General Stair Calculator

How to Use This Stair Calculator

Start with the stair type. The presets fill in a total rise and a width that match how that stair is normally framed, and you can type over either one. They are a starting point drawn from common practice, not a code rule. Your floor structure, your local inspector and the space you have to work with all get a vote.

Total rise is the vertical distance from the finished surface of the lower floor to the finished surface of the upper floor. Measure it in one shot with a level and a straightedge rather than adding up ceiling height and joist depth, because the floor assembly between stories is rarely the number you expect. Tile, hardwood and a thick underlayment all shift the finished surface, and an inch of error at the bottom multiplies across every riser in the flight.

Max riser sets how many risers you end up with. The tool divides the rise by that number and rounds up, then divides again to find the exact riser height. Choosing 7.75 gives you the fewest risers and the longest tread run. Choosing 7 gives you a gentler climb and one or two more risers, which means more floor space and more lumber. That trade runs through the whole layout.

Tread depth is measured from nosing to nosing, which is how the code measures it. The board you actually cut is shallower, and the tool subtracts the nosing to report the tread board width. An 11 inch tread with a 1 inch nosing comes off a 10 inch board.

Floor assembly is the joist depth plus the subfloor, measured from the finished upper floor surface down to the underside of the framing. It only feeds the headroom check, and it is the number most people forget. Ten inches is typical for 2x10 joists with 3/4 inch subfloor, and that ten inches is the clearance you have at the nosing directly under the edge of the stairwell opening.

The three prices drive the budget line at the bottom. Stringers are priced by the linear foot of 2x12, treads and risers by the linear foot of stair width. Set any price to zero to drop that material out of the total.

Stair Code Limits at a Glance

These are the numbers from the International Residential Code that apply to a stair inside a dwelling. They are the limits an inspector will check, and they are also the limits that make a stair feel right underfoot.

Two of these limits catch more DIY stairs than the rest. The 3/8 inch riser variation rule is easy to break at the bottom of a flight, where the last riser meets a floor that turned out to be an inch lower than planned. The headroom rule is easy to break at the top, where a stairwell opening framed from a rough sketch leaves 74 inches instead of 80 and nobody notices until the drywall is up.

Risers, Treads and the Comfort Formula

Code tells you what is legal. The comfort formula tells you what people will actually enjoy walking on. Twice the riser height plus the tread depth should land between 24 and 25 inches, a relationship builders have used for well over a century because it matches the length of a natural stride.

Run the numbers on a few common combinations and the pattern is clear. A 7 inch riser with an 11 inch tread gives 25, which is the classic interior stair and the one most people describe as normal without being able to say why. A 7-1/2 inch riser with an 11 inch tread gives 26, a little steeper. The code maximum of 7-3/4 inches with a 10 inch tread gives 25.5, and that pair is what a lot of basement stairs from the 1960s were built with. It passes inspection and it feels like a ladder after a few trips with laundry.

The other way to read the same geometry is the angle of the flight. A comfortable interior stair sits between about 30 and 35 degrees from horizontal. Steeper than 37 degrees and the stair starts to read as a ship's ladder. Shallower than 30 and you are burning floor space on a stair that feels like a ramp. The calculator reports the angle alongside the material numbers, which is a quick way to sanity check a layout before you commit to framing.

Tread depth deserves its own note, because the 11 inch standard is not arbitrary. Ten inches is the code floor and it works with a 1 inch nosing, but the walking surface people actually land on is measured nosing to nosing, and a 10 inch tread leaves a foot with nowhere to go on the way down. Going to 11 or 11-1/2 inches costs about 14 inches of floor space across a 14 tread flight and it is the cheapest comfort upgrade in the whole project.

Cutting Stringers from 2x12 Stock

The stringer is the notched board that carries the treads, and it is the part of the job that separates a stair that feels solid from one that creaks and flexes. Cut them from 2x12 lumber and nothing smaller. After the notches come out, the remaining material at the throat of each cut is what carries the load, and a 2x10 leaves roughly 3-1/2 inches of wood there, which is not enough over a long span.

Layout runs one of two ways. The traditional method clamps two brass stair gauges to a framing square at the rise and the run, then walks the square down the board marking each notch. It is fast once the gauges are set and it is how most carpenters were taught. The second method marks the top plumb cut, then steps off each tread location along the edge of the board with a tape and connects the marks with a straightedge. It is slower, and it does not accumulate gauge placement error over a long board, which matters on a 15 riser flight where a 1/16 inch slip per step adds up to nearly an inch at the bottom.

Cut the first stringer, then test fit it against both floors before you cut a second one. A stringer that is a quarter inch off sits on the floor with a gap under it, and a gap under a stringer is where a stair starts to bounce. Once one stringer fits, use it as a template for the rest and stack the boards so the notches match.

Spacing follows a simple rule: no more than 16 inches on center. The calculator applies that rule to your stair width and reports the count, and the answer for a standard 36 inch stair comes out at four stringers at 12 inches on center. Three stringers is what many carpenters actually install on a 36 inch closed stair, relying on the risers to stiffen the assembly, and that works fine with glued and screwed risers. If the treads are open, or the stair is wider than 36 inches, or someone in the house is going to haul a piano up it, stay with the 16 inch rule.

