Deck Stairs Calculator

A cut stringer is a sawtooth. Every notch rises by the riser height and runs by the tread depth, and marked out with a framing square every notch is identical and correct.

Then the treads go on. A tread sits on top of the horizontal cut, so each step rises by the notch — except the first, which rises by the notch plus a tread, and the last, which rises by the notch minus one. The middle of the flight is fine: each tread adds at the bottom of a step and subtracts at the top of the one below, and it cancels. The two ends have nothing to cancel against.

The fix is one cut. Take the bottom of the stringer down by exactly one tread thickness before it goes on the landing, and the first step rises by the notch like every other one while the top step meets the deck where it should.

The diagram above draws it. Two sawtooths on one scale — the tread surfaces, and the stringer under them, the same shape dropped by one tread. The gap at the foot of the flight is the drop.

Change the tread build-up and the drop changes with it, because the drop is the tread thickness. A 5/4 tread drops 1" where a 2× one drops 1 ½", and the panel recalculates when you switch.

The limit is ⅜" between the tallest riser and the shortest one in the whole flight, not ⅜" of drift per step (IRC R311.7.5.1). Fourteen steps do not get fourteen allowances. They get one, shared.

The same ⅜" applies twice more — to tread depths (R311.7.5.2) and to nosing projections (R311.7.5.3) — and each is measured on its own, so a flight passes on risers and fails on treads. Where it goes wrong is almost always the bottom step, because the landing is rarely exactly where the plan said it would be and because the stringer has to come down by a tread thickness before it stands up.

Dividing gives every riser the same height. Marking out does not. A tape shows sixteenths, and most rises do not divide into them. A 7' 1" rise over 12 risers is exactly 7.0833 in each. Mark that at the nearest sixteenth — 7 1/16" — stack it 12 times down the board, and the flight finishes ¼" away from the deck. That is inside the limit here. It does not stay inside it on a taller flight.

The calculator reports all three numbers: the exact riser height, the sixteenth to mark, and how far marking at that sixteenth lands from the deck surface. It flags the check when the drift exceeds the limit, which is better than finding out with a level.

Mark from a story pole rather than stacking the same measurement down the board. One stick, marked once from the bottom, carries the exact height into every step instead of accumulating a sixteenth at each one.

Total rise is measured from the deck surface down to the surface you will stand on when the stairs are finished. If the landing pad has not been poured, that surface does not exist yet, and the number you take today is wrong before any arithmetic starts.

A slab poured on grade finishes proud of the ground by its own thickness, and every inch of that comes off the total rise. Measure 2' to bare dirt with 4" of pad still to come and you are building to 1' 8" — 3 risers of 6 11/16" rather than 4 of 6". A pad excavated flush with the ground changes nothing, and a pad on a gravel base raises the landing further still.

The calculator asks which you have and subtracts the pad when the concrete is still to come. It starts at 4", which is our own figure for a slab poured on grade rather than anything published — measure to where the concrete will finish and correct it.

Riser count is a choice, and dividing the rise by the maximum riser makes the wrong one. A 4' 6" (1372 mm) rise comes down in 7 risers of 7 11/16" — legal, under the 7 ¾" ceiling, and the steepest flight the code will let you build. The same rise in 8 risers is 6 ¾" a step.

This calculator aims at 7" first — our own typical for where residential flights land when nobody is fighting for space, not a code figure — and moves off it only to stay under the maximum. The count it returns is the whole number nearest that target.

Override it and watch the riser height move. Adding a step lowers every riser and lengthens the total run; taking one away does the reverse. The calculator will not go below the fewest risers that keep the flight under the maximum, and it says so when it clamps rather than quietly returning a different flight from the one you asked for.

A stringer is laid out with a framing square: the riser height on the tongue, the tread depth on the blade, stepped down the board once per tread. The calculator gives both settings and the number of steps, so the square is transferring numbers rather than deriving them.

