Plan safer stairs: what this tool calculates for you
I’m Adrian Cole, construction cost analyst. This tool answers five things fast: number of risers, riser height, tread (going), total run, and stringer length, given a single total rise. It also shows stair angle so you can check comfort and code.
Use it early to size a straight flight, sanity-check shop drawings, or rough in framing lengths before ordering stringers.
How the Stair Calculator chooses sizes and counts
The calculator searches for an integer count of risers between bounds implied by your Min riser (mm) and Max riser (mm). It then applies a stair comfort rule to set the going within your Min tread (mm) and Max tread (mm), adds any Nosing (mm), and computes geometry.
- Risers: nMin = ceil(Total rise / Max riser), nMax = floor(Total rise / Min riser). Valid n is any integer between them.
- Riser height: Riser = Total rise / n.
- Going: from either 2 × Riser + Going ≈ 630 mm or Riser + Going ≈ 450 mm, clamped to your tread range; then + Nosing.
- Total run: (n − 1) × Going. Stringer: sqrt(Total rise² + Total run²). Angle: atan(Riser / Going).
Assumptions: straight flight, uniform risers and goings, number of treads equals risers minus one.
Worked example with typical residential numbers
Inputs
- Total rise (mm): 2800
- Min riser (mm): 150
- Max riser (mm): 190
- Min tread (mm): 250
- Max tread (mm): 300
- Nosing (mm): 0
- Preferred rule: 2 × Riser + Going ≈ 630 mm
Outputs
- Number of risers: 16
- Riser height: 175 mm
- Tread (going): 280 mm
- Total run: 4,200 mm
- Stringer length: 5,053.9 mm
- Stair angle: 32.7°
Sanity test: Riser 170–180 mm with Going 270–300 mm yields a 30–35° pitch—comfortable and common. We’re in range.
What changes when you tweak a couple inputs?
Scenario A: Increase Max riser (mm) from 190 to 200 while holding other values. You’ll often drop one riser count because taller risers cover the same Total rise with fewer steps. That raises each Riser height and typically increases the Going from the comfort rule, which may push Total run slightly up or down depending on the clamp.
Scenario B: Tighten Min tread (mm) from 250 to 270. The calculator cannot pick a smaller going, so it may increase Going to meet the rule, which increases Total run and stringer length. Stair angle will decrease a bit (flatter stair).
Common limits, clamps, and mistakes that skew results
- Overtight ranges: If nMin > nMax, there is no feasible riser count. Widen either riser or tread limits.
- Nosing confusion: Nosing (mm) increases effective going at the line of travel. If you already entered a full going including nosing, set Nosing to 0.
- Ignoring headroom: Geometry here doesn’t check headroom. Longer goings may conflict with landings or doors.
- Angle expectations: Comfort usually lands around 30–37°. Outside that, re-check constraints.
- Unit slips: All inputs are millimetres. Don’t mix inches.
Field-friendly tips to interpret the numbers and act
- Need a shorter flight? Raise Min tread (mm) only if comfort remains acceptable, or allow a slightly higher Max riser (mm) within code.
- Chasing comfort? Prefer the 2 × Riser + Going rule first; it tracks human stride better.
- Framing allowance: Add 2–3% length allowance on stringers for trimming and seat cuts. Round cuts to the nearest whole millimetre on shop drawings.
- Landing planning: Remember total run excludes landings. Plan landing depth separately.
When to pick the alternative stair sizing rule
Both rules target comfortable gait:
- 2 × Riser + Going ≈ 630 mm: Good default for general residential and most light commercial stairs.
- Riser + Going ≈ 450 mm: Sometimes preferable in tighter spaces or where smaller goings are acceptable by code.
If your tread range clamps too often with the first rule, switch and compare angle and run. Always confirm against local code and manufacturer guidance.
Brief cost and build notes for estimators and crews
- Material budgeting: Total run and stringer length drive stringer stock and tread count. Add 5–10% waste for timber stringers; 3–5% for steel.
- Labor: Uniform risers reduce rework. Expect 8–16 labor-hours for a straight wood flight (carpenter + helper), more with landings.
- Layout: Mark consistent rises on site. Variance should be less than ±3 mm between adjacent risers for a quality feel.
Related terms that users search for
Stair design calculator, step riser calculator, stair run calculator, stair angle calculator, tread and riser calculator, staircase layout tool, stringer length calculator, step height and depth tool.