Stair Stringer Calculator

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.

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.

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.

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