How to use this stair calculator
- Choose inches or millimetres. The selection changes labels and example values; it does not convert numbers already typed. Measure every field in the same system.
- Measure total rise vertically from the finished lower floor to the finished upper floor. Enter the intended number of risers, counting the last vertical rise onto the upper floor.
- Measure the available total horizontal run from the first riser to the upper landing. Do not add the depth of the upper landing as another tread.
- Select Calculate stairs. Read the equal riser height, equal tread depth, tread count, angle and diagonal slope length. Use the reverse check and diagram to catch a misunderstood count before using the measurements elsewhere.
- Change any input and calculate again. Copy Results creates a plain-text summary for notes; it does not create a cut list, construction drawing or saved project.
What the calculation means
A straight flight ending at an upper floor normally has one more riser than tread. Each riser is a vertical change in level. The upper floor supplies the surface after the final rise, so it is not counted again as a separate tread. With 15 risers, this calculator therefore uses 14 treads. This definition is shown beside every result because mixing riser and tread counts is a common source of a one-step error.
Riser height equals total rise divided by the number of risers. Tread depth equals total horizontal run divided by the number of treads. The calculator also multiplies both answers back by their counts. Those reverse totals should match the inputs apart from display rounding. The angle is arctan(total rise ÷ total run), and slope length is the hypotenuse √(rise² + run²). The diagonal is a geometric reference, not a stringer cut length that accounts for mounting, tread thickness or end cuts.
The drawing is a scaled schematic of the entered geometry. It helps show whether the count definition and proportions look plausible, but it is not a fabrication plan. Labels are rounded for display while calculations keep full browser numeric precision. Measure the real finished surfaces rather than copying dimensions from an unfinished opening.
Worked stair examples
Imperial example: a 9-foot rise
Enter 108 in total rise, 15 risers and 140 in total run. The result is 14 treads, a 7.2 in riser, a 10 in tread, an angle of about 37.65° and a slope length of about 176.82 in. The checks return 7.2 × 15 = 108 and 10 × 14 = 140.
Metric example: floor-to-floor layout
Enter 2800 mm total rise, 16 risers and 4500 mm total run. The result is 15 treads, a 175 mm riser, a 300 mm tread, an angle of about 31.89° and a slope length of about 5300 mm. The upper landing is the surface after riser 16; it is not tread 16.
Scope, measurement limits and common errors
- This tool sizes only one straight flight. It does not model landings, winders, spiral or curved stairs, split flights, headroom, railings, nosing, tread thickness, finish buildup, stringer mounting, supports or material quantities.
- It deliberately does not issue a building-code pass or safety verdict. Rules vary by place, occupancy and project type, and may cover dimensions not entered here. Confirm applicable requirements and structural details with the local authority and a qualified professional before construction.
- Use finished-floor measurements and one unit throughout. A mixed inch/millimetre entry can be numerically valid but physically wrong. The accepted range is greater than zero through 1,000,000 units, with 2–200 whole risers.
- The diagram is proportional but not a printable full-scale template. Screen size, SVG scaling and rounded labels make it unsuitable for transferring cut marks. Recheck the site, levels and all arithmetic before ordering or cutting materials.
- Very shallow or steep results are still shown so the geometry can be inspected; they are not recommendations. If a dimension looks unexpected, first confirm total rise, whether the upper floor was counted as a tread, and whether total run was measured to the landing.
Frequently asked questions
Why are there fewer treads than risers?
The final riser arrives at the upper floor, which becomes the next walking surface. For the standard top-floor convention used here, treads = risers − 1.
Does “number of risers” mean visible vertical boards?
No. It means equal vertical changes in level. An open stair may have no physical riser boards and still has the same geometric riser count.
Can this calculator choose the number of steps for me?
No. The approved scope requires you to enter the riser count so alternatives can be compared deliberately. Change the count and recalculate to inspect another layout.
Is the slope length my stringer cutting length?
Not necessarily. It is the right-triangle diagonal only. Real stringer layout depends on mounting, thicknesses, bearing, end cuts and structural design that this page does not calculate.
Does a displayed result meet building code?
No code decision is made. The page only reports geometry. Check current local rules and all relevant dimensions before building.
Are my measurements stored?
No. Calculation and copying happen in this browser for the current visit, with no account or project history.