 550 and 700 mm. Building codes set limits: the US International Residential Code limits risers to 7¾ in and requires treads of at least 10 in, the UK Approved Document K limits private stairs to a 220 mm rise, a 220 mm minimum going and a 42° pitch, and Australia's NCC sets 115–190 mm risers and 240–355 mm goings for most stairs.

All risers in a flight must be equal, within a small tolerance, because people subconsciously expect the next step to match the last. That is why the calculator divides the total rise evenly rather than using a fixed riser height.

From numbers to stringers

Measure the total rise from finished floor to finished floor — including any flooring that will be added — because a missing ¾ in of flooring at the top or bottom creates an uneven first or last step, one of the most common stair errors. The number of treads is one fewer than the number of risers when the top step is the upper floor.

Stringers are cut from wide lumber, usually 2×12 in the US, using a framing square set to the riser and tread. The stringer length shown is an approximation along the pitch line; buy a board a foot or so longer to allow for the top and bottom cuts. Allow for headroom above the stair — codes typically require about 6 ft 8 in (2 m) measured from the pitch line.

Handrails, guards, landings and winders all have detailed code requirements. Use this calculator to plan layout and materials, then check the full rules with your building department or local authority.

Tips & common mistakes

• All risers must be equal; even small differences cause trips. Account for finished flooring at top and bottom.
• Cut one stringer, test-fit it, then use it as a template.
• Building regulations for stairs are safety-critical — confirm with your local building department.

Sources

1. ICC – International Residential Code R311.7 Stairways — checked 2026-10; verify before launch
2. UK Government – Approved Document K — checked 2026-10; verify before launch
3. ABCB – National Construction Code (Australia) — checked 2026-10; verify before launch

Default values (bag yields, coverage rates, densities) are typical figures from manufacturer literature. Always check the label on the product you buy.

Cite or embed this calculator

DIYCalcLab. “Stair Calculator.” Oct 18, 2026. https://diycalclab.com/lumber/stair-calculator/

An embeddable version for blogs and contractor sites is planned.