Attach the stringers to the framing with metal hangers or a ledger rather than notching them over a beam. A notched stringer over a beam concentrates stress at the notch and is one of the few ways a stair actually fails structurally. Simpson and similar manufacturers make adjustable stair stringer hangers that let you set the height on site and nail off in a few minutes.

Headroom and the Floor Opening

Headroom is measured from the sloped plane that runs across the tread nosings up to the underside of the floor structure above, and the code wants 80 inches of it. What makes this tricky is that the number changes as you walk down the flight, so the opening in the floor above is what really gets sized.

The arithmetic is short. Directly under the edge of the floor opening, the clearance equals the floor assembly thickness, which is around 10 inches on a typical wood framed floor. Every tread further down the flight drops the nosing by one riser height, so the clearance grows by that same amount per tread. Subtract 10 from 80 and you have 70 inches to make up. Divide 70 by a 7.7 inch riser and you get about 9.1, which rounds up to 10 treads. Ten treads at 11 inches puts the opening edge 110 inches back from the top nosing, measured horizontally along the flight.

That number is a minimum and it is worth adding a tread of margin. Framing crews measure the opening from the header to the rim, and a stair that lands exactly on the code number leaves no room for a thicker finish floor upstairs or a deeper joist than the plan called for. Ten inches of extra opening is one more tread and it costs one joist.

While you are framing, remember the opening has to be at least as wide as the stair plus the finish on both sides, and the header carries the trimmed joists on each side. A stairwell opening interrupts the floor framing, so the joists that were cut need a double header at the end of the opening and a rim or a bearing wall along the sides. This is the point in the project where a framing plan from an engineer earns its fee.

Worked Example: A 9 Foot Floor to Floor Stair

Take a house with a 108 inch rise from the finished first floor to the finished second floor, which is a common number for 8 foot ceilings plus a joist assembly. The plan is a straight run, 36 inches wide, with 11 inch treads and a 1 inch nosing.

Divide 108 by the 7.75 inch riser maximum and you get 13.9, which rounds up to 14 risers. Divide 108 by 14 and the exact riser height is 7.71 inches. That passes code, since it sits under the 7-3/4 inch cap, and every riser in the flight uses the same 7.71 inch number.

Treads come out at 13, one fewer than the risers, because the second floor surface serves as the top step. Thirteen treads at 11 inches is a 143 inch run, just under 12 feet of floor plan. Subtract the 1 inch nosing and each tread board is cut at 10 inches wide.

For the stringer, square the rise and the run: 11664 plus 20449 comes to 32113, and the square root is 179.2 inches. Add about 12 inches for the top and bottom cuts and each stringer needs roughly 191 inches of stock. That is right at the limit of a 16 foot board, so order 20 foot 2x12 for this flight rather than counting on a perfect cut at each end. A 36 inch wide stair at 16 inches on center takes four stringers, which puts the order at four 20 foot sticks.

Headroom works out to a 110 inch opening from the top nosing with a 10 inch floor assembly, as the earlier section walked through. The comfort check lands at 26.4, which is on the steep side. Dropping the riser target to 7.5 inches gives 15 risers at 7.2 inches, 14 treads, a 154 inch run and a comfort number of 25.4, which is noticeably nicer to climb and costs 11 inches of floor plan.

Materials at $3.00 per foot for 2x12, $9.00 per foot for oak treads and $2.50 per foot for risers run about $636: $180 of stringers, $351 of tread material and $105 of riser stock. A hardwood handrail, newels and balusters are a separate line item and usually cost more than the lumber underneath them.

5 Interior Stair Mistakes

1. Measuring the rise from the subfloor. The finished floor upstairs might be 3/4 inch of hardwood over the subfloor, and the finished floor downstairs might be tile over a mortar bed. Both add height. Measure floor to floor on the finished surfaces, or add the finishes to your subfloor measurement before you divide, because a half inch of error spread over 14 risers puts the top step noticeably out of line with the floor.
2. Letting the bottom riser run short. The bottom riser is the one that gets cut to fit the actual floor, and it is where the 3/8 inch variation rule gets broken. Cut the stringers so the bottom riser lands at the same 7.71 inches as the rest, and set the stair on a landing or a finished floor pad rather than a rough slab you plan to pour later.
3. Framing the floor opening too small. Headroom is the expensive mistake, because fixing it means cutting joists, adding a header and patching a ceiling. Run the numbers before the framing inspection, not after the drywall, and add a tread of margin for the finishes.
4. Using a 2x10 for stringers. The notches remove most of the board's depth and the throat that remains is what carries the load. A 2x10 stringer in a 14 riser flight will flex under a heavy person and it will only get worse as the notches dry and shrink. 2x12 is the standard for a reason.
5. Skipping construction adhesive. Stairs creak because the treads and risers rub against the stringers as the wood moves with the seasons. A bead of subfloor adhesive on every notch, plus screws rather than nails, removes the movement and the noise. It costs about $8 a tube and it is the difference between a stair that is silent in year ten and one that announces every trip to the basement.