Stair Calculator
Design a staircase: number of steps, riser height, run and stringer length from total rise.
Results update as you type.
About this calculator
A stair calculator lays out a staircase from the total vertical rise (floor to floor) and the tread run (the horizontal depth of each step). It divides the total rise by a comfortable target riser height — about 7 inches — to find a whole number of risers, then recomputes the exact riser so every step is equal in height, which building codes require. The number of treads is one fewer than the risers (the top riser lands on the upper floor), the total horizontal run is treads × tread depth, and the stringer length is the diagonal √(rise² + run²).
Worked example: a 108-inch floor-to-floor rise divided by a 7-inch target is 15.4, rounded to 15 risers, giving an exact riser of 108 ÷ 15 = 7.2 inches. That means 14 treads; at a 10-inch run the total run is 140 inches and the stringer is √(108² + 140²) ≈ 176.7 inches. The 2R+T check is 2 × 7.2 + 10 = 24.4 inches — right in the comfortable 24–25 inch range.
It flags whether the riser stays within the common code maximum of 7¾ inches and checks the comfort rule, so carpenters and DIY builders can size a stair, cut stringers, and confirm it will pass inspection before cutting timber.
Frequently asked questions
What is a comfortable riser height?
Around 7 inches is ideal. Most building codes cap the riser at 7¾ inches and set a minimum tread depth near 10 inches. The calculator warns when the riser falls outside a comfortable, code-compliant range.
What is the 2R+T stair rule?
Two times the riser height plus the tread run should total 24–25 inches for a stair that feels natural to climb. Steps that are too steep or too shallow break this rule, and the calculator reports where you land.
Why is there one fewer tread than risers?
The top of the staircase lands on the upper floor, so the final riser has no tread of its own. A stair with 15 risers has 14 treads.
How many steps do I need for a given floor height?
Divide the total rise by a comfortable riser of about 7 inches and round to a whole number. A standard 108-inch (9 ft) rise gives about 15 risers and 14 treads. The calculator then evens out the exact riser height so no step is taller than the rest.
What is the stringer length and why does it matter?
The stringer is the sloped board that supports the treads; its length is the diagonal of the whole staircase, √(total rise² + total run²). Knowing it tells you how long a board to buy and how far the stair will project from the wall.
What are standard riser and tread dimensions?
Residential codes commonly cap the riser at about 7¾ inches and set a minimum tread depth around 10 inches, with all risers within 3/8 inch of each other. Aim for roughly a 7-inch riser and an 11-inch tread for a comfortable, code-compliant stair.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/stair/curl
curl "https://calculator.free/api/v1/stair/?total_rise=108&tread_run=10"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/stair/?" + new URLSearchParams({
"total_rise": "108",
"tread_run": "10"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.