How the stair calculator works
Risers = the total rise divided by the riser height you aim for, rounded to the nearest whole number (never fewer than one). Riser height = the total rise / the number of risers, so every riser is the same. Treads = risers - 1, because the top floor or landing is the last tread. Total run = treads x tread depth. Stringer length = the square root of (rise x rise + run x run), and the angle = atan(rise / run).
Worked example
A total rise of 108 in with a 7.5 in target riser: 108 / 7.5 = 14.4, so use 14 risers. Each riser is 108 / 14 = 7.71 in (about 7 11/16 in). With 13 treads of 10 in, the total run is 130 in. The stringer is the square root of (108 x 108 + 130 x 130) = 169.01 in, and the angle is atan(108 / 130) = 39.72 degrees.
How to calculate stair rise and run
Measure the rise, in inches, from the finished lower floor to the finished upper floor. Divide it by the riser height you aim for, and round to a whole number of risers. Then divide the rise by that number to get the exact riser height. The run of one step is the tread depth, and the total run is the tread depth times the treads. The calculator also shows 2 x riser + tread, for information only.
Stair stringers
The stringer length is the long edge of the sloped board before cutting: 169.01 in for the example flight. The angle is where the stringer meets the floor. We found no source we can cite for the number of stringers, so follow your plans or ask your building authority.
Stair code limits
A City of Saint Paul handout (revised 5 Feb 2015, with IRC section numbers) and a City of Grinnell document (headed 2015 IRC) give these limits for residential stairs. [1] [2] We found no government page that names the 2021 or 2024 edition, and local codes may differ, so check with your building authority. OSHA sets its own range for stairs on construction sites. [3]
| Item | Limit | From |
|---|---|---|
| Tallest riser | 7 3/4 in | Saint Paul and Grinnell |
| Shallowest tread | 10 in | Saint Paul and Grinnell |
| Largest riser minus smallest, in one flight | 3/8 in | Saint Paul and Grinnell |
| Largest tread minus smallest, in one flight | 3/8 in | Grinnell |
| Narrowest stairway, clear width | 36 in | Saint Paul |
| Lowest headroom | 6 ft 8 in | Saint Paul |
| Smallest landing, in the direction of travel | 36 in | Saint Paul |
| Stair angle on a construction site | 30 to 50 degrees | OSHA 1926.1052 |
| Riser or tread variation on a construction site | 1/4 in | OSHA 1926.1052 |
The calculator warns when your stairs break the riser or tread limit above. A warning never blocks the result, and passing the check does not mean your stairs meet your local code. A riser of exactly 7 3/4 in is at the limit and does not warn.
Stair types this calculator does not cover
This tool is for a straight flight with equal risers and equal treads. It does not size spiral, curved, winder or alternating-tread stairs, so ask your building authority about those. For a concrete flight it gives the same rise, run and angle, but it does not calculate the concrete.
How much carpet do stairs need?
This is our own sum (rise plus run), not a figure from a source: carpet covers each tread and each riser, so a runner is about the total rise plus the total run long. For the example flight that is 108 + 130 = 238 in, or 19.83 ft, before extra for the nosing and waste. Take the carpet width from the product label.
How much do stairs cost?
We state no price. Ask local suppliers for the price of the lumber or the treads, and use the risers, treads and stringer length above to list what you need.
Common mistakes and limits
- Measuring to the unfinished floor. Use the finished floors, including carpet or tile.
- Forgetting that the top tread is the floor. A flight has one tread fewer than risers.
- Rounding the riser height by hand. Unequal risers can break the 3/8 inch limit. [1] Let the calculator divide the rise equally.
- Treating the checks as approval. They compare with the 2015 documents we read. Your local code applies.
- What this tool does not cover: headroom, handrails, guards, landings, winders and framing.
Common rises
Example rises, with the flight the calculator gives for each.
| Total rise (in) | Risers | Riser height (in) | Total run (in) | Angle (degrees) |
|---|---|---|---|---|
| 30 | 4 | 7.50 | 30 | 45.00 |
| 48 | 6 | 8.00 | 50 | 43.83 |
| 60 | 8 | 7.50 | 70 | 40.60 |
| 84 | 11 | 7.64 | 100 | 40.03 |
| 96 | 13 | 7.38 | 120 | 38.66 |
| 108 | 14 | 7.71 | 130 | 39.72 |
| 120 | 16 | 7.50 | 150 | 38.66 |
Frequently asked questions
How many stairs do I need?
Divide the total rise by the riser height you aim for and round to the nearest whole number. For a rise of 108 inches and a 7.5 inch target, 108 / 7.5 = 14.4, so use 14 risers, each 7.714 inches (about 7 11/16 inches). That gives 13 treads, because the top floor is the last tread.
How many stringers do I need?
We found no source we can cite for the number of stringers, so follow your plans or ask your local building authority.
What size stairs do I need?
A City of Saint Paul handout and a City of Grinnell document (headed 2015 IRC) give a maximum riser height of 7 3/4 inches, a minimum tread depth of 10 inches, and no more than 3/8 inch difference between the largest and smallest riser in a flight. Your local code may differ, so check with your building authority. [1][2]
The Grinnell document also limits the difference between the largest and smallest tread depth in a flight to 3/8 inch. [2]
Where is stair width measured?
The Saint Paul document says stairways shall be at least 36 inches wide in the clear at all points above the permitted handrail height and below the required headroom height. Handrails may project no more than 4 1/2 inches on either side. Your local code applies. [1]
Sources and references
- City of Saint Paul, Minnesota, Department of Safety and Inspections. Stairways and Handrails for Residential Structures (City handout revised 5 Feb 2015, with IRC section numbers, accessed November 2026). Used for: Maximum riser, minimum tread, the 3/8 inch riser height variation, headroom, width and landings.
- City of Grinnell, Iowa, Building and Planning Department. Residential Stairs, Ramps and Handrails (March 2017, headed 2015 IRC, accessed November 2026). Used for: The same riser and tread limits, and the 3/8 inch variation for riser height and tread depth, from a second city.
- Occupational Safety and Health Administration (OSHA). 29 CFR 1926.1052, Stairways (accessed November 2026). Used for: The 30 to 50 degree angle and the 1/4 inch variation for stairs on construction sites.
How we use sources: methodology.
Estimates only. Confirm quantities with your supplier. See the disclaimer. Found a wrong result? Report an error.