Stair calculator · CostMate

This calculator divides a total rise into equal risers and gives you the run, the stringer length and the pitch. It sets out a flight. It does not decide whether that flight is acceptable, and the difference is deliberate.

What it does: equal risers, every time

Give it the finished floor to finished floor rise, the largest riser you want, and your going, and it back-solves the number of risers that divides the rise evenly.

That even division uses the same solver CostMate's estimating engine uses for balustrade post spacing, because it is the identical problem: a run that must not end in a stub. Seventeen risers at 176.5mm, never sixteen at 180mm plus one odd 100mm step — which is the classic way a flight ends up dangerous.

The run is the going multiplied by the number of goings, and whether that is one fewer than the risers depends on whether the top step is the upper floor itself. That is the switch on the page, and getting it wrong is the other common set-out error.

The stringer length is the hypotenuse of the run and the rise, and the pitch follows from the same triangle. The page also shows the flight with one more riser and one fewer, so you can see what a slightly different set-out would cost you in run.

What it deliberately does not decide

It does not state a maximum riser, a minimum going, or an acceptable band for two risers plus a going. It reports the two-risers-plus-going value and never a range, because the range belongs to D1/AS1 and this tool does not source it.

That is the same line CostMate takes inside the app, where the stairs option flags a D1/AS1 access-route scope check — going, riser, handrail, landing — and asks you to verify the dimensions rather than asserting them for you.

The maximum riser field is your target, not a limit CostMate is setting. Put in the figure you are working to.

What this calculator does not do

• Deciding whether a flight is acceptable. Riser, going, handrail and landing dimensions are D1/AS1's to give — verify against the current document.
• Handrails, balustrades and barrier heights.
• Landings, winders and changes of direction.
• Headroom, and the opening above the flight.
• Sizing the stringer, treads or risers as members, and how the flight is fixed top and bottom.
• Pricing. For the deck a flight comes off, see the decking calculator; for a priced estimate, the CostMate app.

Riser, going, handrail and landing dimensions are design decisions. Verify them against current D1/AS1 and your plans, and measure finished floor levels rather than framing, before you cut a stringer.