Stair codes compared

IRC, IBC and UK Approved Document K side by side — with the section numbers, so you can check the source.

Read this first. Codes are adopted jurisdiction by jurisdiction, often years after publication, and amended locally — New Jersey, for one, amends the IRC. The table below is a starting point for design, not an authority. Confirm the edition in force with your building department before you build to it.

	IRC — residential	IBC — commercial	UK Part K — private
Maximum riser	7¾″	7″	220mm
Minimum riser	—	4″	150mm
Minimum tread / going	10″	11″	220mm
Maximum going	—	—	300mm
Variation within a flight	⅜″	⅜″	—
Minimum width	36″	44″ (36″ if occupant load < 50)	900mm (entrance storey)
Minimum headroom	80″	80″	2000mm
Maximum pitch	—	—	42°
Maximum rise per flight	151″	12 ft	16 risers (utility)
Handrail height	34–38″	34–38″	900–1000mm
Guard opening	no 4″ sphere	no 4″ sphere	no 100mm sphere
Nosing projection	¾″–1¼″	¾″–1¼″	max 25mm

References: IRC 2021 §R311.7 · IBC §1011 · UK Approved Document K. Figures checked October 2026.

Why the numbers differ

The same step drawn against each code: maximum riser by minimum tread. A step has to fit inside the box for the code you are building to.

The explanation is more useful than the table. The three codes assume three different people on the stair.

The IRC governs homes, where the people using the stair use it every day and know it in the dark. It can afford to be the most permissive: 7¾″ risers and 10″ treads make a steepish stair, but a familiar one, and houses have floor space to save.

The IBC governs buildings full of strangers, some of whom will one day be leaving in a hurry and in smoke. Hence 7″ risers, 11″ treads and 44″ of width — a shallower, wider stair that a crowd can descend quickly without knowing it.

UK Part K splits the difference by category rather than building type, with separate limits for private, utility and general access stairs, and adds a hard pitch cap of 42° that the US codes leave implicit.

The rule that matters most

If you take one number from this page, take the ⅜″ variation limit. A stair with a 7¾″ riser is safe. A stair with fourteen 7″ risers and one 7½″ riser is the one people fall on, because the foot learns the rhythm from the first two steps and stops looking.