How to use the stair calculator
- Pick the rules to check against: the US International Residential Code (IRC) or the UK's Approved Document K. Then pick feet and inches or millimeters.
- Enter the total rise: the height from the finished floor at the bottom to the finished floor at the top. For a deck, measure from the deck surface down to the ground or pad where the stair will land.
- Enter the tread depth (called the going in the UK), measured from the front edge of one tread to the front edge of the next.
- Optional: a riser height you would like, the nosing overhang, the headroom, and a landing that splits the stair into two flights.
The calculator shows the number of risers and treads, the exact riser height, the total run, the stringer length, the angle, and a check against the rules you picked. The stair calculator with diagram draws a side view to scale, so you can see the rise and run before you cut anything. Copy result gives you a plain text summary.
How the stair calculator works out rise and run
- Risers = total rise ÷ the largest riser allowed, rounded up to a whole number. If you type a riser height you want, the calculator uses the whole number of risers closest to it instead.
- Riser height = total rise ÷ risers, so every riser is exactly the same.
- Treads = risers minus 1. The upper floor is the last step, so it needs no tread of its own.
- Total run = treads × tread depth (plus the landing, if there is one).
- Stringer length = the square root of (rise² + run²): the straight diagonal from the foot of the bottom riser to the edge of the upper floor. Buy stringer boards longer than this, since they also run past the top and bottom.
- Angle = the arctangent of riser height ÷ tread depth.
Here is the worked example for a 9 ft floor to floor height, 10 in treads and a 1 in nosing, under the US rules:
| Step | Working | Result |
|---|---|---|
| Risers | 108 in ÷ 7.75 in = 13.94, round up | 14 |
| Riser height | 108 ÷ 14 | 7.71 in (7 11/16 in) |
| Treads | 14 minus 1 | 13 |
| Total run | 13 × 10 in | 130 in (10 ft 10 in) |
| Stringer length | √(108² + 130²) | 169 in (14 ft 1 in) |
| Angle | arctan(7.71 ÷ 10) | 37.6° |
| Tread board | 10 in + 1 in nosing | 11 in |
Fractions are rounded to the nearest 1/16 in for marking out. Each riser is shown to that 1/16, but the exact figure is what you divide by, so the total still comes to 108 in.
How many steps do I need?
For the US rules, with the tallest risers allowed (7 3/4 in) and 10 in treads, here is how many steps common heights need. "Steps" here means risers.
| Total rise | Risers | Riser height | Treads | Run |
|---|---|---|---|---|
| 18 in | 3 | 6 in | 2 | 1 ft 8 in |
| 24 in | 4 | 6 in | 3 | 2 ft 6 in |
| 30 in | 4 | 7 1/2 in | 3 | 2 ft 6 in |
| 36 in | 5 | 7 3/16 in | 4 | 3 ft 4 in |
| 4 ft | 7 | 6 7/8 in | 6 | 5 ft |
| 8 ft | 13 | 7 3/8 in | 12 | 10 ft |
| 8 ft 6 in | 14 | 7 5/16 in | 13 | 10 ft 10 in |
| 9 ft | 14 | 7 11/16 in | 13 | 10 ft 10 in |
| 9 ft 6 in | 15 | 7 5/8 in | 14 | 11 ft 8 in |
| 10 ft | 16 | 7 1/2 in | 15 | 12 ft 6 in |
Many people choose one more riser than the minimum for an easier stair: the results panel shows the riser height with one riser fewer and one more, and the run grows by one tread each time.
US rules: IRC section R311.7
The International Residential Code is the model code that many US states and towns adopt for houses, often with their own changes. These are the figures from the 2021 IRC, quoted from the code itself:
| Rule | What the IRC says |
|---|---|
| Riser height (R311.7.5.1) | "not more than 7 3/4 inches (196 mm)" |
| Tread depth (R311.7.5.2) | "not less than 10 inches (254 mm)" |
| Nosing (R311.7.5.3) | "not less than 3/4 inch (19 mm) and not more than 1 1/4 inches (32 mm)", not required "where the tread depth is not less than 11 inches (279 mm)" |
| Headroom (R311.7.2) | "not less than 6 feet 8 inches (2032 mm)" |
| Rise of one flight (R311.7.3) | not greater than "12 feet 7 inches (3835 mm) between floor levels or landings" |
| Landing (R311.7.6) | on a straight run, depth "not less than 36 inches (914 mm)" |
| Handrail (R311.7.8) | on at least one side "of each flight of stairs with four or more risers" |
The code also says the tallest riser and the deepest tread in a flight may not differ from the smallest by more than 3/8 inch (9.5 mm). Dividing the rise evenly, as this calculator does, keeps every step the same; a common slip is a bottom step that ends up a different height once flooring or a concrete pad goes in, so measure to the finished surfaces.
