Stair Rise and Run: Code Limits for the US, Canada, and the UK

Stairs are the one piece of framing where “close enough” fails an inspection. Riser heights must match each other within a few millimetres, the riser-to-going relationship is fixed by code, and a staircase that misses the limits by one riser has to come out. The arithmetic is simple; the part that catches people is that the limits differ by country, and the margins are thin — a tread that clears the American minimum exactly is a millimetre short of Canada’s.

Verify before you build. Section numbers, table values, and demand factors change between code editions, and jurisdictions adopt them with local amendments. Treat this guide — and the app — as a calculation aid, not as the code book: confirm every number against the edition your authority having jurisdiction enforces. How FieldDojo validates its calculations.

Start With Total Rise, Not Riser Height

Every stair layout starts from one measurement: total rise — finished floor to finished floor, not subfloor to subfloor. Get this wrong and every riser inherits the error.

Measure the vertical distance between the two finished surfaces. If the upper floor covering isn’t down yet, add its thickness. A 19 mm (¾ in) hardwood layer you forgot about, spread across 15 risers, is 1.3 mm per riser — invisible. Forgotten under a single top step, it’s a 19 mm trip hazard and a failed inspection, because codes limit the variation between the tallest and shortest riser in a flight.

From total rise, the sequence is always the same:

Number of risers = round(Total rise ÷ Target riser height)
Actual riser height = Total rise ÷ Number of risers
Number of treads = Number of risers − 1
Total run = Number of treads × Tread depth (going)
Pitch = arctan(Riser height ÷ Tread depth)

You choose a target riser height for comfort; the code decides whether the actual one is legal. Because the riser count must be a whole number, the actual height almost never equals the target — and the actual height is what gets checked.

Why Treads = Risers − 1

This is the single most common layout error, and it costs a step.

Walk up a flight. Every step up is a riser. The last riser lands you on the upper floor — and the upper floor is the last tread. So a flight with 15 risers has 14 treads, because the fifteenth “tread” is the landing you’re standing on.

That assumption holds for the standard case where the top riser finishes flush with the upper floor. Build a flight where the top step sits below the floor level (with a separate landing), and the count changes. FieldDojo’s stair calculator uses the flush case — risers minus one — which is what almost every residential flight is.

The consequence is easy to miss when estimating material: a 15-riser flight needs 14 treads, not 15, but you still cut 15 riser boards if the risers are closed.

The Limits, Side by Side

Three countries, three different rulebooks. These are the values FieldDojo checks against, per region:

Source: the limits encoded in FieldDojo’s regional stair calculators — IRC R311.7 (US residential), NBC 9.8 private dwelling stairs (Canada), Approved Document K private stair (UK). All three documents are copyrighted; verify against the edition in force in your jurisdiction.

Three things fall out of that table:

The UK allows the steepest risers but the shallowest goings. A 220 mm riser would fail in Canada by 20 mm and in the US by 24 mm. In exchange, the UK permits a 220 mm going where Canada demands 255 mm.

Only Canada and the UK cap the tread depth. In the US a deeper tread is always acceptable; under NBC and AD K an over-deep tread is a defect, because an unusually long going breaks a walker’s stride as reliably as one that is too short.

Only the UK regulates pitch directly. Elsewhere the pitch is an emergent property of riser and going, and it’s constrained indirectly. In the UK, 42° is a hard ceiling checked on its own.

A subtle detail worth knowing about how that ceiling is enforced: FieldDojo compares the unrounded pitch against 42°. A stair computing to 42.04° displays as “42.0°” but still fails, because rounding a safety limit into compliance is exactly the kind of error that gets built.

The Comfort Rules — and Why They Aren’t the Same Formula

The US and UK both carry a “comfort” relationship, and they are not interchangeable.

US — the 2R + T rule. Twice the riser plus the tread should land between 24 and 25 inches. It comes from the observation that a person’s stride shortens as they climb, and it keeps the riser/tread pair proportional across the legal range.

UK — the 2R + G rule. Twice the rise plus the going, between 550 and 700 mm. Same idea, wider band, metric units.

They look like unit conversions of each other. They aren’t. 24–25 in converts to roughly 610–635 mm — a 25 mm window. The UK’s is 150 mm wide. Run a stair that satisfies AD K’s 2R + G comfortably and it can still sit outside the American 24–25 in band, which is why FieldDojo checks the region’s own rule instead of converting between them.

