Stair calculator
Size a straight stair flight from floor-to-floor rise: riser count, tread depth, total run, stringer length, and Blondel comfort.
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Tool guide
Stair calculator: how it works
Practical information to understand the result and use this tool correctly.
About Stair calculator
This stair calculator sizes a single straight flight. Enter the floor-to-floor rise and a target riser; it rounds to a whole number of risers and shows the actual riser height, total run, stringer length, and slope angle.
For a typical floor-to-floor stair the number of treads is one less than the number of risers, because the upper floor is the last landing. Switch to “treads equal risers” when you are counting a deck or a flight that includes the top tread in the run.
Comfort uses Blondel’s rule: twice the riser plus the tread should land near 63 cm, usually acceptable between 60 and 65 cm. A 17–18 cm riser with a 27–29 cm tread is a common residential target. The stringer length is the hypotenuse of total rise and total run.
It is a planning aid, not a code certificate. Local rules for max riser, min tread, nosing, headroom, and handrails differ. It does not draw winders, landings mid-flight, or spiral stairs, and it ignores nosing overlap.
How to use it
- 1Enter the floor-to-floor rise in centimeters and a target riser.
- 2Set the tread depth and whether treads equal risers or risers minus one.
- 3Apply a preset if you want a comfortable house flight, a 2.7 m floor, or a steeper attic.
- 4Read riser count, actual riser, total run, stringer length, angle, and the Blondel check.
- 5Copy the summary before buying stringers or marking the layout.
Frequently asked questions
- How is the number of steps calculated?
- Riser count is the floor-to-floor rise divided by the target riser, rounded to the nearest whole number (minimum 1). Actual riser height is total rise divided by that count, so every riser is equal.
- Why are there fewer treads than risers?
- On a straight flight between two floors the top floor is the last landing, so treads = risers − 1. Choose the other mode if your run includes a top tread, as on many decks.
- What is the Blondel rule?
- 2 × riser + tread ≈ 63 cm. The tool flags values outside 60–65 cm as less comfortable. It is a proportion check, not a building-code pass.
- Does this replace the local stair code?
- No. Many codes cap residential risers around 18–20 cm and ask for treads of at least 25–28 cm, plus headroom and handrails. Confirm the rule that applies to your project.
- Is the layout uploaded?
- No. Rise, tread, and the result stay in the browser. Nothing is sent to a server.