About the Stair Calculator
A staircase looks simple, but it is one of the most tightly regulated parts of a building, and for good reason: falls on stairs are among the most common causes of injury at home. What makes a stair safe is consistency — every step the same height and depth, so that your feet find each one without thinking — along with steps that are neither too tall nor too shallow for a natural stride.
Everything starts from one measurement: the total rise, from the finished floor at the bottom to the finished floor at the top. From it, and the limits your building code sets, follow the number of steps, their exact height, the length of the staircase and the size of the stringers that carry it.
This stair calculator plans a straight flight: it works out the risers and treads, the total run, the stringer length and the angle, and checks the proportions for comfort.
How to Use the Stair Calculator
Choose metric (millimetres) or imperial (inches).
Enter the total rise, from finished floor to finished floor. Remember to include floor coverings not yet fitted.
Enter the maximum riser allowed by your code — 7¾ in in the US IRC, 220 mm for private stairs in England — or a lower figure for an easier stair. The defaults are 7¾ in and 190 mm.
Enter the tread depth (the going), measured nosing to nosing.
How the Stair Is Worked Out
risers = total rise ÷ maximum riser, rounded up
riser = total rise ÷ risers
treads = risers − 1
run = treads × tread depth
stringer = √(total rise² + run²)
comfort = 2 × riser + treadStep-by-Step Example
A total rise of 2,700 mm, maximum riser 190 mm, treads of 250 mm.
Risers: 2700 ÷ 190 = 14.2 → 15 risers
Each riser: 2700 ÷ 15 = 180 mm
Treads: 15 − 1 = 14 treads
Run: 14 × 250 = 3,500 mm
Stringer: √(2700² + 3500²) ≈ 4,420 mm
Comfort: 2 × 180 + 250 = 610 mm
Angle: atan(180 ÷ 250) = 35.8°In imperial, a 108 in rise with a 7¾ in maximum needs 14 risers of 7.71 in, with 13 treads of 10 in giving a 130 in run and a stringer of about 169 in.
Why There Is One Fewer Tread
Each riser is the vertical face of a step, and each tread the flat part you stand on. The bottom riser rises from the lower floor, and the top riser arrives at the upper floor — which serves as the last "tread". So a stair with 15 risers has 14 treads. Forgetting this is a classic mistake that adds a whole step to the length of the stair.
The Comfort Rule
Since the seventeenth century, stair designers have used the rule that twice the riser plus the tread should roughly equal a person's stride — about 600 to 650 mm, or 23½ to 25½ in. Tall risers need shallow treads and low risers need deep ones. A stair outside that range feels awkward: either choppy and short-stepped, or long and loping.
The angle of the stair follows from the riser and tread. Most domestic stairs are between about 30° and 42°; steeper is tiring and hazardous going down, shallower takes up a lot of floor.
Building Code Limits
Codes differ by country, and by whether the stair is private or public. Examples:
England (Approved Document K, private stairs): risers up to 220 mm, goings at least 220 mm, pitch no steeper than 42°, and 2 × riser + going between 550 and 700 mm; headroom of 2.0 m.
These are summaries only — always check the current rules where you are building, as well as handrail, balustrade and landing requirements.
Stringers and Headroom
The stringer is the long board on each side that carries the treads. A cut stringer has the step profile cut into its top edge; a closed stringer has housings routed into its face. Either way, its length is roughly the hypotenuse of the total rise and run, plus a little at the top and bottom for the connections. Check the headroom too: the opening in the floor above must be long enough that no one hits their head on the way down.
Landings and Winders
A single straight flight is limited in height: many codes require a landing at the top and bottom of every flight and limit how far one flight may rise — the US IRC caps it at 151 in (12 ft 7 in) — and public stairs in England are limited to 16 risers per flight. Stairs that turn use a landing or winders, tapered treads that turn the corner. Winders have their own minimum dimensions, and should be planned so the going measured along the walking line matches the straight treads.
Understanding Your Result
Risers gives the number and exact height of the risers.
Treads and run gives the number of treads and the length of the stair.
Stringer gives the approximate stringer length.
Comfort and angle checks 2 × riser + tread and gives the pitch.
Building codes summarises the common limits.
When Should You Use This Calculator?
Planning a new staircase, loft ladder or deck steps.
Checking whether a stair fits the space available.
Before cutting stringers.
Checking an existing stair against the code.
Common Mistakes
Measuring to the subfloor. Use finished floor levels.
Counting treads as risers. There is one fewer tread.
Uneven first or last steps. Every riser must match.
Forgetting headroom at the top of the flight.
Ignoring the comfort rule. Legal is not the same as comfortable.