IRC R311.7 · R312.1
Deck stair calculator
Your stairs
Measure to where the landing pad will FINISH, not to the dirt you are standing on. Getting this wrong is what makes the bottom step the odd one out.
36" is the code minimum clear width.
10" minimum. 10½" suits two 5½" boards with a gap.
What you would like. The layout rounds to a whole number of risers.
Required at four or more risers.
Clears every IRC limit checked here
6 risers at 7"
IRC R311.7.5.1 max rise 7.75" · R311.7.5.2 min tread 10" · R311.7.1 min width 36"
Risers
6
ground to deck
Treads
5
deck is the top one
Stringers
5
5.9 ft of 2x12 each
Total run
4'-5"
clear this before you dig
| Check | Yours | Code | |
|---|---|---|---|
| Riser heightIRC R311.7.5.1 | 7" (7.00") | ≤ 7.75" | pass |
| Tread depthIRC R311.7.5.2 | 10.50" | ≥ 10" | pass |
| Riser uniformity, as designedIRC R311.7.5.1 | 0.000" — every riser identical | ≤ 0.375" | pass |
| Clear stair widthIRC R311.7.1 | 48" | ≥ 36" | pass |
| HandrailIRC R311.7.8 | on, 6 risers | required at 4+ risers | pass |
| Guard on the deckIRC R312.1.1 / R312.1.2 | required — 3'-6" above grade | ≥ 36" high | note |
| Intermediate landingIRC R311.7.6 | not needed at this rise | engine breaks at 12'-0" of rise | note |
Stringer layout
5 stringers cut from 2x12 stock, 5.9 ft each. 7" rise and 10.50" run, marked from one square setting so nothing drifts — cumulative heights from the bottom:
- 1. 7"
- 2. 14"
- 3. 21"
- 4. 28"
- 5. 35"
- 6. 42"
Then the part that fails inspections: trim the bottom of the stringer by the thickness of one tread. Skip it and the first step comes out a full tread thicker than the rest — usually about an inch and a quarter, which is more than three times the 3/8" tolerance, on a stair whose marks were perfect.
Riser heights are shown to the nearest 1/16" with the exact decimal beside them. Stringer stock includes the 5% the engine adds for the cut.
The rule almost no stair calculator checks
Every stair calculator will tell you a riser is under 7.75 inches. Very few tell you whether your risers match each other, and that is the requirement that fails finished stairs at inspection.
0.375" — greatest riser to smallest riser
IRC R311.7.5.1The tolerance is three eighths of an inch across the whole flight. Not per step, not "about right". A stair with a 7" rise everywhere and a 7½" bottom step is out of tolerance and gets rejected, even though every individual riser is legal. The reason is not paperwork: people climb by feel, and one odd step is what puts someone on their face.
There are two ways it goes wrong on real jobs, and both are avoidable. The first is measuring the rise to the dirt instead of to the finished landing surface, so the bottom riser ends up short or tall by whatever the pad turns out to be. Pour or set the pad first, or decide its finished height before you cut anything.
The second is forgetting to drop the stringer. Treads sit on top of the notches, so unless you trim the bottom of the stringer by one tread thickness, the first step is a full tread thicker than the rest — usually about an inch and a quarter, which is more than three times the tolerance. The layout on the results panel above assumes uniform risers; the drop is what makes that true once the treads are on.
Deck stair code, in one table
| Rule | Limit | Section |
|---|---|---|
| Maximum riser height | 7.75" | IRC R311.7.5.1 |
| Minimum tread depth | 10" | IRC R311.7.5.2 |
| Riser uniformity (greatest minus smallest) | 0.375" (3/8") | IRC R311.7.5.1 |
| Minimum clear stair width | 36" | IRC R311.7.1 |
| Handrail required at | 4 or more risers, 34"–38" high | IRC R311.7.8 |
| Guard required above | 30" above grade | IRC R312.1.1 |
| Minimum guard height | 36" | IRC R312.1.2 |
| Intermediate landing — this engine breaks a flight at | 12'-0" of rise | IRC R311.7.6 |
Residential values, and your Authority Having Jurisdiction still governs — some jurisdictions amend guard height, and commercial work follows a different code entirely. One number above is the engine's, not the code's: IRC R311.7.6 caps the rise of a single flight and requires a landing beyond it, and this engine breaks a straight run at 12 feet of rise — inside the code limit, deliberately. Confirm the exact figure in your adopted edition. Inside a saved design, turned runs split every 6 feet of rise so each landing stays a sane size.
