How to use the stair calculator
Type the total rise, measured from the finished floor or deck down to the surface where the bottom step lands. Type the tread depth you want from the front edge of one tread to the next, or leave it blank for the minimum of the rules you picked. You get the risers, riser height, treads, run, slope, stringer length and stringer count. The other tabs lay the stringer out, mark it with only a tape, compare every number with its rule and chart common rises. Nothing leaves your device unless you make a share link.
The formula
riser height = total rise ÷ risers
treads = risers − 1 (or = risers when a tread is level with the floor above)
run = treads × tread depth
stringer length = √(rise² + run²)
Rounding up keeps every riser at or under the limit, and dividing the rise equally keeps them the same height. The code lets the tallest and shortest riser in a flight differ by no more than 3/8 inch. With the floor or deck above as the top step there is one fewer tread than riser.
A worked example
A deck is 48 inches above the ground. The model code's 7 3/4 inch limit gives 48 ÷ 7.75 = 6.19, rounded up to 7 risers of 6 7/8 inches. The deck is the top step, so there are 6 treads; at 11 1/4 inches each the run is 67.5 inches. The stringer is √(48² + 67.5²) = 82.8 inches, so an 8 foot board does it. A 2×12 is 11 1/4 inches deep and the notches take 5.86, which leaves 5.39 inches. A 48 inch wide stair at 16 inches on center takes 4 stringers. The page opens on this example.
What the model code says
These numbers are from IRC Section R311.7 (Stairways) and R312 (Guards), as read in the 2018 and 2021 editions and in building department handouts. Your jurisdiction decides which edition applies, and many amend the numbers.
| Rule | Model code | Section |
|---|---|---|
| Riser height | 7 3/4 in at most | R311.7.5.1 |
| Tread depth | 10 in at least | R311.7.5.2 |
| Variation in a flight | 3/8 in | R311.7.5.1, .2 |
| Nosing, on treads under 11 in | 3/4 to 1 1/4 in | R311.7.5.3 |
| Clear width | 36 in at least | R311.7.1 |
| Headroom | 6 ft 8 in | R311.7.2 |
| Rise of one flight | 151 in (12 ft 7 in) | R311.7.3 |
| Landing | as wide as the stairs, 36 in deep | R311.7.6 |
| Handrail, 4 or more risers | 34 to 38 in high | R311.7.8 |
Some places allow a taller riser or a shallower tread. New York State's Department of State stair page, for the 2015 IRC with the 2017 Uniform Code Supplement, lists 8 1/4 inches at most and 9 inches at least, so you can pick the rules to compare with or type your own. Your code office says which edition applies.
Cutting the stringer
A notched stringer is a right triangle repeated along a board. With a framing square set to the riser and the tread, each step is √(riser² + tread²) along the board's edge, about 13 3/16 inches here. The tread boards add thickness, so the first riser is cut shorter by one tread board: 5 7/8 inches with a 1 inch tread. The stairs rise 7.31 inches for every 12 of run, 31.4 degrees. Layout and cut order are shop practice, not code.
Laying out a stringer with only a tape measure
You do not need a framing square or an angle gauge to find an inside corner. Each notch is a right triangle whose legs are the tread and the riser, with its long side along the board's straight edge. Mark that edge every step length (13 3/16 inches in the example), always reading from the first mark so a slip stays on one mark. A is where a tread starts, B is the next mark up, and C is the inside corner: the one point a tread length from A and a riser length from B. Hook the tape on a nail at A and swing the tread length, move the nail to B and swing the riser length, and the arcs cross at C. Join A to C and C to B; with the board at the pitch, A to C is level and C to B is plumb. A framing square is faster if you own one. With every reading off by 1/16 inch the wrong way, a corner can miss by about 0.15 inch and risers can differ by about 0.19 inch, inside the code's 3/8 inch. The Mark it with a tape tab does the arithmetic and makes a PDF cut sheet.
How many stringers, and how much board is left
The American Wood Council's deck guide, DCA 6, calls for stringers of at least 2×12, at least three when only cut stringers are used, and no more than 18 inches on center when cut and solid stringers are mixed on a stair wider than 36 inches. The page counts the width divided by the spacing, plus one, and never fewer than three. DCA 6 also limits what is left under the notches and how far a stringer can span, but those figures did not come through as readable text, so the limit under the notches is a number you can change.
Landings, handrails and guards
The bottom needs a firm, level landing and the top one as wide as the stairs and 36 inches deep, with door exceptions. The model code allows 151 inches of rise between landings, and DCA 6 asks for a landing above 12 feet on a deck stair. A handrail 34 to 38 inches above the nosings is required on four or more risers, and a guard on an open side more than 30 inches up. For a whole deck with stairs, DeckIQ designs it and prices the materials.
Common mistakes
- Measuring the rise to the lawn. Measure to the pad or lower deck where the bottom step sits.
- Forgetting the tread board. Cutting every riser equal leaves the first step taller than the rest.
- Rounding each riser by eye. Seven risers of 6 7/8 add 1/8 inch; mark from the table instead.
- Skipping the rail. Four or more risers needs a handrail.
Questions
How many steps do I need for a 30 inch deck?
30 ÷ 7.75 = 3.87, so 4 risers of 7 1/2 inches, and 3 treads when the deck is the top step.
What is the maximum riser height?
7 3/4 inches in the model code (IRC R311.7.5.1). Some places allow more, so pick the rules on this page and confirm with your code office.
What does 2R + T mean?
Twice the riser plus the tread, about 25 inches (63 to 65 cm): a comfort rule of thumb, not code.
Can I use metric?
Yes. Press Metric to type centimeters and meters. The code limits stay in inches because the codes are written in inches, and the page shows them in your units too.
Can I share my stairs?
Yes. Share makes a link with a picture for Facebook, a link to copy, and a TikTok video made on your device. The link keeps only the rise, tread, width, top step and limits, for 90 days.