Stair stringer calculator: rise, run and cuts

How it works

Enter the finished-floor rise and the horizontal run from the first riser face to the last. The calculator selects the most balanced layout that fits the published dimensions, or shows the closest layout and the rule checks that fail.

Formula

• Divide the rise into equal steps

  Riser = total rise ÷ number of risers

  Every riser in a flight has the same calculated height.

• Divide the run between risers

  Goings = risers − 1, going = total run ÷ goings

  This calculator defines total run between the first and last riser faces. Landing dimensions and projections are not added.

• Check the step-line pitch

  Pitch = arctan(riser ÷ going)

  Pitch is calculated from one repeating rise and going, matching the stair line between tread nosings.

• Estimate the stringer line

  Stringer length = √(total rise² + total run²)

  This is the theoretical sloping length only. Add end cuts, overhangs and waste when choosing stock.

• Adjust the bottom seat

  Bottom seat rise = calculated riser − tread thickness

  The lowest tread rests on the stringer, this layout adjustment accounts for its thickness.

Worked example

1. 1

   Enter the floor-to-floor dimensions

   Rise 2,700 mm · run 3,600 mm · tread 25 mm

   Use finished surfaces and measure the horizontal run between the first and last riser faces.

2. 2

   Review the suggested layout

   15 risers at 180 mm · 14 goings at 257.1 mm

   The dimensions are evenly divided, one going is not counted at the upper landing.

3. 3

   Check the rule and material estimate

   2R + G = 617.1 mm · pitch = 35.0° · stringer line = 4,500 mm

   These values fall within the ordinary NCC table ranges. The bottom seat rise is 155 mm before site-specific checking.

Assumptions and limits

This check uses NCC 2022 Amendment 2 Housing Provisions Table 11.2.2a for ordinary non-spiral stairs. It does not apply the separate allowance for infrequently used stairs to non-habitable rooms. It checks equal geometry and listed ranges, but not flight-to-flight landings, headroom, handrails, barriers, structure, fixings, timber grade or local approval. Have a qualified builder or engineer check the finished stair and stringer design.

Frequently asked questions