What the square gives you and the calculator cannot is the line. Stair gauges clamped to the square keep every notch identical, and the plumb cuts at the top and bottom come off the same setting. A story pole beats stacking the same measurement fourteen times down a board.

The throat is what is left of the board behind the notches, measured square to the stair line, and it decides whether a stringer is a stringer or a row of triangles. This flight leaves 6 1/16" of 2 x 12 behind its cuts. The same notches in 2 x 10 would leave 4 1/16", which is why stringers are cut from 2 x 12 — and why AWC DCA 6 requires nothing smaller. The 5" minimum this calculator checks against is dimensioned on DCA 6's stringer figure (Figure 28), which also caps a cut stringer's horizontal span at 6' before it needs a post or a landing under it. This calculator does not check that span yet; the drawing is free from the American Wood Council, and your jurisdiction may amend it.

Stringer spacing is a control here, not an answer. The calculator offers 12, 16, 18 in on centre and starts at 16 in, the conservative end of common practice. What your treads carry between supports is published by whoever milled them, and it is their figure rather than ours.

A single flight is capped at 12' 7" of vertical rise between floor levels or landings (IRC R311.7.3). Past that the flight has to be broken, and what breaks it is a landing. AWC DCA 6, the prescriptive deck guide most permit offices work from, is stricter: it requires an intermediate landing once the total rise passes twelve feet even. The check above uses the code figure; a deck stair built to DCA 6 is held to the shorter one.

A landing is a small deck rather than a big step. It has footings, it has framing, it has its own connection to whatever it meets, and it goes on the permit drawing like the rest of the structure. The run below it starts again from its surface, so a landing does not shorten the ground the stairs need — it moves some of that ground sideways.

Total run is the figure that surprises people. 5' 10 ⅞" (1800 mm) of clear ground in front of a 4' 6" deck, for 7 treads at 10 ⅛" nosing to nosing. That is a patio's worth of yard, and running out of it is what turns a straight flight into an L. Measure the ground before you settle the riser count, because the two decide each other.

Tread depth is where that run comes from, and it has a floor of 10" nosing to nosing (R311.7.5.2). A tread shallower than 11" has to overhang, which is how a shallow tread buys its foot room back (R311.7.5.3).

A handrail is required at 4 or more risers (IRC R311.7.8), on at least one side, and it has to be graspable for the full length of the flight. Height is 34" to 38" measured from the sloped plane through the tread nosings (R311.7.8.1). Four risers is a short flight off a low deck, which is why the default flight here already crosses the trigger.

A guard is triggered by height rather than by step count: more than 30" above the floor or grade below, measured anywhere within 36 inches horizontally of the open side (IRC R312.1.1). Measure to the ground, not to the next level down, and check it along the whole deck rather than only at the stairs — a deck that sits low at the steps and high at the far corner still needs a guard at the far corner.

The two guard heights are measured from different places, and that is where stair guards go wrong. At deck level it is 36" from the walking surface. On the open side of the stairs it is 34" from a line through the tread nosings (R312.1.2) — a different datum, and the same number as the minimum handrail height without being the same rule.

Open risers carry their own opening limit. Leave the risers off and the gap between treads must not pass a 4" sphere, except where the total rise of the stair is 30" or less (R311.7.5.1). That is separate from the guard, and it is the one people miss on a low flight they left open on purpose.

It gives you a shape. Four things decide whether that shape stands up, and it answers none of them:

• The footing under the foot of the stringer. Soil bearing and frost depth set it, and both are local.
• The connection at the top. How the stringers hang off the deck, and whether the band they hang from carries them. That is hardware and framing, not geometry.
• Anything turned. Winders and L-shaped flights change the calculation rather than extending it.
• Your jurisdiction. The limits here are the 2021 model code. Editions and amendments are adopted locally, and the building department has the version you will be inspected against.

Minimum stair width is 36" clear above the handrail height (R311.7.1) and the calculator checks it, but handrails projecting into that width change what is left of it. Confirm the reduction with your building department rather than with this page. The methodology page lists every limit above with its section and the document it was read from.