Deck stair calculator
Deck stairs follow the same R311.7 rules: the section's first exception covers only "Stairways not within or serving a building, porch or deck", so a stair that serves a deck is covered. A 30 in high deck with 11 in treads needs 4 risers of 7 1/2 in, 3 treads and a 33 in run; the stringer along the nosings is about 3 ft 8 5/8 in, and with 4 risers a handrail is needed. Because the tread is 11 in, no nosing is required.
UK rules and the stair calculator in metric
In England, Approved Document K (2013 edition) gives these limits for a private stair, meaning a stair "intended to be used for only one dwelling":
| Rule | What Approved Document K says |
|---|---|
| Rise (Table 1.1) | 150 mm minimum, 220 mm maximum |
| Going (Table 1.1) | 220 mm minimum, 300 mm maximum |
| Pitch | "The maximum pitch for a private stair is 42°." |
| 2R + G | "twice the rise plus the going (2R + G) equals between 550mm and 700mm" |
| Headroom (Diagram 1.3) | at least 2 m; loft conversions may have less where there is not enough space (Diagram 1.4) |
| Open risers in dwellings (1.9) | "Overlap treads by a minimum of 16mm", and a 100 mm sphere must not pass through |
| Long stairs (1.17) | more than 36 risers in consecutive flights need at least one change of direction |
Pitch and 2R + G work together: a 220 mm rise with a 220 mm going passes 2R + G (660 mm) but is 45°, too steep. With a 250 mm going, typical floor to floor heights work out like this:
| Floor to floor | Risers | Rise | Total going | 2R + G | Pitch |
|---|---|---|---|---|---|
| 2,400 mm | 11 | 218.2 mm | 2,500 mm | 686 mm | 41.1° |
| 2,500 mm | 12 | 208.3 mm | 2,750 mm | 667 mm | 39.8° |
| 2,600 mm | 12 | 216.7 mm | 2,750 mm | 683 mm | 40.9° |
| 2,700 mm | 13 | 207.7 mm | 3,000 mm | 665 mm | 39.7° |
| 2,800 mm | 13 | 215.4 mm | 3,000 mm | 681 mm | 40.7° |
Wales, Scotland and Northern Ireland have their own building regulations. Utility stairs and general access stairs have tighter limits in the same table, for example a 170 mm maximum rise for a general access stair.
Stair calculator with landing
Tick Split the stair into two flights to add a landing. The calculator keeps one riser height for the whole stair, puts half the risers (or the number you choose) below the landing, and works out each flight's treads, run and stringer. The landing replaces a tread, so two flights have one tread fewer in all than a single flight.
Example: a 13 ft 4 in (160 in) rise is too much for one flight under the IRC (the most is 12 ft 7 in). With a landing it becomes 21 risers of 7 5/8 in: 11 below the landing, which sits 83 13/16 in up, and 10 above it. With 10 in treads and a 36 in landing, the straight run is 19 × 10 + 36 = 226 in (18 ft 10 in).
For an L or U shaped stair, read each flight's run on its own: the landing's size then depends on the stair's width, which is the rule in both codes (the IRC asks for a landing at least as wide as the flight, Approved Document K for one at least as wide and long as the flight's width).
Questions and answers
How many steps do I need for a 9 ft ceiling?
Measure floor to floor, which is the ceiling height plus the floor structure above. For exactly 108 in under the US limit of 7 3/4 in, you need 14 risers of about 7 11/16 in and 13 treads.
What is the difference between rise and run?
The rise is the height a stair climbs, either for one step (riser height) or for the whole stair (total rise). The run is the horizontal depth, for one step (tread depth or going) or the whole stair (total run).
How long should a stair stringer be?
The straight diagonal from the foot of the bottom riser to the edge of the upper floor is the square root of (total rise² + total run²). For a 108 in rise and 130 in run that is about 169 in. Buy a longer board, because the stringer also extends past the top and bottom.
Do deck stairs follow the same rules?
Under the IRC, yes: section R311.7 covers stairways serving a deck, so the 7 3/4 in riser, 10 in tread and handrail rules apply. Your town may add its own rules for decks.
What is the best stair angle?
The rules set limits rather than a best angle: Approved Document K caps a private stair at 42°, and the IRC limits work out to about 37.8° at a 7 3/4 in riser and 10 in tread. Lower risers and deeper treads make an easier stair but need more floor space.
Who decides if my stair is allowed?
Your local building department in the US, or your building control body in the UK. Local codes vary and change these figures, so check before you build.
Sources
Checked on 8 October 2026. If a rule has changed, please tell us.