Stairs are the most measured, most regulated, most inspected element of a house. A wall an inch out of plumb is a cosmetic conversation; a riser 20 mm out of step is a fall waiting for the person carrying the laundry. The saving grace is that almost every comfort and compliance question about a straight stair resolves with one short calculation and a few typical ranges.
This guide covers the vocabulary, the comfort formula that ties riser to tread, typical residential ranges, headroom and landings, the arithmetic behind total run and the floor opening above, and the compact solutions for when the space simply is not there.
Comfort lives at 2R + T ≈ 600–640 mm, where R is the riser and T the going. Typical residential ranges: riser 125–190 mm, tread 250–320 mm, pitch around 30–38°. Divide the floor-to-floor height by a whole number of risers, never the reverse; treads come to one fewer than risers. Headroom minimums of around 2 m are common in many countries — but regulations differ by country and region, so check your local building regulations before you fix anything. The Stair Calculator runs the full arithmetic from your floor-to-floor height.
Vocabulary First: Rise, Going, Run, Headroom
Stair talk is precise because the geometry is. Six terms cover most of it:
| Term | Meaning |
|---|---|
| Total rise | The floor-to-floor (or floor-to-landing) vertical distance — the starting number for everything |
| Riser (R) | The vertical face of a step; its height is the "rise" |
| Going / tread (T) | The horizontal nose-to-nose depth of a step (the tread board may add a nosing overhang) |
| Total run | Sum of all goings, from the face of the first riser to the last nosing |
| Headroom | Clear vertical height above the pitch line joining the nosings |
| Stringer | The inclined member (timber or steel) carrying the treads |
One counting rule does more work than any other: a flight has one fewer tread than risers. The last riser steps onto the upper floor, which supplies its own "tread". Sixteen risers, fifteen goings — always.
The Comfort Formula: 2R + T
Legs have a stride, and legs are the same everywhere, which is why a rule from 17th-century France still sizes stairs in Japan and Chile: twice the riser plus the going should come to about 600–640 mm (Blondel's original figure, about 24–25 inches in the imperial form). The physics behind it: a short riser lets you take a long stride, so it pairs with a deep tread; a tall riser forces a short stride, so the tread can be shallower. The sum tracks the length of a comfortable pace.
- R = 190, T = 250: 2 × 190 + 250 = 630 mm — in the band, but steep: a compact stair for a tight spot.
- R = 150, T = 300: 2 × 150 + 300 = 600 mm — gentle and generous: a principal stair with room to spare.
- R = 169, T = 280: 2 × 169 + 280 ≈ 618 mm — the comfortable middle where most domestic stairs land.
Drift outside the band and you feel it before you can name it: 2R + T much below 600 climbs like a ship's ladder; much above 640 and every step wants to be two.
Typical Residential Ranges
| Dimension | Typical residential range |
|---|---|
| Riser height | 125–190 mm (5–7.5 in) |
| Going (tread depth) | 250–320 mm (10–12.6 in) |
| Pitch (angle) | about 30–38° |
| Clear stair width | around 850–1,000 mm in dwellings |
| Headroom | minimums around 2,000 mm are common in many countries |
Every figure in that table is typical, not universal. Maximum riser heights, minimum goings, uniformity tolerances, handrail and guard requirements, and the headroom rule all differ by country and are amended locally — and stairs are exactly where building control looks hardest. Treat the table as the shape of a comfortable stair, then check your local building regulations for the numbers that carry legal weight, and permit the work where permits are required.
One rule is close to universal because it is physics: every riser in a flight must be equal. The tolerance is tight everywhere — a few millimetres — and the classic failure is at the ends, where floor finishes of different thickness quietly change the first or last riser.
Worked Example: 2,700 mm Floor-to-Floor
A standard two-storey house, 2,700 mm from finished floor to finished floor:
- Pick the riser count, not the riser. 2,700 ÷ 15 = 180 mm — legal in many places but steep (2R + T = 640 at a 280 going, the top of the band). 2,700 ÷ 16 = 168.75 mm — comfortably mid-range.
- Choose 16 risers at 168.75 mm each.
- Treads: 16 − 1 = 15, at a 280 mm going.
- Comfort check: 2R + T = 2 × 168.75 + 280 = 617.5 mm — inside the band.
- Total run: 15 × 280 = 4,200 mm (4.2 m).
- Pitch: tan−1(168.75 ÷ 280) ≈ 31° — a comfortable domestic angle.
The Stair Calculator produces exactly this table from a floor-to-floor height, including the alternative riser counts either side, so you can see the trade-off between step count and run length instantly.
Width, Landings and the Opening Above
Width is set by who and what uses the stair: around 850–1,000 mm clear suits most dwellings, with wider stairs where furniture or accessibility demands it, and minimum widths for escape routes fixed by regulation. Landings belong at the top and bottom of every flight, wherever the stair changes direction, and near doors that swing onto it; many guidance documents also cap the risers per flight — commonly in the teens — after which you build two flights with a landing between.