Canada’s NBC 9.8 has no comfort formula. The riser and run ranges do that work. If a Canadian stair meets the dimensional limits, there’s no third relationship to satisfy — so the calculator reports no comfort result for Canada rather than inventing one.

Worked Example — US (IRC R311.7)

A basement stair, total rise 112 in (9 ft 4 in) floor to floor, using 10½ in treads and targeting a 7 in riser.

Risers    = round(112 ÷ 7)  = 16
Riser     = 112 ÷ 16        = 7.00 in
Treads    = 16 − 1          = 15
Total run = 15 × 10.5       = 157.5 in  (13 ft 1½ in)
Pitch     = arctan(7 ÷ 10.5) = 33.7°
2R + T    = (2 × 7) + 10.5   = 24.5 in

Every check passes: 7.00 in ≤ 7¾ in, 10.5 in ≥ 10 in, and 24.5 in sits inside the 24–25 in comfort band.

The number that matters for framing is the 157.5 in total run — the horizontal floor space the flight consumes. Compare it against the opening you actually have before cutting the stringers. A stair that meets every code limit and doesn’t fit the stairwell is still wrong.

Worked Example — Canada (NBC 9.8)

Total rise 2750 mm, 280 mm treads, targeting a 180 mm riser.

Risers    = round(2750 ÷ 180) = 15
Riser     = 2750 ÷ 15         = 183.33 mm
Treads    = 15 − 1            = 14
Total run = 14 × 280          = 3920 mm
Pitch     = arctan(183.33 ÷ 280) = 33.2°

Riser 183.33 mm sits inside 125–200 mm; the 280 mm tread inside 255–355 mm. No comfort check applies.

Note what the rounding did. The target was 180 mm, the actual is 183.33 mm — 3.33 mm taller, on every riser. That’s fine here, but it’s the mechanism behind most failures: a target chosen close to the maximum rounds up past it. Targeting 195 mm on this rise gives 14 risers at 196.43 mm (7.73 in) — legal in Canada, and inside the US 7¾ in limit, but by only 0.02 in. The real trap is the step where the riser count drops: target 204 mm and the division rounds to 13 risers at 211.54 mm (8.33 in), which fails Canada’s 200 mm limit and the US 7¾ in limit at once. A 9 mm change in target moved the actual riser by 15 mm. Always check the riser the count produces, never the target you asked for.

Worked Example — UK (Approved Document K)

Total rise 2600 mm, 250 mm going, targeting a 190 mm rise.

Risers    = round(2600 ÷ 190) = 14
Rise      = 2600 ÷ 14         = 185.71 mm
Goings    = 14 − 1            = 13
Total run = 13 × 250          = 3250 mm
Pitch     = arctan(185.71 ÷ 250) = 36.6°
2R + G    = (2 × 185.71) + 250   = 621.4 mm

All four checks pass: rise inside 150–220 mm, going inside 220–300 mm, pitch under 42°, and 2R + G inside 550–700 mm.

Now move that staircase. The 185.71 mm rise is legal in Canada (under 200 mm) and in the US (7.31 in, under 7¾ in) — but the going travels badly. At 250 mm it is 9.84 in, which falls short of the US 10 in minimum, and 5 mm short of Canada’s 255 mm minimum. One staircase, fully compliant at home, non-compliant in both other countries on the same dimension. That is the argument for checking the code in force where you are building rather than a remembered rule of thumb.

Headroom Is Measured on the Pitch Line

Headroom is the limit people discover last, usually after the stringers are cut.

It’s measured vertically from the pitch line — the imaginary line joining the nosings — to the underside of whatever is above: ceiling, landing edge, or trimmer. Not from the tread surface, which would flatter the measurement by the height of the nosing projection.

• US: 6 ft 8 in (2030 mm), the highest of the three
• UK: 2000 mm
• Canada: 1950 mm

The pinch point is nearly always the floor opening at the top, where the trimmer joist crosses over the flight. If the opening is too short, headroom fails no matter how well the risers work — and the fix is enlarging the opening, which means structural work.

Common Mistakes

Measuring subfloor to subfloor. Guarantees the top or bottom riser is off by the flooring thickness.

