How many steps do I need?

Divide the total rise by the maximum riser your code allows and round up. A 108 inch rise under the IRC needs at least 14 risers, because 13 would put each one over 7¾ inches. Note that the number of treads is always one fewer than the number of risers, since the top floor forms the last landing.

Does the nosing count towards tread depth?

Under the IRC, tread depth is measured from nosing to nosing, so the overhang does count toward the required 10 inches. A nosing of 3/4 to 1¼ inches is required where the tread is less than 11 inches deep, and is optional above that.

What headroom do I need above a stair?

The IRC and IBC both require 6 feet 8 inches, measured vertically from the sloped plane of the nosings. The UK requires 2 metres, relaxed to 1.9 m at the centre for a loft conversion. This is the constraint that most often forces a stair to move rather than change shape.

Is a steeper stair allowed for a loft or attic?

Sometimes, under specific relaxations, and it varies by jurisdiction. The UK allows alternating tread stairs for loft conversions in defined circumstances. Do not assume a relaxation applies without checking, because a stair is one of the few things an inspector will always look at.

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Stair calculator: rise, run, and whether it passes code

This stair calculator takes your total floor-to-floor height and works out the riser height, tread depth and stringer length. It then checks the result against the building code you select, because a stair that is comfortable is not automatically a stair that is legal.

Total rise, floor to floor

Units

Building code

Tread depth / going (blank = auto)

Nosing overhang

What are the legal limits on stair dimensions?

They differ by country and, in the United States, by whether the building is a home or not. This is the part most stair calculators leave out, and it is the part that fails an inspection.

Code	Max riser	Min tread / going	Other
IRC 2021 (US home)	7¾ in (196 mm)	10 in (254 mm)	Max 3/8 in variation between risers
IBC 2021 (US commercial)	7 in (178 mm)	11 in (279 mm)	Stricter than the residential code
UK Part K (private)	220 mm	220 mm	Pitch max 42°, 2R+G 550-700 mm
NCC 2022 (AU house)	190 mm	240 mm	2R+G 550-700 mm, 5 mm uniformity

Code IRC 2021 section R311.7; IBC 2021 section 1011.5.2; UK Building Regulations Approved Document K; NCC 2022 ABCB Housing Provisions Part 11.2. Checked August 2026.

Why does the US have two different stair codes?

Because homes and commercial buildings are regulated separately. The International Residential Code governs houses and allows a steeper stair: a 7¾ inch riser against the commercial code's 7 inches, and a 10 inch tread against 11.

The practical consequence catches people out. A stair built legally in a house can be illegal in a shop or office, so a garage conversion or a home business fit-out may need the stair rebuilt. If you are unsure which applies, ask the building department before cutting a stringer.

What is the 3/8 inch rule and why does it fail inspections?

Under the IRC, the difference between the tallest and shortest riser in a flight cannot exceed 3/8 of an inch. It is one of the most common reasons a stair fails.

It usually goes wrong at the top or bottom step, where the finished floor thickness was not accounted for. Tile at the top and carpet at the bottom can easily shift one riser by half an inch. Measure from finished floor to finished floor, not subfloor to subfloor, and the problem disappears.

Australia is stricter still: the NCC allows 5 mm between adjacent risers and 10 mm across the whole flight.

What is the rise plus run rule?

A comfort guide, not a legal requirement in the US. The common forms are that rise plus run should total 17 to 18 inches, or that twice the rise plus the run should total 24 to 25 inches.

The second has a real pedigree. It descends from the French architect François Blondel, who set it out in his Cours d'architecture in the 1670s, and it survives today as the 2R+G range of 550 to 700 mm written directly into both the UK and Australian codes. So in Britain and Australia it is law; in the United States it is advice.

Convention in the US, Code in the UK and Australia.

How do you work out stringer length?

The stringer is the diagonal, so it is the hypotenuse of a triangle whose sides are the total rise and the total run. Stringer = √(rise² + run²).

The figure above is the theoretical length along the cut line. Cut the stock longer, because you lose material at both ends: the plumb cut at the top and the seat cut at the bottom, less the thickness of the tread at the bottom step.

Common questions

How many steps do I need?: Divide the total rise by the maximum riser your code allows and round up. A 108 inch rise under the IRC needs at least 14 risers, because 13 would put each one over 7¾ inches. Note that the number of treads is always one fewer than the number of risers, since the top floor forms the last landing.
Does the nosing count towards tread depth?: Under the IRC, tread depth is measured from nosing to nosing, so the overhang does count toward the required 10 inches. A nosing of 3/4 to 1¼ inches is required where the tread is less than 11 inches deep, and is optional above that.
What headroom do I need above a stair?: The IRC and IBC both require 6 feet 8 inches, measured vertically from the sloped plane of the nosings. The UK requires 2 metres, relaxed to 1.9 m at the centre for a loft conversion. This is the constraint that most often forces a stair to move rather than change shape.
Is a steeper stair allowed for a loft or attic?: Sometimes, under specific relaxations, and it varies by jurisdiction. The UK allows alternating tread stairs for loft conversions in defined circumstances. Do not assume a relaxation applies without checking, because a stair is one of the few things an inspector will always look at.