Stair Calculator

Cutting stair stringers is the one carpentry job where close enough is not good enough — a single riser that is off by half an inch becomes a trip hazard you feel every time you climb. The math itself is simple: divide the total rise by your ideal riser height, round to whole steps, then spread the rise evenly so every riser is identical. The number of treads is always one fewer than the number of risers, because the upper floor (or deck surface) acts as the final tread. This stair calculator runs that layout for you: riser count, exact riser height, tread count, total run (the floor space the stair occupies), stringer length from the Pythagorean theorem, and the stair angle. It works the same for indoor staircases and deck stairs — a deck stair is just a short staircase, so if you searched for a deck stair calculator, you are in the right place.

How to use this calculator

Measure the total rise — the vertical distance from the finished lower surface to the finished upper surface. For a deck stair, that is from the ground (or patio) up to the top of the deck boards, not the top of the joists. Measure twice; every number below hangs on this one.
Pick an ideal riser height. Seven inches (about 18 cm) climbs comfortably for most people. If you want the shallowest comfortable stair, you can go lower — the calculator will simply give you more steps. Do not plan above 7.75 in (19.7 cm): that is the widely used code maximum for residential stairs.
Pick a tread depth. Eleven inches (about 28 cm) fits a full footstep; 10 in is the common code minimum when the tread has a nosing (a slight overhang). The classic comfort check is 2 × riser + tread ≈ 24–25 in (61–64 cm).
Read the riser count and the actual riser height. The calculator rounds your ideal height to a whole number of risers, then divides the total rise evenly — so every riser is exactly the same, which is what keeps stairs safe. If rounding would push a riser over the 7.75 in code maximum, it adds one more riser automatically.
Check the total run against your available space. Total run is treads × tread depth — the horizontal footprint of the staircase. A stair that does not fit the room or yard is the most common layout mistake.
Use the stringer length to buy lumber. It is the diagonal hypotenuse of the rise and run — order boards at least this long, plus a little extra for the top and bottom cuts.
Lay out the stringer with a framing square, stepping off one riser and one tread at a time, and cut one stringer first as a template. Test-fit it before cutting the rest.

Formula

Start with risers: total rise ÷ ideal riser height, rounded to the nearest whole number (adding one more riser if that would push any riser over the 7.75 in / 19.7 cm code maximum). Actual riser height = total rise ÷ number of risers — every riser identical. Treads = risers − 1, because the upper floor or deck surface serves as the final tread. Total run = treads × tread depth. Stringer length = √(total rise² + total run²), the Pythagorean theorem on the whole flight. Stair angle = arctan(total rise ÷ total run). Comfort check: 2 × riser + tread should land near 24–25 in (61–64 cm).

Worked example

Take a deck stair: 36 in of total rise, 7 in ideal riser height, 11 in treads. Step one — risers: 36 ÷ 7 = 5.14, which rounds to 5 risers. Step two — actual riser height: 36 ÷ 5 = 7.2 in per riser, under the 7.75 in code maximum. Step three — treads: 5 − 1 = 4 treads. Step four — total run: 4 × 11 = 44 in (3 ft 8 in) of horizontal space. Step five — stringer: √(36² + 44²) = √3232 = 56.9 in (4 ft 8.9 in) — buy 6 ft boards to be safe. Step six — stair angle: arctan(36 ÷ 44) = 39.3°. Comfort check: 2 × 7.2 + 11 = 25.4 in — right in the comfortable zone.

Metric version: 2.5 m total rise, 18 cm ideal riser, 28 cm treads. Risers: 250 ÷ 18 = 13.9 → 14 risers. Actual riser: 250 ÷ 14 = 17.9 cm. Treads: 13. Total run: 13 × 0.28 = 3.64 m. Stringer: √(2.5² + 3.64²) = 4.42 m. Stair angle: 34.5°.

Measurement tips

Measure rise between finished surfaces. For indoor stairs that means finished floor to finished floor — including flooring thickness. A measurement taken to bare subfloor builds every riser wrong.
Cut one stringer first and test-fit it. It is your template — only cut the rest after the first one sits correctly at top and bottom.
Keep every riser identical, including the first and last. The top riser often needs trimming for the deck board or flooring thickness; forgetting it makes one step taller than the rest.
Use the 2R + T comfort check: twice the riser plus the tread near 24–25 in (61–64 cm) feels natural. Far outside that range, stairs feel either tippy or exhausting.
For deck stairs, set the bottom of the stringer on a solid footing — a concrete pad or pavers — not bare soil, which settles and tilts the whole stair.
Buy stringer boards longer than the calculated length. The diagonal cut, the top plumb cut and the bottom level cut all eat length; a foot of extra board is cheap insurance.

