How to use the stair stringer calculator
- Measure the total rise from the finished lower floor to the finished upper floor, and enter it with the unit you measured in.
- Enter a target riser height. The tool picks the whole number of risers that lands closest to it, then divides the rise evenly so every riser is the same.
- Choose how the tread depth is set. Type a depth you want, or pick the run option and enter the horizontal space you have so the tool divides it across the treads.
- Read the layout, the scaled side view and the cut sheet. Use Copy result, Print cut sheet or Download PDF to take the numbers to your saw. Open More options to change tread thickness, board depth, stringer count and the check limits.
Formula
Risers = total rise / target riser height, rounded to the nearest whole number
Riser height = total rise / risers
Treads = risers - 1 (the upper floor is the top step)
Total run = treads x tread depth, or tread depth = available run / treads
Angle = arctan(riser height / tread depth)
Stringer length = square root of (total rise^2 + total run^2)
Throat = board depth - (riser x tread / square root of (riser^2 + tread^2))
Bottom riser cut = riser height - tread thickness
The throat is the wood left under the sharpest corner of a notch, measured square to the board edge. The stringer length is the usual layout figure from the overall rise and run. The level and plumb end cuts add a little, so buy a board at least that long.
Worked example: a 105 inch rise with 10 inch treads
A target riser of 7.5 inches gives 105 / 7.5 = 14 risers, each exactly 7.5 inches. That means 13 treads.
With 10 inch treads the total run is 13 x 10 = 130 inches, which is 10 ft 10 in. The angle is arctan(7.5 / 10) = 36.9 degrees.
The stringer length is the square root of (105^2 + 130^2) = square root of 27,925 = 167.11 inches, so a 14 ft board covers it.
On a 2x12 with an actual depth of 11.25 inches the throat is 11.25 - (7.5 x 10 / 12.5) = 5.25 inches. If you nail 1.5 inch treads on top of the stringer, the bottom riser is cut 7.5 - 1.5 = 6 inches so the first step matches the others once the tread is on.
How the layout is chosen
Equal risers matter more than almost any other stair dimension, because people adjust their stride to the first few steps and trip when one is different. The tool never rounds a riser to a convenient number. It picks a whole number of risers and divides the rise by it, so the height you read is the height you mark.
Rounding to the nearest sixteenth of an inch is only for display and for setting a framing square. If you mark with a story pole or a digital angle gauge, use the decimal value in parentheses.
Code checks are your responsibility
Stair dimensions are regulated by the building code your local authority has adopted, and amendments differ from place to place. Model residential codes have published limits such as a maximum riser height near 7 3/4 inches and a minimum tread depth near 10 inches. This tool starts its two flag limits at those figures so you can see when a layout drifts outside them, and you can change them to match your jurisdiction.
A layout with no flags is not an approval. Landings, handrails, guards, headroom, stair width, nosing profile and tread slope are not checked here. Ask your building department which edition applies and what the inspector will measure.
Cutting from the cut sheet
The step table lists the height of every riser top above the lower floor and the distance from the first riser face. Those two columns let you check a stringer against the real opening before you cut, and they give a second way to verify a layout if you use a story pole instead of a framing square.
Cut one stringer, test it in place, then trace it onto the remaining boards. Choose straight, dry lumber and confirm the span and spacing for your stringers with a span table or an engineer, because strength is not calculated here.
Assumptions and limits
- The top step is the upper floor, so the number of treads is one fewer than the number of risers. Stairs that end in a landing tread need a different layout.
- Every riser is equal and every tread is equal. Winders, curves and open risers with different rules are not modeled.
- Stringer length is calculated from total rise and total run and is a layout figure. The real board length also depends on how you cut the top and bottom ends.
- The 3.5 inch throat check is a rule of thumb for solid wood stringers, and the riser and tread flag limits are editable starting points. None of them is a code approval.
- Lumber strength, spans, stringer spacing, fasteners and connections are not calculated. Use a span table or ask an engineer.
- Measurements are only as good as the rise you enter. Measure the finished floors on site and re-measure after flooring is installed.
Frequently asked questions
How do I calculate the number of steps for a stair?
Divide the total rise by the riser height you want and round to a whole number. That is the number of risers. A stair with 14 risers has 13 treads when the upper floor serves as the top step.
What is total rise on a staircase?
Total rise is the vertical distance from the finished lower floor to the finished upper floor. Measure it after the flooring is installed or add the thickness of any flooring still to come.
Why is the bottom riser cut shorter than the others?
When a tread is fastened on top of the stringer, it adds its own thickness to the first step. Cutting the first riser on the stringer shorter by that thickness keeps every finished riser the same height.
What is the throat of a stringer?
It is the solid wood left between the inside corner of a notch and the bottom edge of the board. A thin throat is the weakest point, so the tool reports it and warns when it falls below a common rule of thumb.
Does this tool tell me if my stairs meet code?
No. It flags riser height and tread depth against limits you can edit, but it does not approve a design. Check the code your local building department has adopted and ask about landings, handrails and headroom.
Can I use metric measurements for a stair layout?
Yes. Pick millimeters, centimeters or meters and the results switch to millimeters. Changing the unit converts the numbers already in the form so the same stair stays described.