Stair calculator
Size a straight staircase from the total rise and available run: number of risers, actual riser height, going, angle, stringer length and Blondel check.
Straight staircase sizing
Enter the total rise and available run. The calculator adjusts the number of risers around the target riser height.
Was this tool useful?
How does this tool work?
Before ordering a straight staircase or marking out a stringer, you need to know how many steps fit between two floors and whether the available floor space allows comfortable steps. This calculator takes the total rise, the available run and a target riser height, then returns the number of risers and treads, their actual dimensions, the Blondel value, the stair angle and the theoretical stringer length.
The number of risers is the total rise divided by the target riser height, rounded to the nearest whole number; the actual riser height is then recalculated so every step is equal: 260 cm with an 18 cm target gives 14 risers of 18.57 cm.
A straight flight has one tread fewer than risers, because the upper floor acts as the last step. The run is therefore divided by (risers − 1) to get the actual going.
Blondel is 2 × riser + going. The angle is arctan(total rise ÷ run) and the stringer is the diagonal √(rise² + run²). The result is flagged as comfortable when the riser stays between 16 and 20 cm and Blondel between 60 and 64 cm.
Limits: the calculation assumes a straight flight with no landing, with the run measured from the nosing of the first step to the edge of the upper floor. It does not handle quarter-turn or spiral stairs, headroom, width, tread thickness or balustrades. Code requirements (Approved Document K in England, IRC in the US, NF DTU 36.3 in France or other local codes) are references to check on the project.
The relationship between riser height and going can be checked with the Blondel relation 2h + g, but it is only a proportion check. Headroom, width, guarding, number of steps and local requirements still need to be checked on the real design.
How to use this tool
Measure the total rise
Measure from finished lower floor to finished upper floor, in centimetres, adding the thickness of floor coverings still to come.
Measure the run
Enter the floor length available between the foot of the stair and the edge of the stairwell or upper floor.
Choose the target riser height
Start from 17 to 18 cm for a house stair; the tool adjusts the value so all steps are equal.
Check comfort and stringer
Read Blondel, the angle and the comfort indicator, then note the stringer length for cutting or ordering.
Straight stair geometry
The calculation splits the total rise into equal risers, then the run into goings.
Number of risers
n = round(H ÷ target), h = H ÷ n
H total rise, h actual riser height.
Going
g = R ÷ (n − 1)
R available run; there is one tread fewer than risers.
Blondel and pitch
2h + g; angle = arctan(H ÷ R)
The stringer is √(H² + R²) long.
The calculation applies to a straight flight. Winding stairs are measured on the walking line, and accessibility or public-building rules impose their own dimensions.
Use cases
Stair to a loft conversion
Check whether an opening and a 3 to 3.5 m run allow a comfortable straight stair for a 2.60 m rise.
Ordering a kit staircase
Compare the calculated number of steps and angle with the models a manufacturer offers before buying.
Outdoor or basement stair
Split a 1 to 3 m rise into equal steps and check Blondel before building formwork or cutting the stringer.
Examples
Straight stair, 2.80 m rise over 4 m
280 ÷ 17.5 = exactly 16 risers. 400 ÷ 15 = 26.7 cm going, and 2 × 17.5 + 26.7 = 61.7 cm stays within 60–64 cm.
Input
Total rise 280 cm, available run 400 cm, target riser 17.5 cmResult
16 risers of 17.5 cm, 15 treads of 26.7 cm, Blondel 61.7 cm, 35° pitch, stringer ≈ 4.88 m, comfortableSteeper stair, 2.60 m rise over 3.30 m
260 ÷ 18 = 14.4, rounded to 14 risers of 18.57 cm. The shorter run gives a steeper stair that still fits the comfort range.
Input
Total rise 260 cm, run 330 cm, target riser 18 cmResult
14 risers of 18.6 cm, 13 treads of 25.4 cm, Blondel 62.5 cm, 38.2° pitch, stringer ≈ 4.20 m, comfortableTips and best practices
- If Blondel exceeds 64 cm, the going is too long or the riser too high: use less of the run or add a step.
- An angle above 40° gives a stair close to a ship’s ladder: keep it for secondary access.
- The stringer length shown is a theoretical diagonal: add the plumb and seat cuts at top and bottom before ordering timber.
- Try two target heights, for example 17 and 18 cm: one step more or less often changes the going by several centimetres.
Sources and references
Frequently asked questions
What is Blondel’s formula?
It is a proportion check between riser height H and going G, usually written 2H + G. The calculator shows this value so you can compare stair layouts.
Why is there one going fewer than risers?
On a straight stair that arrives on a floor, the upper floor acts as the last stepping surface, so the number of goings is usually one less than the number of risers.
Does the result guarantee compliance?
No. It is a preliminary geometric calculation. Regulatory dimensions and safety rules depend on the building and the project.
What riser height is comfortable?
Domestic stairs often aim for 16 to 18 cm risers with 25 to 30 cm goings. The calculator flags as comfortable a riser between 16 and 20 cm combined with a Blondel value of 60 to 64 cm. Building codes, such as UK Approved Document K, set the legal limits.
How do I measure the total rise?
Measure from finished floor to finished floor, coverings included. Forgetting future tiles or boards shifts the top step by several centimetres.
Is headroom checked?
No. Clear headroom above the treads depends on the stairwell opening and floor thickness; about 2 m is a common target. Check it on the section drawing.
Related tools
Keep going with these tools
Tools similar or complementary to this one.
Similar tools
Convert slope between percent, degrees, rise, length and ratio.
Construction & renovationUse this tool
Estimate studs, plates, and timber length for a framed wall.
Construction & renovationUse this tool
Calculate fence panels, posts and footing concrete.
Construction & renovationUse this tool
Read next
GuideBest practicesIntermediate
Calculate stairs: risers, rise, tread depth and slope
Learn how to size a staircase from total rise, number of risers, tread depth, available run and the Blondel relationship.
30 August 20262 min
GuideBest practicesIntermediate
Wall, roof and framing materials: studs, rafters, rebar and mortar
A practical guide to dimensions, spacing, lengths and waste allowances when estimating studs, rafters, rebar and mortar.
21 September 20263 min
GuideBest practicesBeginner
How much concrete for a slab, foundation or posts?
Calculate concrete volume, site allowance and the quantity to order for slabs, footings, walls or cylindrical posts.
30 August 20262 min
TutorialBest practicesBeginner
Plan a construction project: calculate materials before buying
A practical method for estimating concrete, screed, bricks, render, fencing, decking and other materials before you order.
30 August 20264 min