Common mistakes

Measuring to the wrong surfaces. Rise must be finished-to-finished — subfloor to subfloor misses the flooring thickness and shifts every riser.
Letting risers vary. Code typically allows only 3/8 in (9.5 mm) of variation between the tallest and shortest riser in a flight — and even legal variation trips people, because feet expect uniformity.
Counting treads equal to risers. There is always one fewer tread than risers — the upper floor or deck top is the last step. Counting one extra tread stretches the run and misplaces the stringer.
Forgetting the run will not fit. People compute beautiful stairs that need 12 ft of floor space in a 10 ft hallway. Check total run against the room before cutting.
Planning risers over the code maximum. Anything above 7.75 in (19.7 cm) per riser fails most residential codes and feels like climbing a ladder — the calculator adds a riser automatically if your numbers land there.
Skipping the permit check. Many places require a permit for new stairs, and deck stairs often fall under deck permit rules. A five-minute call to the building department beats rebuilding.

Factors that affect accuracy

Total rise. Taller rises need more risers; the count — and the lumber bill — scales directly with height.
Ideal riser height. A lower target (6.5 in) gives more, shallower steps and a longer run; a higher target (7.5 in) gives fewer steps in less space but a steeper climb.
Tread depth. Deeper treads need more run per step — 11 in treads stretch a flight noticeably compared with 10 in ones.
Available run. Hallways, landings and yard space cap the total run, which can force a steeper riser/tread combination than you would prefer.
Code limits. The 7.75 in max riser and 10 in min tread are the envelope most residential stairs must fit inside — local amendments can differ, so verify.
Top and bottom conditions. Deck board thickness, flooring build-up and footing height all shift the first and last riser if you do not account for them.

Sources & assumptions

Risers = total rise ÷ ideal riser height, rounded to whole steps (one more added if any riser would exceed the 7.75 in / 19.7 cm code maximum); actual riser = total rise ÷ risers.
Treads = risers − 1; total run = treads × tread depth; stringer = √(rise² + run²); stair angle = arctan(rise ÷ run).
Code limits per IRC guidance: 7.75 in max riser, 10 in min tread with nosing; comfort rule 2R + T ≈ 24–25 in per MiniWebtool.
Layout method follows SmartOnlineCalculator's stair method; stringer math per CoolConversion.

Safety note

Stairs must meet your local building code (max riser height, min tread depth, handrail height, headroom) — this calculator lays out the geometry but does not approve a design. Cutting stringers is structural work: get a professional for anything beyond a simple low deck stair.

Please note: material estimates are approximate. Always confirm product coverage with the manufacturer and buy a little extra for cuts, waste and future repairs.

What this calculator doesn't do

This lays out straight staircases only — it doesn't design landings, winders, or spiral stairs.
It is a planning aid, not a code approval. Stair rules (max riser, min tread, permits) are set by your local building code — always check before you build.
It doesn't size the stringer lumber or check spans — cutting stringers and framing stairs is structural work; get a professional for anything beyond a simple low deck stair.
Headroom, stair width and handrail rules aren't checked here; they matter just as much for a legal staircase.

Frequently asked questions

How many steps do I need for my stairs?

Divide the total rise by your ideal riser height (7 in / 18 cm is comfortable) and round to the nearest whole number — that is your riser count. Then divide the total rise by that count for the exact, even riser height. A 36 in rise at 7 in per riser gives 5 risers at 7.2 in each.

Is this the same as a deck stair calculator?

Yes. A deck stair is just a short straight staircase, and the math is identical: total rise from the ground to the deck surface, risers rounded to whole steps, treads one fewer than risers, stringer from the Pythagorean theorem. Enter your deck height as the total rise and this calculator lays out the stair.

Why is there always one fewer tread than risers?

Because the upper floor — or the deck surface — acts as the final tread. Each riser lifts you one step, but you only need a board to stand on for every step except the last one, where the floor itself does the job.

What is the ideal riser height and tread depth?

The widely used comfort pairing is a 7 in (18 cm) riser with an 11 in (28 cm) tread — it satisfies the classic 2R + T ≈ 25 in comfort rule. Building codes commonly cap risers at 7.75 in (19.7 cm) and require at least 10 in (25.4 cm) of tread with a nosing.

How do I calculate stair stringer length?

With the Pythagorean theorem: stringer = √(total rise² + total run²), where total run = (risers − 1) × tread depth. A 36 in rise with a 44 in run needs a 56.9 in stringer. Buy boards longer than the result — the top and bottom cuts consume length.

What angle should stairs be?

Most comfortable residential stairs land between about 30° and 40° of pitch. The 7 in riser / 11 in tread pairing gives roughly 32–33° per step. Steeper than 40° starts feeling ladder-like; shallower than 30° eats a lot of floor space.

Do I need a permit for stairs?

Often, yes — many building departments require permits for new or rebuilt stairs, and deck stairs usually fall under the deck permit. Rules on max riser height, min tread depth, handrails and headroom vary by location, so check with your local building department before you cut lumber.

How much do stair stringers cost to make?

This calculator gives you the cut dimensions, not a price — lumber prices vary too much by region and species. Take the stringer length and riser/tread counts to your lumberyard: you will need one board per stringer (typically 3 stringers for stairs up to 36 in wide), plus tread boards.

Next step

Cut one stringer first and test-fit it before cutting the rest — then check your local building code for riser, tread, handrail and permit rules before you build.

Save this estimate: use the Copy link button in the results box above to share these exact inputs, or Print results for a paper copy to take to the store.