Stair Stringer Calculator
To lay out a stair stringer, divide the total rise by 7-3/4 inches and round up to get the number of risers, divide the rise by that number for the exact step height, multiply one fewer tread by the tread depth for the total run, and the stringer length is the square root of rise² + run². This calculator does all of it against the IRC R311.7 limits. It works out the stringer itself, not just the staircase: the cut length, how many stringers, the framing-square settings for marking them, and — the part most calculators skip — whether the board you plan to use still has enough material left after notching.
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What you'll need
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Prices and savings as of Oct 25, 2026, 4:32 AM UTC. Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.com at the time of purchase will apply to the purchase of this product.
Shopping for someone? See our guide to best gifts for deck builders & diy deck builders.
What will the stairs cost? Try the deck stairs cost calculator.
Four or more risers need a handrail — size it with the stair handrail calculator (length, angle, stock to buy).
How to calculate a stair stringer
Five measurements and one square root. Here they are for the calculator's default deck, 42 inches high:
- Measure the total rise from the finished deck surface to the finished landing: 42".
- Count the risers: divide by your preferred step height and round — 42 ÷ 7 = 6 risers. If that leaves a step over the 7-3/4" maximum, add one more riser.
- Find the exact rise: 42 ÷ 6 = 7.000" per step. Every step gets this figure; never let the last one absorb the remainder.
- Count the treads and the run: one fewer tread than risers, because the deck is the top step — 5 treads × 10.5" = 52.5" of run.
- Stringer length = √(rise² + run²) = √(42² + 52.5²) = 5.60 ft (67.2"), so you buy 96" (8 ft) boards.
IRC stair code limits for deck stairs (R311.7)
Most US towns build deck stairs to the International Residential Code. These are the 2021 IRC limits the calculator checks; local amendments can be stricter, so confirm with your building department before you cut.
| Rule | 2021 IRC limit | Section |
|---|---|---|
| Riser height | 7-3/4 in maximum | R311.7.5.1 |
| Riser variation in one flight | 3/8 in maximum between tallest and shortest | R311.7.5.1 |
| Tread depth | 10 in minimum, nosing to nosing | R311.7.5.2 |
| Tread variation in one flight | 3/8 in maximum | R311.7.5.2 |
| Nosing | 3/4 in to 1-1/4 in; not required on treads 11 in or deeper | R311.7.5.3 |
| Clear width | 36 in minimum | R311.7.1 |
| Headroom | 6 ft 8 in minimum | R311.7.2 |
| Rise of one flight | 151 in maximum between landings | R311.7.3 |
| Handrail | Required with four or more risers, 34 in to 38 in above the nosings | R311.7.8 |
Source: International Code Council, 2021 International Residential Code, Section R311.7 Stairways, checked on Oct 2, 2026 against the IRC Chapter 3 text published by Routt County, Colorado.
Stair stringer chart by deck height
Worked out by this calculator at a 7" target rise and a 10.5" tread on 2x12 stringers. Find your total rise for a quick answer, then enter the exact figure above.
