Stair stringer calculator: rise, run and cuts

How it works

Choose the nation whose rules apply, then enter finished-floor rise and horizontal run. The calculator chooses a balanced layout from that nation’s private-stair guidance and lists each dimensional check.

Formula

• Divide the rise into equal steps

  Riser = total rise ÷ number of risers

  Each step in the flight uses the same calculated rise.

• Divide the run between risers

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

  Run is measured between the first and last riser faces. Upper and lower landings are excluded.

• Check the step-line pitch

  Pitch = arctan(riser ÷ going)

  This uses the repeating tread-nosing line for the selected nation’s private-stair pitch limit.

• Estimate the stringer line

  Stringer length = √(total rise² + total run²)

  The result is a theoretical length before end cuts, overhangs and material waste.

• Adjust the bottom seat

  Bottom seat rise = calculated riser − tread thickness

  This cut-layout adjustment makes room for the lowest tread thickness.

Worked example

1. 1

   Enter a measured stair opening

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

   Measure between finished floor surfaces and riser faces.

2. 2

   See how the dimensions divide

   With 15 risers: 180 mm each · 14 goings at 257.1 mm

   The selected nation can produce a different suggested riser count, because its limits differ.

3. 3

   Read the nation-specific check

   Pitch = 35.0° · stringer line = 4,500 mm

   This example fits the private-stair checks in all four nation choices. Wales and Northern Ireland also apply a 2R+G range, England and Scotland do not use that check for this private stair table.

Assumptions and limits

The selected rules are for domestic/private stairs, not public or shared stairs. England uses Approved Document K Table 1.1, with rise, going and pitch checks. Wales uses its alternative rise/going bands plus the 2R+G relationship. Scotland uses its private-stair rise, going and pitch table and normally limits flights to 16 rises, lower-count exceptions depend on location. Northern Ireland uses rise, going, pitch and 2R+G checks and allows certain single-step exceptions. The calculator does not check the full regulations, landing/headroom, handrails, barriers, structure, timber grade, fixings or permissions. Ask a qualified builder or building-control professional to check the finished design.

Frequently asked questions