Measuring your stairs before you calculate
Approved Document K works from one number outward: total rise. Get that right, pick the correct stair category, and the rest of the checks follow.
Measure total rise
Plumb down from the finished floor level you're leaving to the finished floor level you're arriving at. On an existing opening, drop a plumb line from the top nosing to the floor below and read it in millimetres.
Pick the correct category
Part K sets different limits for a private stair inside one dwelling, a common/utility stair serving more than one dwelling or a non-domestic building, and an institutional/assembly stair for large public gatherings. Picking the wrong one is the most common way a page like this gets misapplied.
Note your available going
Measure the horizontal floor space you actually have, from the base of the rise to the nearest wall or obstruction. This tells the calculator whether the resulting stair will physically fit.
Read the compliance chips
Green means the result sits inside the Part K limit for that category. Amber means the underlying figure needs local verification with Building Control before you rely on it. Red means adjust total rise, riser or going before you cut anything.
The formulas behind every result
Nothing on this page is a black box. Here's exactly what runs when you change a number.
Step Count
Total rise divided by a target riser height, rounded to the nearest whole step — you can't build a fractional stair.
Actual Riser Height
Rounding to a whole step count means the real riser is rarely the exact target. This is the number every riser gets cut to, uniformly, within a single flight.
Going (2R+G target)
Approved Document K's comfort relationship: twice the riser plus the going should land between 550mm and 700mm. This calculator's Auto mode aims for the 625mm midpoint, then floors the result at the category's minimum going.
Pitch & 2R+G Check
Pitch is the angle of the stair from horizontal. The 2R+G value must fall between 550mm and 700mm regardless of category, and pitch must stay under the category's maximum.
Total Going & Stringer
One fewer tread than the step count (the top floor doesn't need its own tread). Stringer length is the Pythagorean hypotenuse of total rise and total going.
Handrail Sides
Approved Document K requires a handrail on at least one side of every flight with three or more risers, and on both sides once the stair reaches 1 metre clear width.
Walking through a real 2,600mm rise
Same formulas, filled in step by step, for a private stair climbing 2.6m floor to floor — the default this calculator loads with.
Step count
2,600mm total rise, targeting a 190mm riser: 2600 ÷ 190 = 13.68, which rounds to 14 steps. You can't cut a fractional step, so the calculator always rounds to the nearest whole number.
Actual riser height
Re-divide total rise by that whole step count: 2600 ÷ 14 = 185.7mm per riser. That's the number every riser gets cut to — not the 190mm target, which was only a starting point, and it's comfortably under the private-stair 220mm maximum.
Going, via the 2R+G target
625 − (2 × 185.7) = 253.6mm going. That's above the private-stair 220mm minimum, so no flooring is needed here — 2R+G comes out at exactly 625mm, mid-band and well inside the 550–700mm rule.
Pitch, total going, and stringer
13 treads (one fewer than the step count) at 253.6mm each gives a total going of 3,296mm. Pitch works out to atan(185.7 ÷ 253.6) ≈ 36.2° — comfortably under the 42° private-stair maximum. Stringer length is √(2600² + 3296²) ≈ 4,198mm, the board length a stringer has to be cut from before notching.
Here's where the rules start to interact: in a tighter space — a low-headroom loft conversion with a shorter available rise, say — the Auto calculation pushes the riser up toward the 220mm private-stair maximum to keep the step count down. That simultaneously drags the going down toward the 220mm minimum, which pushes 2R+G toward the top of its 700mm ceiling and the pitch toward the 42° cap, all at once. Loosening any one of total rise, available going or the riser target eases all three limits together, because they're locked to the same two numbers.
Approved Document K, quick reference
This calculator checks your result against these Part K limits by category. Private-stair figures are corroborated across multiple sources; the non-domestic figures below vary between summaries of the regulation and are flagged for local verification.
| Category | Max Rise | Min Going | Max Pitch |
|---|---|---|---|
| Private (single dwelling) | 220mm | 220mm | 42° |
| Common / Utility [VERIFY] | 190mm [VERIFY] | 250mm [VERIFY] | 38° |
| Institutional / Assembly [VERIFY] | 150mm [VERIFY] | 280mm [VERIFY] | 33° |
All categories: 2R + G must fall between 550mm and 700mm. Reference only — always confirm the exact figures for your project against the current Approved Document K and your local Building Control body before building.
