Stair Stringer Calculator

What it calculates and who it’s for

What it estimates and who it is for

This calculator lays out a straight run of stairs from one measurement: the total rise. It finds the fewest risers that stay under your maximum riser height, the exact riser height, the treads, the total run, the stringer length and angle, and it checks the result against the riser and tread limits in the International Residential Code (IRC). It’s for deck, porch, and basement stair builders who want the numbers before they mark a stringer with a framing square.

How to use

Step by step

Measure the total rise from the finished surface at the bottom (patio, landing pad, or floor) to the finished surface at the top (deck boards or floor), in inches.
Enter the tread depth you want, the horizontal run of each step. The IRC minimum is 10 in.
Keep the 7 3/4 in maximum riser unless your local code amends it.
Open Advanced to split the stair with a landing or change the nosing.
Read the results: risers, riser height, treads, stringer length, and the IRC check.

The formula

Every variable and unit

Risers = Total rise (in) ÷ Maximum riser (in), rounded up to a whole number
Riser height (in) = Total rise ÷ Risers
Treads = Risers − 1 (the top “step” is the deck or floor itself)
Total run (in) = Treads × Tread depth
Stringer length (in) = √(Total rise² + Total run²), the diagonal along the step corners
Stair angle = arctan(Riser height ÷ Tread depth)
Tread board depth (in) = Tread depth + Nosing

With a landing, the risers are split as evenly as possible into two flights (for example 13 risers become 7 and 6), the landing takes the place of one tread, and the stringer length and drawing are for the longer flight.

Uniformity check: most people mark riser heights to the nearest 1/16 in. The calculator rounds your riser height to 1/16 in, multiplies by the number of risers in the flight, and compares it with the true rise. Any difference ends up in one step, so it has to stay within the IRC’s 3/8 in limit on the difference between the largest and smallest riser.

Worked example

Real numbers, worked by hand

A deck 60 in above the patio, 10 in treads, 7 3/4 in maximum riser, no landing:

Risers: 60 ÷ 7.75 = 7.74, rounded up to 8 risers
Riser height: 60 ÷ 8 = 7 1/2 in
Treads: 8 − 1 = 7 treads
Total run: 7 × 10 = 70 in
Stringer length: √(60² + 70²) = √8,500 = 92.2 in (about 7 ft 8 in)
Angle: arctan(7.5 ÷ 10) = 36.9 degrees
Check: 7 1/2 in is under 7 3/4 in, 10 in meets the 10 in minimum, and 7 1/2 in marks exactly on a tape, so the rounding left over is 0.

How to read your results

Rounding and buying tips

Risers is the number of vertical steps up. Treads is one fewer, because you step onto the deck at the top.
Riser height is shown to the nearest 1/16 in for marking. The rounding left over is listed under the IRC check.
Stringer length is the diagonal of the total rise and run. The board must be longer than this to leave room for the top and bottom cuts, so the shortest even-length board is listed, and many builders buy the next size up.
IRC check shows Yes or No for each limit with both the 2021 and 2024 section numbers. A No on the riser line means your riser is over 7 3/4 in, which only works where a local amendment allows it.

Assumptions and limits

What this estimate does not cover

The stair is a straight flight (or two straight flights with one landing) between level surfaces, with every riser the same height and every tread the same depth.
The check covers only riser height, tread depth, and riser uniformity. It does not check headroom, stair width, handrails, guards, open risers, winders, stringer size or spacing, or the depth of wood left after notching. Those have their own rules; confirm them with your local building department.
Local codes can amend the IRC limits. Use the maximum riser field for your local figure, and follow your local code where it differs.
This is a layout estimate, not a permit, an inspection, or an engineered design.

FAQ

5 questions

What is the stair rise and run code?

In the 2021 IRC, R311.7.5.1 limits risers to 7 3/4 in and R311.7.5.2 requires treads of at least 10 in, with no more than 3/8 in between the largest and smallest riser or tread in a flight. The 2024 IRC keeps the same limits under new numbers, R318.7.5.1 and R318.7.5.2. Your town may have amended them.

How do I calculate stair stringer length?

Find the total rise and the total run (treads × tread depth), then take the square root of rise squared plus run squared. For a 60 in rise with seven 10 in treads, that’s √(60² + 70²) = 92.2 in. Buy a board long enough for the top and bottom cuts.

How long is a 6 step stair stringer?

With 7 1/2 in risers and 10 in treads, six risers give a 45 in rise and a 50 in run, so the stringer is about 67.3 in along the step corners. The table below lists 2 to 12 risers.

How do I lay out stairs with a landing?

Turn on the landing under Advanced. The risers are split between two flights as evenly as possible, and the landing replaces one tread. Each flight still has to meet the riser, tread, and uniformity limits on its own.

Why does my riser come out at an odd fraction?

The total rise rarely divides into a tidy number. The calculator gives the exact height to 1/16 in and shows how much rounding is left over, so you can split it across steps instead of leaving it all on the top or bottom step.

Stringer lengths for 2 to 12 risers

Reference

All rows use 7 1/2 in risers and 10 in treads (a 36.9 degree stair). Stringer length is the diagonal of the total rise and run; the board column is the shortest even-length board that covers it.

Risers	Total rise (in)	Treads	Total run (in)	Stringer length (in)	Shortest board (ft)
2	15	1	10	18.03	2
3	22.5	2	20	30.10	4
4	30	3	30	42.43	4
5	37.5	4	40	54.83	6
6	45	5	50	67.27	6
7	52.5	6	60	79.73	8
8	60	7	70	92.20	8
9	67.5	8	80	104.67	10
10	75	9	90	117.15	10
11	82.5	10	100	129.64	12
12	90	11	110	142.13	12

Your results

Layout

Landing

IRC check