Targeting a riser height at the legal maximum. Rounding pushes the actual height past it. Aim near the middle of the range and let the arithmetic land inside.

Cutting stringers before checking total run. The flight must fit the opening, not just the code.

Assuming treads equal risers. Costs a tread, or a re-cut.

Carrying a rule of thumb across a border. Some travel and some don’t. The American “7-11” habit (7 in riser, 11 in tread) happens to satisfy all three codes — 177.8 mm and 279.4 mm sit inside both the Canadian and UK ranges. But build to the US minimum tread of 10 in and you land on 254 mm, one millimetre under Canada’s 255 mm floor. A rule of thumb that clears one code by a wide margin can miss another by almost nothing.

Ignoring the maximum tread. Under NBC and AD K, too deep is a defect, not extra generosity.

Frequently Asked Questions

How many risers do I need for a 9-foot ceiling?

For a total rise of 112 in (9 ft 4 in floor to floor, which is what a 9 ft ceiling usually works out to once the floor assembly is included), 16 risers at exactly 7.00 in each. That gives 15 treads and, at 10½ in per tread, a total run of 157½ in. Always calculate from the measured floor-to-finished-floor dimension rather than the nominal ceiling height — the floor assembly adds several inches.

What is the maximum stair riser height?

It depends on the country. Under the US IRC R311.7 the maximum is 7¾ in (196 mm) for residential stairs. Canada’s NBC 9.8 caps private dwelling risers at 200 mm and also sets a 125 mm minimum. Approved Document K allows the tallest riser of the three at 220 mm, with a 150 mm minimum. A riser legal in the UK can therefore be illegal in both North American codes.

Why are there one fewer treads than risers?

Because the upper floor serves as the final tread. Every step up is a riser, and the last riser delivers you onto the landing — so a 16-riser flight has 15 treads. This holds when the top riser finishes flush with the upper floor, which is the standard residential arrangement. Miscounting here is the most common stair layout error and leaves you one tread short.

What is the 2R+T rule for stairs?

It is the US comfort relationship: twice the riser height plus the tread depth should fall between 24 and 25 inches. It keeps the riser and tread proportional to a natural climbing stride. The UK uses a related but distinct rule, 2R + G between 550 and 700 mm, which is a much wider band than the American one converts to (roughly 610–635 mm). Canada’s NBC has no comfort formula — its riser and run ranges do that job.

What is the minimum headroom above a staircase?

The US IRC requires 6 ft 8 in (2030 mm), Approved Document K requires 2000 mm, and Canada’s NBC 9.8 requires 1950 mm for private dwelling stairs. In all three it is measured vertically from the pitch line — the line joining the tread nosings — to the underside of the structure above, not from the tread surface. The usual failure point is the trimmer joist at the top of the floor opening.

Can I use US stair dimensions in Canada or the UK?

No, and the differences are large enough to fail an inspection rather than merely look unusual. A 7½ in (190 mm) riser is fine under the IRC, legal under NBC, and legal under AD K — but a 10 in (254 mm) tread that satisfies the IRC minimum falls 1 mm short of NBC’s 255 mm minimum, and a 250 mm going that passes AD K is under the minimum in both North American codes. Check the flight against the code actually in force where you are building.

How FieldDojo Handles This

FieldDojo’s stair calculator is region-aware. Set your region and it applies that country’s rulebook:

• United States — IRC R311.7: riser ≤ 7¾ in, tread ≥ 10 in, 2R + T comfort band, 6 ft 8 in headroom
• Canada — NBC 9.8: riser 125–200 mm, run 255–355 mm, 1950 mm headroom
• United Kingdom — Approved Document K: rise 150–220 mm, going 220–300 mm, pitch ≤ 42°, 2R + G comfort band, 2000 mm headroom

Enter total rise, tread depth, and a target riser; it returns the riser count, actual riser height, tread count, total run, and pitch, with each limit individually flagged pass or fail rather than a single verdict — so you can see which rule is tight and adjust the one input that fixes it. Imperial and metric both work in every region. Works offline, on the stringer, with no signal.

Get FieldDojo on the App Store → or on Google Play → — free on both.

Related trade guides:

• How to Calculate Concrete for Footings, Slabs, and Columns — the landing pad and footing under a basement flight
• Residential Electrical Load Calculation: NEC Article 220 — stairwell lighting circuits belong in the dwelling load