Landings, headroom, handrails & winders
A stair can pass every rise and going check and still fail Building Control on the items in this section — flight length, headroom, handrail count, and how tapered treads are measured.
When you need a landing / change of direction
Landings are required at the top and bottom of every flight regardless of length, at least as deep as the stair is wide. In a dwelling, if consecutive flights exceed 36 risers with no landing, a change of direction of at least 30° is required. Non-dwelling stairs have a tighter limit between landings — commonly cited as around 12–16 risers depending on stair use [VERIFY] — check the specific figure for your stair type.
Checking headroom
Measured perpendicular from the pitch line — the notional line through the tread nosings — not straight up and down. This applies along the whole stair and at landings. A reduced allowance can sometimes apply in genuinely space-constrained loft conversions, but the exact reduced figure and the conditions attached to it need confirming with Building Control [VERIFY] — do not rely on a reduced number without that sign-off.
Handrails
Required on any flight of three or more risers, fixed between roughly 900–1,000mm above the pitch line. Handrails are not required on the bottom two steps of a flight. Widen a stair past 1 metre clear and you take on a second handrail run.
Guarding, open risers & winders
Guarding is needed at the open sides of flights and landings with two or more risers, wherever a fall of 600mm or more is possible, and must stop a 100mm sphere passing through any gap — including under an open riser, where the tread overlap must be at least 16mm. Tapered (winder) treads are measured at a set distance in from the narrow end [VERIFY exact measurement point and minimum narrow-end going] — this calculator assumes straight-flight geometry and doesn't size winders directly.
These are planning-stage estimates, not a signed-off Building Regulations submission. A loft conversion, an unusually steep site, or any departure from Table 1.1 figures should be agreed with your local Building Control body before work starts.
Common mistakes when planning to Part K
Most Part K rejections trace back to one of these five planning errors, not to bad joinery.
Reaching for a US rule of thumb
The American 7-11 rule and IRC riser/tread limits are a different code, in inches, with different numbers. Part K works in millimetres and uses the 2R+G relationship, not a fixed 7"/11" pairing.
Measuring going from the wrong point
Going is measured along the pitch line between nosings, not the full physical tread depth including any overhang. Confusing the two makes an otherwise-compliant tread look short — or a non-compliant one look fine.
Ignoring the 36-riser rule
A long, straight flight in a dwelling with no landing and no change of direction is non-compliant past 36 risers — a real constraint on tall houses or multi-storey extensions with a single stair run.
Measuring headroom straight up instead of on the pitch line
Headroom is measured perpendicular from the pitch line to the obstruction above, not vertically from the tread. This is the single most common reason a compliant-looking loft stair fails inspection.
Sizing handrails off the wrong width threshold
Assuming one handrail is always enough. Once clear stair width reaches 1,000mm, Part K expects handrails on both sides — easy to miss on a generously wide feature stair.
Locking the going before checking the pitch
A going that satisfies the category minimum on its own can still push pitch over the category maximum once the riser is fixed — always check the resulting pitch, not just rise and going individually, as in the worked example above.
When to use a Part K stair calculator
Part K compliance is checked at the very start of almost every UK stair project, before a single stringer is cut.
Loft Conversions
Fitting a compliant stair into a fixed, often tight, roof-space opening.
New Build Dwellings
Setting private-stair layout before framing, matching the approved plan set.
Staircase Replacement
Replacing a worn or non-compliant staircase during a home renovation.
Blocks of Flats
Checking a common stair serving more than one dwelling against the tighter utility limits.
Small Commercial Fit-Outs
Offices and shops sizing a general-access stair before a Building Regulations submission.
Assembly Buildings
Early-stage stepped access planning for theatres, halls and similar public venues.