| Total rise | Risers | Rise per step | Treads | Total run | Stringer length | Buy | Handrail |
|---|---|---|---|---|---|---|---|
| 12" | 2 | 6.000" | 1 | 0.88 ft (10.5") | 1.33 ft (15.9") | 96" (8 ft) | No |
| 18" | 3 | 6.000" | 2 | 1.75 ft (21.0") | 2.30 ft (27.7") | 96" (8 ft) | No |
| 24" | 4 | 6.000" | 3 | 2.63 ft (31.5") | 3.30 ft (39.6") | 96" (8 ft) | Yes |
| 30" | 4 | 7.500" | 3 | 2.63 ft (31.5") | 3.63 ft (43.5") | 96" (8 ft) | Yes |
| 36" | 5 | 7.200" | 4 | 3.50 ft (42.0") | 4.61 ft (55.3") | 96" (8 ft) | Yes |
| 42" | 6 | 7.000" | 5 | 4.38 ft (52.5") | 5.60 ft (67.2") | 96" (8 ft) | Yes |
| 48" | 7 | 6.857" | 6 | 5.25 ft (63.0") | 6.60 ft (79.2") | 96" (8 ft) | Yes |
| 54" | 8 | 6.750" | 7 | 6.13 ft (73.5") | 7.60 ft (91.2") | 120" (10 ft) | Yes |
| 60" | 9 | 6.667" | 8 | 7.00 ft (84.0") | 8.60 ft (103.2") | 120" (10 ft) | Yes |
| 72" | 10 | 7.200" | 9 | 7.88 ft (94.5") | 9.90 ft (118.8") | 144" (12 ft) | Yes |
| 84" | 12 | 7.000" | 11 | 9.63 ft (115.5") | 11.90 ft (142.8") | 168" (14 ft) | Yes |
| 96" | 14 | 6.857" | 13 | 11.38 ft (136.5") | 13.91 ft (166.9") | 192" (16 ft) | Yes |
How to cut a stair stringer
- Set a framing square with stair gauges: the rise on the tongue, the tread on the blade — the two "Framing square" lines in the result.
- Mark every step from the top, sliding the square along the crown edge of a straight 2x12. Mark the top cut where the stringer meets the rim joist.
- Drop the bottom by the tread thickness (the "Bottom cut" line) so the first step is not taller than the rest once the treads are on.
- Cut with a circular saw and stop at the inside corner; finish the corner with a handsaw so you do not overcut and weaken the throat.
- Test-fit the first stringer, then trace it onto the others so every step matches.
- Check the throat: at least 5" of solid wood behind the notches. That is why a 2x10 usually fails and deck stringers are cut from 2x12.
How many stringers, and what goes with them
Three stringers minimum, then one for about every 16" of width with wood treads or 12" with composite — the result gives the count for your width. Four or more risers need a handrail; size it with the stair handrail calculator. To price the whole staircase, use the deck stairs cost calculator; for steps, total run and landing size at a glance, the deck stair calculator. Building the deck as well? Start from the deck cost calculator.
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Frequently asked questions
- What size board do I need for a stair stringer?
- Almost always a 2x12. After you notch a stringer, what matters is the material left behind the notches — the throat — and it has to be at least 5 inches. At a typical 7 inch rise and 10-1/2 inch tread, a 2x12 leaves about 5.2 inches and passes, while a 2x10 leaves about 3.2 inches and fails. That is why deck stairs are cut from 2x12 even though a 2x10 looks big enough.
- How many stringers do I need?
- Three minimum for any staircase, then one more for roughly every 16 inches of width with wood treads. Composite and PVC treads flex more, so their manufacturers usually require 12 inch spacing — a 48 inch wide stair needs 4 stringers in wood and 5 in composite. Check the specific tread's instructions, because the spacing requirement is a condition of the warranty.
- How do you calculate stringer length?
- It is the hypotenuse of the total rise and the total run. Square the total rise, square the total run, add them and take the square root. For a 42 inch rise over a 52-1/2 inch run that is about 67 inches, or 5 feet 7 — so you would buy 8 foot boards and have room for the cuts at each end.
- Why is the rise per step not a round number?
- Because the total height rarely divides evenly. The right approach is to pick the number of risers, then divide the total height by it and cut every step to that exact figure, even if it is 6.857 inches. Rounding to a round number and letting the last step absorb the difference is what creates the uneven bottom step people trip on — and codes limit the variation between steps to 3/8 inch.
- Do I measure to the ground or to the concrete pad?
- To the finished surface the bottom step will land on. If you are pouring a pad, measure to where the top of that pad will be. Measuring to bare soil and then pouring concrete afterwards raises the landing and shortens the bottom step, which is a common way to end up outside code after everything is already cut.
- Should the bottom step be shorter to allow for the tread thickness?
- Yes — this is the classic stringer mistake. When stringers sit on the landing and treads are added on top, the bottom riser ends up one tread thickness taller than the rest unless you drop the bottom of the stringer by that amount. Cut the bottom of the stringer down by the thickness of your tread material, usually 1 to 1-1/2 inches.
