Calculators · Stair Calculator
Lumber & FramingStair Calculator
Enter the total rise to get the number of steps, riser height, tread depth, total run, stringer length and stair angle.
Stair layout
—
Riser and tread table for your rise
The row marked "yours" is the layout above. The rows around it show one or two more or fewer risers for the same rise, so you can compare riser height, tread depth and stringer length.
| Risers | Riser (in) | Tread (in) | Run (in) | Stringer (in) | IRC limits |
|---|
How to calculate stairs
A staircase is a rise to climb and a run to climb it in. The total rise is the vertical distance between the finished floors. Each step has a riser, the vertical part, and a tread, the part you step on. All the risers must be equal, so the calculator first picks a whole number of risers and then divides the rise by it:
| What you get | Formula |
|---|---|
| Number of risers | total rise ÷ riser height you want, rounded to a whole number |
| Riser height | total rise ÷ number of risers |
| Number of treads | number of risers − 1 |
| Total run | number of treads × tread depth |
| Stringer length | √(total rise² + total run²) |
| Stair angle | arctangent of (riser height ÷ tread depth) |
There is one tread fewer than risers because the last step up lands on the upper floor. Use the tab "Available run" when the space decides the design. The calculator then divides your run by the number of treads to get the tread depth, and tells you if it falls under the code minimum.
How to measure the total rise
Measure from the finished surface of the lower floor to the finished surface of the upper floor. If the flooring is not installed yet, add its thickness to the measurement on both levels. On a deck, measure from the top of the deck surface to the finished surface where the stair will land, such as a paving slab or compacted base. An error in the total rise spreads over every riser, so measure twice.
Example: a stair with a 105 in rise
The total rise is 105 in (8 ft 9 in). You want risers of about 7 in and treads 10 in deep. These are the steps, and you get the same figures by typing those numbers into the calculator.
| Step | Operation | Result |
|---|---|---|
| Number of risers | 105 ÷ 7 | 15 |
| Riser height | 105 ÷ 15 | 7 in |
| Number of treads | 15 − 1 | 14 |
| Total run | 14 × 10 | 140 in (11 ft 8 in) |
| Stringer length | √(105² + 140²) = √30,625 | 175 in (14 ft 7 in) |
| Stair angle | arctangent of (7 ÷ 10) | 34.99° |
Stair stringer length: why the board must be longer
The stringer figure above is the straight line from the lower floor to the upper floor. It tells you how long the stair is along its slope, but it is not the length of lumber to buy. The board is cut at both ends, and the notches need solid wood left below them, so the board has to be longer than the figure. Buy the next full length your supplier sells above it, and ask the supplier or your local building department how much wood must remain below the notches. That depends on the lumber and the design, so this page gives no number for it.
If the treads sit on top of the stringer, the first tread adds its own thickness to the first step. To keep every riser equal, the stringer must be cut shorter at the bottom by that thickness. Type the thickness in the optional field and the result shows the bottom riser cut, which is the riser height minus the thickness.
Stair rise and run chart
This chart uses a 7 in target riser height, rounded to the nearest whole number of risers, and 10 in treads. Your own numbers may differ, so use the calculator for the exact layout.
| Total rise | Risers | Riser height | Run with 10 in treads | Stringer length |
|---|---|---|---|---|
| 36 in (3 ft) | 5 | 7.20 in | 40 in | 53.81 in (4 ft 5 13/16 in) |
| 48 in (4 ft) | 7 | 6.86 in | 60 in | 76.84 in (6 ft 4 13/16 in) |
| 60 in (5 ft) | 9 | 6.67 in | 80 in | 100.00 in (8 ft 4 in) |
| 72 in (6 ft) | 10 | 7.20 in | 90 in | 115.26 in (9 ft 7 1/4 in) |
| 84 in (7 ft) | 12 | 7.00 in | 110 in | 138.41 in (11 ft 6 3/8 in) |
| 96 in (8 ft) | 14 | 6.86 in | 130 in | 161.60 in (13 ft 5 5/8 in) |
| 108 in (9 ft) | 15 | 7.20 in | 140 in | 176.82 in (14 ft 8 13/16 in) |
| 120 in (10 ft) | 17 | 7.06 in | 160 in | 200.00 in (16 ft 8 in) |
Residential stair limits in the IRC
The calculator compares your layout with the stair rules of the International Residential Code (IRC), which we read in the 2024 edition and the 2021 edition in October 2026. The values below are the same in both, only the section numbers changed. Riser height is measured vertically between the leading edges of adjacent treads, and tread depth is measured horizontally between the foremost edges of adjacent treads. Both are measured without carpets, rugs or runners.
