Open menu

Stair Calculator

Enter the floor-to-floor rise and the tread depth you want. The calculator finds the number of risers, the exact riser height, total run, stringer length and stair angle, and can check the result against the US IRC or the Australian NCC limits. A 108 in rise with 10.5 in treads gives 14 risers of 7.71 in, inside the IRC maximum of 7¾ in.

Estimates only. Structural elements such as beams, footings, stairs, retaining walls over 1 m (4 ft) and rebar design must be designed or approved by a licensed engineer and comply with your local building code and permits. Results are "within typical limits", never "approved" — verify with your building authority.

Measurements

Units

Horizontal depth of each tread, nosing to nosing.

Vertical clearance above the stair's slope line.

Results

Number of risers

14

Riser height

7.71 in

Number of treads
13
Total run
11.37 ft
Stringer length
14.5 ft
Stair angle
38.4 °
2 × riser + going
25.93 in
  • Within limit: riser at most 7.75 in
  • Within limit: tread depth at least 10 in
Show the working
  1. Risers⌈108 in ÷ 7.75 in⌉ = 14
  2. Riser height108 in ÷ 14 = 7.71 in
  3. Total run13 × 10.5 in = 11.37 ft
  4. Stringer length√((9 ft)² + (11.37 ft)²) = 14.5 ft
  5. Slope relationship2 × 7.71 in + 10.5 in = 25.93 in
Paper and orientation
Open the list

How to use the stair calculator

  1. Choose Metric or Imperial. You can switch at any time; your numbers are converted.
  2. Pick a building code preset: US IRC, Australian NCC, or no preset if you only need the geometry.
  3. Enter the total rise, measured from finished floor to finished floor. Measure at the stair location, because floors are rarely perfectly level.
  4. Enter the tread depth (going) you want. For the IRC, 10 in is the minimum; many builders use 10½ to 11 in for comfort.
  5. Optionally enter the headroom you have, measured vertically from the line of the nosings to the ceiling.
  6. Read the results, then open Show the working to see each step of the calculation.

How the calculation works

The calculator divides the total rise by the maximum riser height allowed by the selected code and rounds up to get the number of risers. Without a preset it aims for a riser close to 175 mm (about 6.9 in). The exact riser height is then the total rise divided by that count, so every step is identical, which is what codes require. The total run is the number of treads multiplied by the going. The stringer length comes from Pythagoras: √(rise² + run²). The angle is the arctangent of rise over run.

Worked examples

US basement stair (IRC). Total rise 108 in, treads 10.5 in. 108 ÷ 7.75 = 13.94, so 14 risers of 7.71 in. Thirteen treads × 10.5 in give a total run of 11 ft 4½ in. The stringer is about 14 ft 6 in long and the stair angle is about 38°. Both IRC checks pass.

Metric stair (no preset). Total rise 2,800 mm, going 280 mm. The calculator chooses 16 risers of 175 mm and 15 treads, for a total run of 4.2 m. 2R + G is 2 × 175 + 280 = 630 mm, right in the comfortable range.

Australian NCC. A 3.8 m rise with 250 mm goings needs 20 risers of 190 mm. That breaks the 18-risers-per-flight rule, so the calculator flags it: you would add a landing and split the stair into two flights.

How to read the checks

A green tick means the value is within the limit of the selected preset; a warning sign means it is outside. The checks cover only the geometry listed. Handrail height, guard openings, nosing projection and winder rules are not checked.

Tips and common mistakes

  • Measure finished floors. Adding a floor covering later changes the top and bottom risers, which then differ from the rest and become a trip hazard.
  • Keep risers uniform. The IRC allows a variation of only ⅜ in between the largest and smallest riser in a flight.
  • Check headroom along the whole flight, not just at the bottom; a beam over the middle of the stair is a common problem.
  • Plan the landing. A landing at the top and bottom needs floor space at least as wide as the stair.
  • Remember the stringer cut. The useful length of a stringer board must be longer than the calculated slope length.

Assumptions and limitations

The calculator assumes a straight flight with equal risers and treads. It does not design winders, spiral stairs or structural stringers. Stairs are a structural element: have the design checked by a qualified professional and get the permits your local authority requires. Country presets for Germany (DIN 18065), Spain (CTE DB-SUA), Russia (SP 1.13130) and Mexico are not included because we have not yet verified their limits against the official texts.

Frequently asked questions

How many steps do I need for a 9 ft rise?

A 9 ft (108 in) rise divided by the IRC maximum riser of 7.75 in gives 13.9, so you need 14 risers of about 7.71 in each. With a landing at the top floor, that is 13 treads.

What is the difference between risers and treads?

Risers are the vertical steps up; treads are the horizontal surfaces you stand on. A straight flight that ends at a floor always has one fewer tread than risers, because the upper floor acts as the last tread.

What does 2R + G mean?

It is the comfort rule used by many codes: twice the riser height plus the going (tread depth). The Australian NCC requires 550–700 mm. Values around 600–640 mm match a normal walking stride.

Which codes does the calculator check?

The US IRC preset checks the 7¾ in maximum riser, 10 in minimum tread and 6 ft 8 in headroom. The Australian NCC preset checks risers of 115–190 mm, goings of 240–355 mm, 2R + G of 550–700 mm and 2 to 18 risers per flight. Other countries are not preset yet, so check your local regulations.

Is a passed check the same as code approval?

No. The checks show whether the geometry is within typical limits for the selected code. Handrails, guards, landings, nosings, winders and local amendments also apply, and your building authority has the final word.

How is the stringer length calculated?

The stringer runs along the slope, so its length is the square root of the total rise squared plus the total run squared. Add extra length for the cut at the floor and any connection detail at the top.

Sources