Stair Calculator — Rise, Run, Slope & Comfort Check

Real-world use cases

• Plan a basement staircase in a DIY self-build

  You are framing stairs from the basement slab to the ground floor and measured 2900 mm of total rise. Type that in, set a target rise of 180 mm, and the tool lands on 16 steps at 181 mm each with a 268 mm tread — a flight that passes Blondel at 630 mm and sits at 34° pitch. You now have the exact riser height to mark on the stringer stock before the first cut, instead of guessing and discovering on step 14 that the top riser is 40 mm taller than the rest.

• Check a duplex / loft staircase against comfort rules

  A split-level or loft conversion often forces a steeper flight to fit a tight opening. Enter your rise and the step count the opening allows, and watch the pitch-angle row: if it goes amber past 37° or red past 42°, you know the flight will feel like a ladder and you should either steal more run length or add a step. Catching that here is cheaper than rebuilding after the building inspector flags it.

• Size outdoor deck or garden steps

  Exterior steps usually want a gentler rise (130-160 mm) and a deeper tread for wet-weather footing. Switch to that target rise, and the tool recomputes the step count and the total horizontal run so you know how far the steps will project into the yard — critical when the landing has to clear a path or a drain.

• Verify a contractor's stair quote

  A builder hands you a drawing: 14 steps, 195 mm rise, 240 mm tread. Plug it in. The convenience and Blondel rows will tell you instantly whether those numbers are within comfortable range (a 195/240 step gives 2R+T = 630, which is fine, but 195 mm is on the tall side). You walk into the conversation knowing whether to push back.

• Convert an imperial plan to metric on site

  The architect's drawing is in inches but your tape measure and the lumber yard are metric. Enter the rise in inches, read the rise/run back in mm, and the same comfort verdict applies — no separate conversion step, no rounding drift between the two unit systems.

Common pitfalls

• Counting treads instead of risers. A flight that climbs one storey has one more riser than it has treads — the top riser lands on the floor above and has no tread. If you divide total rise by the number of treads you will get a rise that is slightly too small and a flight that overshoots the floor. This tool divides by risers (the "steps" count) and computes total run over treads.

• Forgetting that rise and run trade off. People fix a "nice" rise and a "nice" run independently, then wonder why the stairs feel wrong. The Blondel rule ties them together — raise the step and the tread must shrink to keep 2R+T near 620. Adjusting one number in isolation is the most common reason a code-legal flight still feels exhausting.

• Mixing units mid-calculation. Reading rise off an imperial drawing and run off a metric one, then comparing to a comfort number in the wrong system, produces nonsense. Pick one unit in this tool; the comfort thresholds are converted consistently behind the scenes so a verdict in inches means the same as in mm.

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