How the layout is worked out
The riser count comes first. Divide the measured rise by your target — 7 inches is a comfortable step — and round to a whole number. Then divide the rise by that count again: that quotient is the riser height every step will actually be. If it lands above 7.75 inches, add a riser and divide once more. The calculator does exactly this, and it tells you when it added one.
Treads are one fewer than risers, because the deck surface is the top tread and the landing pad is the bottom one. That off-by-one is the most common arithmetic mistake in stair layout, and it is why a stair ordered from a riser count sometimes arrives with one tread too many.
Stringers are cut from 2x12 stock — 3 of them for a stair up to 36 inches wide, and past 36 inches the width divided by 16, rounded up, plus two. That is a step function, not a gentle slope: a 37-inch stair already takes 5, the same as a 48-inch one, and 6 carries you to 64 inches. A notched stringer has far less material left than the board it started as, which is why they sit closer together than the joists above them. The stock length shown includes 5% for the cut.
Common questions
What is the maximum riser height for deck stairs?
7.75 inches, measured vertically between tread surfaces (IRC R311.7.5.1). The minimum tread depth is 10 inches (R311.7.5.2). Both are ceilings and floors on individual steps — the uniformity rule below is a separate requirement that a stair can fail even when every single riser is inside those limits.
What is the 3/8 inch stair riser rule?
Within one flight, the greatest riser height may not exceed the smallest by more than 0.375 of an inch — 3/8" (IRC R311.7.5.1). The same tolerance applies to tread depth. It exists because people climb stairs by feel, not by looking, and a single step that is half an inch off is what people trip on. It is also the most common reason a finished deck stair gets rejected, usually because the bottom step was not adjusted for the tread thickness.
How do I calculate stair stringers for a deck?
Measure the total rise from the finished deck surface to the finished landing surface. Divide by roughly 7 inches to get a starting riser count, then round to a whole number and re-divide — that quotient is the actual riser height, and it must land at or below 7.75 inches. Multiply the tread count (one fewer than the riser count) by the tread depth for the total run. Mark all stringers off one layout so every riser is identical, then trim the bottom of the stringer by one tread thickness.
How many stringers does a deck stair need?
3 cut 2x12 stringers up to 36 inches wide. Past 36 inches the count is the width divided by 16, rounded up, plus two — so a 48-inch stair gets 5, a 60-inch stair 6, and a 72-inch stair 7. Note that crossing 36 inches jumps straight from 3 to 5; there is no four-stringer stair in that rule. Cut stringers lose a lot of their section at the notches, which is why they sit closer together than the joists on the deck itself.
Do deck stairs need a handrail and a guard?
They are two different things. A handrail is the graspable rail you hold, required once a flight has four or more risers, mounted 34 to 38 inches above the tread nosings (IRC R311.7.8). A guard is the barrier that stops a fall, required where the walking surface is more than 30 inches above grade (R312.1.1) and at least 36 inches high for a residential deck (R312.1.2). A tall stair usually needs both.
How much room do deck stairs need?
More than most people plan for, and the run grows faster than the height. At a 10.5 inch tread, a deck 3 feet above grade takes 4 treads and runs out 3'-6" from the deck; at 5 feet it is 8 treads and 7'-0"; at 8 feet, 13 treads and 11'-5" — all before the landing pad at the bottom. That is why the total run sits on the results panel: it is the number that decides whether the stair fits the yard, hits a fence, or has to turn.
The deck those stairs hang off needs sizing too — start with the joist span calculator and the beam and footing calculator.
Keep going
Joist span calculator
How far a 2x6, 2x8, 2x10 or 2x12 deck joist spans at 12", 16" or 24" on center — plus the overhang the same cell allows.
DCA6-2015 Table 2, p.4
Beam & footing calculator
Beam size, post spacing and footing diameter for one deck — including the post/footing combinations the table marks Not Permitted.
DCA6-2015 Table 3A + Table 4 · IRC R507
Ledger fastener spacing
On-centre spacing for ½" lags and through-bolts in a deck ledger, by the joist span it carries and the band joist behind it.
DCA6-2015 Table 5, p.15
Now do the whole deck
The calculator sizes one member. The free designer runs the same tables across a real footprint — every joist, beam, post and footing sized together, with the full material list underneath. No signup.
Source & limits
AWC DCA 6-2015, Prescriptive Residential Wood Deck Construction Guide (based on the 2015 IRC), © 2018 American Wood Council. The guide is published free by AWC.
Estimate only — not an engineered design. Spans, sizes, and quantities are screened against the AWC DCA 6-2015 / IRC prescriptive baseline at 40 psf live + 10 psf dead load. Local codes and amendments vary; your Authority Having Jurisdiction (AHJ) and a licensed design professional govern. Verify everything with a permit and a licensed pro before ordering or building.