Stair Calculation: Number of Steps, Riser Height and Tread Depth
How many steps does a staircase need, and how high and deep should each one be? Start from the floor-to-floor height, apply the 2h + b = 63 cm rule and check the result in the calculator.
Step 1: Measure the total height
Measure from the finished floor below to the finished floor above (H). Include screed, tiles or parquet that are not laid yet, because they change the first and last riser. Example: H = 270 cm.
Step 2: Number of risers and riser height
Divide H by an ideal riser height of about 17.5 cm and round to a whole number N. Then h = H ÷ N. For H = 270 cm: 270 ÷ 17.5 = 15.4, so N = 15 and h = 270 ÷ 15 = 18.0 cm.
Comfortable risers are 15–19 cm high, and every riser in one flight must be equal.
Step 3: Tread depth with the 2h + b rule
Blondel's rule: 2h + b = 63 cm (comfortable range 60–66 cm). With h = 18 cm: b = 63 − 2 × 18 = 27 cm. Treads are usually 24–32 cm deep.
A straight flight has N − 1 treads, because the upper floor is the last step: 14 treads × 27 cm = 378 cm of horizontal run.
Worked example: height 180 cm
N = 180 ÷ 17.5 = 10.3, so 10 risers and h = 18.0 cm. Then b = 27 cm, 9 treads, horizontal run 9 × 27 = 243 cm, slope 33.7°. A 180 cm rise needs a stair about 2.4 m long.
Riser height and matching tread depth
Tread depth b = 63 − 2h. Above 19 cm the stair is steep, and above 20 cm it is outside usual recommendations.
| Riser h (cm) | Tread b (cm) | Slope (°) |
|---|---|---|
| 15 | 33 | 24.4 |
| 16 | 31 | 27.3 |
| 17 | 29 | 30.4 |
| 18 | 27 | 33.7 |
| 19 | 25 | 37.2 |
| 20 | 23 | 41.0 |
Common mistakes
- Forgetting the floor finishes: they change the first and last riser.
- Unequal risers: differences above about 5 mm make people trip.
- Counting treads as risers: a straight flight has one tread fewer than risers.
- Ignoring headroom: keep at least 200 cm clear above the steps.
- Copying a tread depth without checking 2h + b.
Frequently asked questions
What is the best riser height?
Usually 16–18 cm. The calculator recommends 15–19 cm and a tread of 24–32 cm, with 2h + b around 63 cm.
How many steps are needed for a height of 280 cm?
280 ÷ 17.5 = 16 risers of 17.5 cm, with 15 treads of 28 cm (2h + b = 63 cm).
Are the rules the same everywhere?
No. Building regulations differ by country and building type, so check the local rules. The calculator's ranges are guidance.
Open the calculator: Staircase Calculator