UK Staircase Stringer Calculator: Rise, Going and Length

A UK staircase stringer calculator estimates the number of rises, individual rise and going, staircase angle, and the sloping length of a stringer from the floor height and available run. Enter measurements in one consistent unit—usually millimetres—and use the results as a layout guide, then check the design against current building requirements and the actual site before cutting timber.

Formula

The calculator divides total floor height by the number of risers to find the rise of each step. It treats the going as the horizontal distance from one riser to the next, then uses right-triangle geometry to estimate the sloping length.

• Rise per step, r = h ÷ n. Here, h is total vertical floor-to-floor height in millimetres and n is the number of risers.
• Stair angle, a = arctan(r ÷ g). The angle is measured above horizontal in degrees; g is the going in millimetres.
• Sloping length, L = n × √(r² + g²). This is the theoretical distance along the repeated step profile, before allowances for end cuts, bearing, or the way a particular stringer is marked and cut.
• Horizontal run = n × g. This assumes the same going is used for every rise in the calculation.

For the length formula, each repeated rise-and-going pair forms a right triangle. The calculator multiplies that diagonal by the number of risers. A physical stringer’s cut length can differ: the first and last connections, floor thicknesses, landings, and whether the top step is a tread or finished floor all affect the timber layout.

Worked Example

Using the block’s defaults: floor height h = 2,600 mm, n = 15 risers, and going g = 250 mm.

1. Rise: r = 2,600 ÷ 15 = 173.333… mm, displayed to one decimal place as 173.3 mm.
2. Angle: a = arctan(173.333… ÷ 250) = arctan(0.693333…) ≈ 34.7°.
3. Sloping length: L = 15 × √(173.333…² + 250²) ≈ 15 × 304.2 mm ≈ 4,563 mm, rounded to the nearest millimetre.
4. Horizontal run: 15 × 250 = 3,750 mm.

These are geometric estimates. Do not use the displayed rounded rise to calculate the final length; the formula uses the unrounded value internally.

How to Choose the Number of Risers

Measure vertically from the finished lower floor to the finished upper floor, not from an unfinished slab to a framing surface unless those levels will remain unchanged. Choose a trial riser count, divide the height by it, and see whether the resulting rise and the available horizontal run produce a practical layout. If the rise is unsuitable, test a different count rather than rounding individual steps to different heights.

For example, the 2,600 mm height can be divided into 15 equal rises of about 173.3 mm. If the number of rises changes, the rise changes too. The going must also fit the floor plan: at 250 mm and 15 calculated goings, the run is 3,750 mm. Check whether that space exists after accounting for walls, landings, doors, and headroom.

Do not assume the count of risers always equals the count of separate treads. In many layouts, the upper floor serves as the final walking surface, so the number of physical treads can be one fewer than the number of risers. Confirm how the stair terminates before setting out cuts.

Using the Result for a Timber Stringer

A stringer (sometimes called a carriage) supports the stair treads and risers. The calculator’s sloping length is a useful early estimate for material planning, not a cut list. A sawtooth or housed stringer’s actual dimensions depend on its section, the tread and riser thicknesses, the top and bottom fixing details, and the required bearing at each end.

• Confirm the finished floor levels and whether floor coverings or tread coverings change them.
• Set out a full-size pattern or make a trial template before cutting every stringer.
• Check that the first and last steps retain the intended rise after the stair is fitted.
• Allow for the support and fixing method specified for the project; do not assume the calculated diagonal is the length to cut.

For a straight stair with a landing or a turn, calculate each flight separately. A winder or curved arrangement cannot be fully described by one total run and one constant going; its walking line and geometry need separate layout checks.

UK Rules, Safety and Common Errors

A calculator cannot determine whether a stair complies with UK requirements. Requirements depend on the building and project, and can differ between England, Wales, Scotland, and Northern Ireland. Check the current applicable building regulations and guidance, including the rules for rise, going, pitch, headroom, handrails, guarding, and landings. For a new build or alteration, ask the relevant building control body or a suitably qualified building professional if the requirements are unclear.

Common errors include measuring between the wrong floor surfaces, mixing metres and millimetres, using a tread’s overall depth as its going, and treating the stringer length as the horizontal run. Going is the horizontal step-to-step distance; a tread may project beyond the riser as a nosing. A nosing does not automatically add to the going measurement. Also avoid rounding each rise separately: small discrepancies accumulate and can leave the top or bottom step noticeably different.

Uneven steps are a trip hazard. Before construction, verify the complete flight, including finished surfaces and landings, and ensure the support structure and fixings are suitable for the loads. Use appropriate tools and safe cutting practices, and obtain professional advice where structural capacity or compliance is uncertain.

Frequently Asked Questions

How do I calculate the rise of UK stairs?
Measure the finished floor-to-floor height and divide it by the number of risers. Keep the result unrounded for layout calculations, and check that the proposed rise meets the requirements that apply to your project.

Is stringer length the same as stair run?
No. Run is the horizontal distance; stringer length follows the slope and is longer. The calculator gives a theoretical sloping length, not necessarily the timber’s final cut length.

Does a staircase have one fewer tread than risers?
Often, when the upper floor is the final walking surface, there is one fewer separate tread than risers. Landings and other stair arrangements can change the count, so define the top and bottom conditions first.

What units should I enter?
Enter height and going in the same unit. This calculator uses millimetres, so enter, for example, 2,600 rather than 2.6 for a 2.6 m floor height. Convert any imperial measurements before using it.

Can I use the result as a timber cutting plan?
No. It estimates the stair geometry. End cuts, tread thickness, floor build-up, support details, and the stringer design affect the actual cuts and should be resolved in a full-size setting-out plan.

Will these dimensions automatically meet UK building regulations?
No. The calculator does not check compliance. Verify current rules for the relevant UK nation and building type, including the complete stair, headroom, handrails, guarding, and landings.

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in h | Total floor height | mm | 2600 | 0
in n | Number of risers | - | 15 | 1
in g | Going per step | mm | 250 | 0
out r | Rise per step | mm | h / n | 1
out a | Stair angle | degrees | atand(r / g) | 1
out L | Sloping stringer length | mm | (n * sqrt(r ^ 2 + g ^ 2)) | 0
out run | Horizontal run | mm | n * g | 0