The floor opening is where stair geometry meets structural framing, and it is pure arithmetic with the pitch line. For the example stair, assuming about 250 mm of upper-floor structure (underside of the floor at roughly 2,700 − 250 = 2,450 mm above the lower floor) and a 2,000 mm headroom target:
- Worst case: at the lower edge of the opening, the pitch line may sit no higher than 2,450 − 2,000 = 450 mm above the lower floor.
- Where the pitch line reaches 450 mm: the first nosing sits at about 169 mm, and each 280 mm of going climbs 168.75 mm — so (450 − 169) ÷ 169 × 280 ≈ 470 mm of run.
- Opening length: 4,200 − 470 ≈ 3.7 m — nearly the whole run, plus a practical margin.
That is why straight stairs over a lower storey tend to get a long stairwell or a mid-flight landing with a turn — the turn recycles headroom by rotating the flight inside a shorter opening. Change the assumptions — floor thickness, the headroom rule in force locally, a bulkhead — and the opening changes with them, so draw the section before anyone cuts a joist.
Tight Spaces: Alternating Treads, Ships Stairs, Spirals
When 2R + T cannot be satisfied in the space available, the compact options each trade comfort for footprint. Alternating-tread devices halve the plan area by letting each foot use only its own half of a double-depth tread. Ships stairs and steep fixed ladders accept a near-vertical pitch with handrails doing the safety work. Spiral stairs rotate the run into a circle, with every tread tapered — comfortable at the walk line, narrow at the newel. All three are typically restricted by regulation to loft access, utility rooms or secondary routes within a dwelling rather than a main escape stair, and all demand more care from the user. Choose them because the space forces it, not because the main stair looked hard.
Safety and When to Bring In a Professional
Layout arithmetic is a planning exercise; a stair is a structure. Cut-stringer timber stairs, concrete stairs, cantilevered flights and anything supporting guard loads along an opening edge are structural design work — member sizes, fixings and connections come from an engineer or a manufactured system, not from a guide. Handrails and guards have their own dimensional rules in every jurisdiction. And because stairs concentrate falls like no other element, they attract the closest inspection on any permit: build to the local regulations, not to a table on a website, and where the rules are unclear, ask the building department before the stringers are cut.
Common Mistakes
- Dividing to a "nice" riser instead of a whole riser count. 2,700 ÷ 170 = 15.9 risers does not exist; risers are integers.
- Counting treads equal to risers. The run comes out one going long, and the top of the stair arrives in the wrong place.
- Ignoring floor finish thickness. A 20 mm thicker floor finish on the upper storey turns a 168.75 mm top riser into 188.75 — a trip hazard in the making.
- Measuring floor-to-ceiling instead of floor-to-floor. The total rise includes the upper floor structure.
- Fixing the stairwell opening before checking headroom on a section drawing. The worst stair mistake to discover is found with a saw already in the joists.
- Designing to a remembered rule from another country. Riser and going limits genuinely differ; look up the one that governs your build.
Plan It With the Stair Calculator
Enter the floor-to-floor height and a target going, and the Stair Calculator returns the viable riser counts, the riser heights, total run, pitch and the 2R + T comfort check for each — the full worked example above, live for your numbers.
Open the Stair CalculatorFrequently Asked Questions
What is the 2R + T rule for stairs?
A comfort formula usually credited to François Blondel: twice the riser plus the going should total roughly 600–640 mm (24–25 in). It matches the stair to a comfortable human stride. It is a comfort guideline — your local building regulations set the legal limits.
What is a comfortable riser height for a home stair?
Typical residential risers run 125–190 mm, with 150–180 mm the comfortable middle. Whatever the height, every riser in the flight must match within a tight tolerance — unequal end risers are both a trip hazard and a common inspection failure.
How do I work out how many steps I need?
Divide the floor-to-floor height by whole numbers of risers until one lands in the comfortable range: 2,700 mm gives 180 mm at 15 risers or 168.75 mm at 16. Then treads = risers − 1, and total run = treads × going.
How much headroom does a staircase need?
Around 2,000 mm of clear height above the pitch line is the common minimum in many countries, but the exact figure and measurement method vary by jurisdiction. Headroom is what sets the floor opening length, so check the local rule before the opening is framed.
Are treads the same number as risers?
No — one fewer. The final riser lands on the upper floor, which acts as its own tread. A 16-riser flight has 15 goings, so total run = (16 − 1) × going.
Related MyHouseLogic Guides
The stair sets the rhythm of the floors above and below it.
- Timber sizes around the world — what stringer and tread stock is actually called in your local yard.
- Flooring quantity guide — the landing and upper floor the stair delivers you onto.
- Compact home planning, Japanese style — how tight-footprint housing handles stairs, storage and circulation together.
- All Concrete & Building guides — structure, quantities and dimensioning.
The bottom line: pick a whole number of risers that divides the floor-to-floor height into 150–180 mm steps, pair it with a going that puts 2R + T at 600–640 mm, and check headroom on a section before the opening is cut. Then verify every dimension against your local building regulations — stairs are the one element where "typical" is never a defence.