Stair Stringer Calculator

What do I need to know about UK Building Regulations?

UK Building Regulations (Approved Document K) set out the dimensions any new domestic staircase must meet, and getting these right is both a legal and a practical safety requirement. For domestic stairs: the maximum riser height is 220 mm and the minimum tread (going) depth is 220 mm — the absolute boundary values, but most comfortable stairs use considerably gentler dimensions. The maximum pitch angle is 42° for domestic stairs (steeper for loft conversions, up to 60° for paddle stairs in special circumstances). Headroom must be at least 2 metres measured from the pitch line. The '2R + G' rule (twice the riser height plus the going) should equal between 550 and 700 mm for comfortable use — based on the natural human stride length on stairs. Ideal riser height is 175–190 mm and ideal going is 220–250 mm — settings that give a comfortable rhythm to climb and descend without over- or under-striding. A worked example: a total floor-to-floor rise of 2,600 mm divided into 14 risers gives 185.7 mm per riser, with a 220 mm going, gives 2(185.7) + 220 = 591 mm — comfortably within the 550–700 range. The total horizontal run for 14 treads would be 13 × 220 = 2,860 mm (one less tread than risers, since the top landing is the final step). Tread nosings (the rounded front edge that projects past the riser) should be 16–25 mm; balustrades on open sides need to be at least 900 mm high in dwellings and have spacing such that a 100 mm sphere cannot pass through. For loft conversions, slightly different rules apply (steeper pitch allowed). Always check current Approved Document K when planning a new staircase; this calculator gives compliant dimensions when configured within the standard limits.

What do I need to know about The Stringer Length Formula?

The stringer is the angled timber on either side of a staircase that supports the treads and risers, and getting its length right is essential for ordering material and cutting. Stringer length = √(total rise² + total run²), straight from Pythagoras' theorem — the stringer is the hypotenuse of a right triangle whose legs are the staircase's total vertical rise and total horizontal run. A worked example: a staircase with 2,600 mm total rise and 2,860 mm total run has stringer length √(2,600² + 2,860²) = √(6,760,000 + 8,179,600) = √14,939,600 ≈ 3,865 mm. Add 50–100 mm for the top and bottom connections (where the stringer meets the landing and the floor), so order 4,000 mm of stringer timber. Standard stringer timber comes 285–300 mm wide to accommodate the cut-out for treads and risers — at 220 mm going + 185 mm riser, the diagonal cut-outs need careful marking out so the remaining timber depth (the 'effective depth' below the cut-outs) stays at least 80–100 mm for structural strength. Use 38–47 mm thick stringer timber for open-tread or simpler stairs, 45–63 mm for closed-tread stairs carrying full loads. Always order an extra length (10%) above the calculated minimum to allow for cuts and any errors. Two stringers are needed for a typical staircase (one each side), or three for wider stairs over 900 mm wide for additional support. This calculator gives all the key dimensions including stringer length; for the cut-out marking, a staircase template (a metal jig that marks the riser and going at the right angle on the stringer) saves much measurement and is widely available.

What's the key thing to understand about Consistent Risers Are Critical?

All risers in a staircase must be exactly the same height — even a 3–4 mm variation is a real trip hazard and a building code violation that may require remedial work. The reason is biomechanical: people walk up and down stairs largely without looking once they've internalised the rhythm, so a single odd step breaks the expectation and causes trips and falls. This is also why people sometimes fall on the bottom step of an unfamiliar staircase — the position they expect the floor doesn't match where it actually is. The calculation discipline: total floor-to-floor rise must be divided exactly into an integer number of risers, giving a precise per-riser height. Worked example: if your total rise is 2,615 mm and you choose 14 risers, each must be 186.79 mm — not 186 mm for thirteen of them and 195 mm for the top one. For practical building, this often means the bottom riser is cut slightly shorter or the top riser uses a thinner finish to absorb 0–3 mm of variation between calculated and floor-finish realities. Always measure the total rise from the finished floor surface to the finished floor surface above (including the floor coverings, since carpet adds about 10 mm and tile/wood floor 15–20 mm). Building Control will check this on inspection. The going (tread depth) should also be consistent — variations cause trips for the same reason — but minor differences in going are less dangerous than riser variations because people see treads as they descend. For loft conversions, where ceiling height often forces compromises, getting consistent risers within the available space takes careful planning; sometimes the answer is to reduce the floor finish by a millimetre or two, or to extend the staircase length slightly to add a riser. This calculator computes exact riser heights from your total rise input.

What's the key thing to understand about Safety and Regulations?

Most structural and electrical DIY work in England and Wales must comply with Building Regulations and is notifiable to local Building Control. Work that is not notifiable (like-for-like replacements, cosmetic changes) can be done without notification. Notifiable work done without approval is technically illegal and can cause problems when selling the property. Electrical work in most rooms requires a Part P competent person or Building Control inspection. Gas work must always be performed by a Gas Safe registered engineer — it is illegal for anyone else to carry out gas work in the UK, and using an unregistered installer invalidates both your home insurance and any warranty on the work. For structural changes (removing load-bearing walls, altering the roof structure), always get a structural engineer's assessment before starting, even for jobs that look straightforward.

Headroom and a Full Worked Example

Alongside riser and going limits, UK Building Regulations require a minimum headroom of 2,000 mm, measured vertically from the pitch line (or from the floor, on landings) to the underside of any ceiling, beam, or the structure above — this is what stops a tall adult hitting their head partway up a staircase built under a sloped ceiling or landing. Worked example: a staircase needs to rise 2,400 mm floor to floor. Choosing a riser near the comfortable middle of the regulatory range, 200 mm, gives 2,400 ÷ 200 = 12 risers exactly. That leaves 11 goings (always one fewer than the number of risers). Picking a 250 mm going for each of the 11 goings gives a total run of 11 × 250 = 2,750 mm. Pitch angle = arctan(200 ÷ 250) = arctan(0.8) ≈ 38.7°, safely under the usual 42° maximum. Stringer length (the hypotenuse connecting total rise and total run) = √(2,400² + 2,750²) = √(5,760,000 + 7,562,500) = √13,322,500 ≈ 3,650 mm, so a standard 3.6-3.9 m length of stringer timber would need to be ordered.

Riser/Going Combinations and Resulting Pitch Angle

Because pitch angle = arctan(riser ÷ going), the same total rise can be achieved at very different steepnesses depending on how riser and going are balanced against each other. The table below shows how pitch angle changes as the riser-to-going balance shifts, which is useful for a quick compliance check against the 42° regulatory maximum for private stairs before running a full calculation.

Winder Staircases and Extra Regulation Requirements

A winder staircase — one that turns a corner using angled, pie-shaped treads instead of a flat landing — has its own additional building regulation requirements beyond the standard rise-and-going rules, because the tread width varies across its length. UK Building Regulations require the 'going' of a winder tread to be measured at a defined point (typically 270mm in from the narrow end, where most people's foot naturally lands while turning), not at the widest or narrowest point of the pie-shaped tread, to ensure a genuinely safe footing is available where it's actually used. Winders are also restricted in how many consecutive turning treads can replace a landing, and handrail continuity through the turn has its own separate requirement. Because of this added complexity, winder staircase designs are more often checked against regulations by a qualified stair designer or building control than a standard straight flight would be.