| Item | IRC limit | 2021 | 2024 |
|---|---|---|---|
| Riser height | Not more than 7 3/4 in | R311.7.5.1 | R318.7.5.1 |
| Tread depth | Not less than 10 in | R311.7.5.2 | R318.7.5.2 |
| Variation inside a flight | Tallest and shortest riser within 3/8 in. Same for treads | R311.7.5.1, R311.7.5.2 | R318.7.5.1, R318.7.5.2 |
| Nosing | 3/4 in to 1 1/4 in. Not required if the tread is 11 in or more | R311.7.5.3 | R318.7.5.3 |
| Rise of one flight | Not more than 12 ft 7 in between floors or landings | R311.7.3 | R318.7.3 |
| Headroom | Not less than 6 ft 8 in | R311.7.2 | R318.7.2 |
| Clear width | 36 in above the handrail height. 31 1/2 in at and below it with one handrail, 27 in with two | R311.7.1 | R318.7.1 |
| Landing | At the top and bottom, as wide as the flight and 36 in deep on a straight run. Exceptions apply | R311.7.6 | R318.7.6 |
| Handrail | At least one side of a flight with four or more risers | R311.7.8 | R318.7.8 |
The calculator makes all the risers equal, so the variation limit only matters when you cut. A small error in the cut adds up quickly against 3/8 in. The IRC leaves out stairways that are not within or serving a building, porch or deck, so stairs to a deck are covered. Spiral stairs and winders have their own rules, which this calculator does not handle. Your state or city adopts a particular edition and may amend it, and commercial buildings follow other codes. Check with your local building department before you cut.
Stairs with a landing, and deck stairs
A stair with a landing is two flights. Run the calculator once for the rise from the lower floor to the landing and once for the rise from the landing to the upper floor. Each flight gets its own number of risers. The landing is a floor, so it is the top of one flight and the bottom of the next. For a straight run the IRC asks for a landing 36 in deep in the direction of travel.
For deck stairs, the work is the same. Measure the rise from the finished deck surface to the finished surface at the bottom and enter the thickness of the treads if they sit on the stringer. The calculator does not check the footing, the way the stringers attach to the deck or the lumber size. Those are structural questions for the deck manufacturer's guide and your local code. For the deck itself, see the deck calculator.
Frequently asked questions
How do you calculate stairs?
Divide the total rise by the riser height you want and round to a whole number. That is the number of risers. Divide the total rise by that number to get the exact riser height. The number of treads is the number of risers minus one, because the upper floor works as the last tread. Multiply the treads by the tread depth to get the total run. For a 105 in rise with a 7 in target you get 15 risers of 7 in, 14 treads, and with 10 in treads a run of 140 in.
How do you calculate stair stringers?
Start with the total rise, the number of risers, the riser height and the tread depth. Those give you the size of every notch, the number of notches and the straight-line stringer length, which is the square root of rise squared plus run squared. The calculator does all of that. The marking and cutting of the board is a separate job, and a manufacturer guide such as Trex's guide to deck stair stringers walks through it. Use the optional tread thickness field to get the shorter cut for the bottom riser.
How long of a stair stringer do I need for 5 steps?
It depends on the rise and the tread depth. Take a 35 in rise with 5 risers of 7 in and 10 in treads. There are 4 treads, so the run is 40 in and the stringer is √(35² + 40²) = 53.15 in, which is 4 ft 5 1/8 in. The board you buy has to be longer than that figure, because both ends are cut and the notches need wood left below them.
How long of a 2x12 do I need for a stair stringer?
Calculate the stringer length first, then buy a board longer than that number. For the 105 in rise in the example, the stringer is 14 ft 7 in, so the board must be longer than that. We do not list lumber lengths because they vary by supplier. Ask for the next full length above your figure. Trex's deck guide describes stringers cut from 2x12 pressure-treated lumber, but the lumber size, the notch depth and the number of stringers are structural choices to confirm with your local code.
What is the maximum riser height and minimum tread depth?
In the International Residential Code the riser height is limited to 7 3/4 in and the tread depth must be at least 10 in. Within a flight, the tallest riser and the shortest riser cannot differ by more than 3/8 in, and the same goes for the treads. We read these values in the 2021 and 2024 editions in October 2026. Your state or city may use another edition or add its own rules.
Can I calculate stairs with a landing?
Yes, calculate each flight on its own. Use the rise from the lower floor to the landing for the first flight, and from the landing to the upper floor for the second. The calculator handles one straight flight at a time. It does not size the landing itself. For a straight run the IRC asks for a landing at least as wide as the flight and 36 in deep in the direction of travel, with exceptions for some doors and exterior stairs.
Does this calculator work for deck stairs?
Yes, for the geometry. Measure the total rise from the finished deck surface down to the finished surface at the bottom, such as a landing pad or compacted ground. If the treads sit on top of the stringer, type their thickness and the calculator shows how much shorter the bottom riser must be cut. The IRC limits shown also apply to stairs serving a deck, but the footing, the stringer attachment and the lumber are not checked here.
Source for the lumber reference in the FAQ: Trex, How to Calculate & Cut Stair Stringers, read in October 2026.
More calculators
- Deck Calculator Count the deck boards, joists and screws for your deck size, with real board widths, gap and joist spacing.
- Roof Pitch Calculator Convert rise and run to pitch, angle and slope factor, and find rafter length.
- Square Footage Calculator Add rooms, circles or triangles and get the total in sq ft, sq yd, m² and acres.