Stair Calculator

R Run r t

What this calculator does

The Stair Calculator designs a straight staircase for you. Enter the total rise (the height from the finished lower floor to the finished upper floor) and your preferred tread depth and target riser height; the calculator works out the number of risers, the exact riser height, the number of treads, the total run (floor space the staircase occupies), the stringer length and the stair angle. It also applies the classic "Blondel" comfort formula, 2 × riser + tread, to check whether the staircase will feel comfortable to walk on.

All risers must be equal by building codes, which is why the calculator divides the total rise evenly rather than simply stacking your target height. The page is available in English, Arabic, and Spanish.

How to use the Stair Calculator

  1. Measure the total rise — the vertical distance from the finished floor below to the finished floor above — in centimeters or inches.
  2. Enter the tread depth you want, typically 25–30 cm (10–12 in). This is the horizontal depth of each step, excluding any nosing overhang.
  3. Enter a target riser height, typically 17–19 cm (6.5–7.5 in). The calculator rounds the step count up or down so every riser is equal and as close as possible to your target.
  4. Click "Calculate" to see the step count, exact riser height, total run, stringer length, angle and a comfort check based on the Blondel formula.

Formulas

With total rise R, target riser h₀, tread depth t and n risers:

Number of risers: n = round(R ÷ h₀) (minimum 2) Exact riser: r = R ÷ n Number of treads: n − 1 (the upper floor is the last "tread") Total run: G = (n − 1) × t Stringer length: S = √(R² + G²) Stair angle: θ = arctan(R ÷ G) Blondel check: 2r + t (comfortable: 600–650 mm / 24–26 in)

Common comfort guidelines: risers of 15–19 cm (6–7.5 in), treads of at least 25 cm (10 in), and stair angles between roughly 30° and 37°. If the Blondel value falls outside 600–650 mm, adjust the tread depth and recalculate.

Examples

Frequently asked questions

Why is the number of treads one less than the number of risers?

Because the upper floor landing is the final "tread". You step up n times onto n+1 levels, but only n−1 of those levels are actual stair treads; the last one is the floor itself.

What is a comfortable riser height?

Most building codes and guidelines target 15–19 cm (about 6–7.5 in). Shorter risers make shallower, easier stairs but need more floor space; taller risers save space but feel steeper.

What is the Blondel formula?

A rule attributed to François Blondel: 2 × riser + tread should equal roughly 600–650 mm (24–26 in), the average comfortable stride. It balances effort going up with safety coming down, and it is why very deep treads need shorter risers.

Does the total run include the top landing?

No. The total run here covers only the treads of the flight itself (n − 1 treads). If you have a separate landing platform at the top or bottom, add its depth to your floor-plan layout.

How long should the stringer be?

The calculated stringer length is the hypotenuse of the stair triangle — the structural slope length. In practice, buy a board longer than this to allow for cutting the step profile, and check your local code for stringer attachment details.

What about curved or winder stairs?

This calculator designs straight flights only. Winders and curved stairs follow special rules (such as minimum tread width at the walk line) that vary by code.

Is my data saved or sent anywhere?

No. Every calculation runs in your browser, and nothing you type is stored or sent to a server.

Related calculators

Other free calculators commonly used with this tool: Concrete Calculator, Tile Calculator, and Irregular Surface Area Calculator.

Want a deeper walkthrough? Read our stair design guide.