Stair dimensions: rise, run, headroom and handrails

The numbers, IRC and UK Part K side by side#

Item	IRC 2021 (US dwellings)	UK Approved Document K (private stairs)
Maximum riser or rise	7.75 in (197 mm)	220 mm (8.66 in)
Minimum tread or going	10 in (254 mm)	220 mm (8.66 in)
Maximum pitch	not stated directly	42 degrees
Minimum headroom	6 ft 8 in (2032 mm)	2000 mm (6 ft 7 in)
Minimum width	36 in (914 mm)	not set for private stairs; 900 mm (35.4 in) where Part M applies
Handrail height	34 to 38 in (864 to 965 mm) above nosings	900 to 1000 mm (35.4 to 39.4 in) above pitch line
Guard height at stairs	34 in (864 mm) min from nosings	900 mm (35.4 in)
Guard opening	no 4 in (102 mm) sphere passes	no 100 mm (3.9 in) sphere passes
Riser variation within a flight	3/8 in (9.5 mm) max	consistent rise required
Comfort formula	2R + T = 24 to 25 in	2R + G = 550 to 700 mm

Note the difference in philosophy. The IRC constrains riser and tread independently and lets the pitch fall where it may. Part K constrains the pitch directly at 42 degrees and gives a wide 2R + G band, which permits a steeper individual step but caps how steep the flight as a whole can be. A UK stair at 220 mm rise and 220 mm going is 45 degrees at the step but would exceed the 42 degree pitch limit, so in practice private stairs land nearer 200 mm rise and 240 mm going.

Rise and run: the trade you are actually making#

Every stair is a negotiation between comfort and floor area, and the exchange rate is fixed.

Riser count is always a whole number, so the riser height is fixed by your floor to floor dimension, not chosen freely. Divide, round up, divide again. Measuring floor to floor means finished floor to finished floor: if the upstairs gets 0.75 in (19 mm) of engineered board after the stair is built, every riser is now 0.05 in out and the top one is out by 0.75 in, which breaks the 3/8 in variation rule at the most dangerous step on the flight.

Headroom eats more floor than the run does#

Headroom is measured vertically from the pitch line (the line joining the nosings) to whatever is above. To find how long the opening in the upper floor has to be, work backwards.

Width, handrails and guards#

36 in (914 mm) is the minimum stair width above handrail height. Handrails may project up to 4.5 in (114 mm) into that width on each side, so a 36 in stair with two handrails still gives 27 in (686 mm) of clear passage at hand height. That is enough to walk, and it is not enough to carry a mattress. If a stair is the only route to bedrooms, 40 to 44 in (102 to 112 cm) is worth having, and it makes the difference between furniture arriving and furniture going back on the van. The relevant geometry is in measuring the delivery path.

The handrail itself:

• Height 34 to 38 in (864 to 965 mm) above the nosings, continuous for the full flight
• Circular Type I grip 1.25 to 2 in (32 to 51 mm) in diameter, or a non-circular profile with a 4 to 6.25 in (102 to 159 mm) perimeter and a cross-section no more than 2.25 in (57 mm)
• 1.5 in (38 mm) clear between the rail and the wall

The 1.25 to 2 in range is not arbitrary. It is the band in which a hand can close around the rail far enough to arrest a fall. A 3 in (76 mm) square newel-style rail looks generous and cannot be gripped, which is why it fails the graspability test even though it is larger.

Guards on the open side of a stair are 34 in (864 mm) minimum measured vertically from the nosings, and no opening may pass a 4 in (102 mm) sphere. The triangle formed by riser, tread and bottom rail is permitted a 6 in (152 mm) sphere. Landing guards rise to 36 in (914 mm) because they are measured from a flat floor rather than a slope.

Landings#

A landing must be at least as wide as the stair it serves and at least 36 in (914 mm) long in the direction of travel. A door may not swing over a stair flight, and where a door opens onto a landing at the top, the landing has to accommodate the swing.

A quarter-turn landing costs 36 x 36 in (91 x 91 cm) of floor and buys two things: it shortens the straight run, and it interrupts a fall. Winders can replace it and save floor, but a winder tread must be at least 10 in (254 mm) deep measured 12 in (305 mm) from the narrow edge and never less than 6 in (152 mm) at any point, and they are measurably harder to descend in the dark.

What a staircase costs in floor area#

At 36 in (914 mm) wide, a 130 in (3.3 m) run occupies 32.5 sq ft (3.0 sq m) on each of the two storeys it connects, so a straight flight between two floors removes about 65 sq ft (6.0 sq m) from the house. Add the landing and the figure passes 80 sq ft (7.4 sq m). That is a small bedroom's worth of floor, which is why stair position is a first-order plan decision and not a detail, and why the space beneath a flight is worth designing rather than accepting: the head end gives you a full-height cupboard, the foot end gives you a shoe drawer.

Checking an existing stair in ten minutes#

1. Measure every riser from tread surface to tread surface. Note the largest and smallest. If they differ by more than 3/8 in (9.5 mm), that flight has a trip hazard, almost always at the top or bottom step.
2. Measure the tread from nosing to nosing, not the visible board.
3. Compute 2R + T. Under 23 in (58 cm) means a shuffling stair, over 26 in (66 cm) means a climbing one.
4. Stand on the third step and measure straight up to the ceiling, then repeat where the flight passes any beam or bulkhead. Anything under 6 ft 8 in (2032 mm) is a head strike waiting to happen.
5. Measure the handrail height at the top nosing and the bottom nosing. Rails fitted parallel to a wall rather than to the pitch line drift out of range at one end.
6. Push a 4 in (102 mm) ball through the balusters at the widest point, which is usually mid-flight where a rail has bowed.

Where the numbers fail and rebuilding is not possible, the ranked mitigations are: fit a second handrail on the open side, add tread-edge contrast so the nosing line is readable, then light the flight from the side at low level rather than from a ceiling fitting overhead, which throws your own shadow onto the treads. A layered lighting plan matters more on a stair than anywhere else in the house.