Staircase Calculator

How does the tool decide how many risers there are?

It divides the total rise by your target riser height and rounds to the nearest whole number, with a minimum of one. The actual riser height is then the total rise divided by that count. A 2,700 mm rise at a 180 mm target gives exactly 15 risers of 180.0 mm; 2,600 mm at 170 mm gives 15 risers of 173.3 mm, because 15.29 rounds down.

How is the horizontal run calculated?

This tool uses run = tread depth × (risers − 1) + nosing. The top floor is treated as the last tread, so a 15-riser flight has 14 treads. The nosing value is added once to the total run. For the defaults that is 260 × 14 + 25 = 3,665 mm. If you measure your run differently, compare it with this figure before cutting anything.

How is the stringer length calculated?

It is the straight-line length from the bottom to the top of the flight, the square root of rise² + run². The default flight comes to 4.552 m. It is a geometric length along the slope; it does not include the extra timber needed for the top and bottom cuts, trimming, or any notch or housing depth.

What is the 2R + T check?

It adds twice the actual riser height to the tread depth. For 180 mm risers and 260 mm treads that is 620 mm. The tool marks the result as passing between 600 and 640 mm and warns otherwise. These are figures built into the tool as a general comfort rule of thumb, not requirements from any code.

Which limits does it use for the warnings?

It flags a riser outside 150–220 mm, a tread outside 220–350 mm, and 2R + T outside 600–640 mm. They are generic figures built into this tool. Local building codes vary and a qualified professional or building inspector decides what applies to your stair, so a flight with no warnings is not an approval, and one with warnings is not necessarily non-compliant.

Why does stair width not change any result?

The width field is accepted but this version does not use it in any calculation, so changing it leaves the riser count, run and stringer length the same. Treat it as a note for your own planning, and work out tread material, stringer spacing and load requirements separately.

What if my rise is small or the target riser is larger than the rise?

The count is held at a minimum of one riser, so the calculation always completes. A 100 mm rise with a 500 mm target returns one riser of 100.0 mm and shows the warnings. Check any such result by eye, since it describes a single step.

Does it cover landings, winders, spirals or headroom?

No. It treats the stair as one straight flight with equal risers and equal treads. Landings, turns, winders, spiral stairs, headroom, handrails and guarding are not checked, and nothing here is a structural design, so rely on a qualified professional or inspector for those.