Step safely.

Stairs are the one thing on your project the inspector will measure

Most of a build has some give in it. A wall an inch off, a joist a little long, nobody notices. Stairs are different. The building code sets the rise and the run to fractions of an inch, an inspector shows up with a tape measure, and a staircase that misses the numbers does not get signed off. It gets torn out. So this is the one place where getting the math right before you cut is not about tidiness. It is about not building the thing twice.

The good news is the math is not hard, and the calculator above does it for you. Give it your total rise, the vertical distance from one finished floor to the next, and it hands back the number of steps, the exact rise and run of each one, the stringer length, and how many stringers you need. The rest of this page explains what those numbers mean and where people go wrong, starting with what every part of a staircase is actually called.

The parts of a staircase

Before the numbers make sense, the vocabulary has to. A staircase is a handful of parts with specific names, and every stair calculator, code book, and lumber list uses them. Here is the whole thing in one drawing.

The tread is the part you step on. The riser is the vertical face between one tread and the next, the height you lift your foot. The nosing is the small lip where the tread overhangs the riser below it, which gives your foot a little extra depth without adding to the run. The stringer is the sawtooth-cut board, usually a 2x12, that carries the whole staircase on a diagonal. And the throat is the narrowest strip of wood left in the stringer after the step notches are cut out. That last one matters more than people expect, because if you cut the notches too deep the stringer loses its strength.

Two more terms run through every calculation. Total rise is the full vertical height the stairs climb, floor to floor. Total run is the full horizontal distance they cover. Divide those by the number of steps and you get the single-step rise and run, the two numbers the code cares about most.

Rise and run: the numbers the code sets

Start with total rise. Measure floor to floor, in inches, and be exact, because everything downstream depends on it. Divide that by a target riser height, and the sweet spot is 7 to 7.5 inches per step. Round up to a whole number of steps, then divide the total rise back by that step count to get the actual rise per step. A 48-inch total rise divided by 7.5 is 6.4, which rounds up to 7 steps, giving an actual rise of about 6.86 inches each.

Every riser has to be the same height. Not close. The same. The code allows a maximum variation of three eighths of an inch between the tallest and shortest riser in a flight, and that is the rule most do-it-yourselfers fail. If your first step is 7.5 inches and your last is 7 inches because the floor was not level, that half inch fails inspection and it is also a real trip hazard, because a foot expecting one height and finding another is how people fall. The calculator keeps every riser identical, which is the whole point of doing the math instead of eyeballing it.

What angle should stairs be

Angle is just rise and run wearing a different hat. A staircase built to the 7-11 rule sits at about 32 degrees from horizontal, which is the comfortable middle most interior stairs land on. The code does not state an angle directly, it states the rise and run limits, but those limits box you into a range of roughly 30 to 37 degrees for a normal staircase. Go steeper than that and you are into ship ladders and attic pull-downs, which have their own separate rules. Go shallower and you are building a ramp.

The practical takeaway: you do not pick an angle, you pick a comfortable rise and run, and the angle falls out of them. If someone asks you the angle of a staircase, the honest answer is that a good one is about 32 to 35 degrees, and anything a person would call comfortable to climb lives in that band.

How many stair stringers do I need, and how long

The stringer is the backbone. Its length comes straight out of the Pythagorean theorem, because the stringer is the hypotenuse of a triangle whose legs are your total rise and total run. Square each, add them, take the square root. A staircase climbing 108 inches over 130 inches of run needs a stringer at least the square root of 108 squared plus 130 squared, which is about 169 inches, or 14 feet and an inch. Buy 16-foot 2x12 boards for a run like that. Anything shorter will not reach, and anything narrower than a 2x12 does not survive the notching.

How many stringers is a separate question, and it comes down to width and tread material. A 36-inch-wide staircase, the code minimum, takes 3 stringers: one under each edge and one down the middle. Every extra 16 to 18 inches of width adds a stringer, so a 48-inch stair takes 4. If your treads are composite or thin decking rather than solid 2x lumber, tighten that spacing to every 12 to 16 inches, because those materials flex more and a tread that bounces underfoot is a tread that needs more support beneath it.

The IRC numbers every staircase has to hit

These are not preferences. They are the limits an inspector checks, and a staircase outside them does not pass. Keep this table handy, because it is the whole rulebook for a standard residential stair.

Treads, landings, and deck stairs

The tread count is always one less than the riser count, because the top tread is the floor you are stepping onto. A 14-riser staircase has 13 treads. For interior stairs the tread is usually a 1-inch hardwood board about 11.25 inches deep, giving a 10-inch run plus a 1.25-inch nosing. For deck and exterior stairs, two 2x6 pressure-treated boards or a single composite stair tread do the same job.

A landing becomes mandatory once a straight run climbs more than 12 feet of total rise, roughly 19 risers, and landings are also how L-shaped and U-shaped stairs turn a corner. A landing has to be at least 36 inches deep in the direction you walk and at least as wide as the stairs. When a landing splits a staircase, treat each flight as its own separate calculation with its own total rise, run, and stringer, and frame the landing as a platform in between.

Deck stairs follow the same code as interior stairs with a couple of extras. Any deck more than 30 inches above grade needs stairs, the total rise is measured from the deck surface down to the ground, and the whole run projects 5 to 6 feet out from the deck, which is often the thing that decides whether the stairs even fit the yard. If the stairs are part of a bigger deck build, the deck calculator and the deck planner roll the stair numbers into the full material list, and the lumber calculator converts the stringer and tread stock into board feet for pricing.

Where stairs go wrong

The failures repeat, and every one of them is a math or planning miss rather than a skill problem. The unequal risers top the list, usually because the total rise got measured off a floor that was not level, and the fix after the fact means rebuilding the whole flight. Cutting the stringer notches too deep is next, leaving a throat under 3.5 inches and a stringer that flexes or cracks under load.

After those come the planning misses. Forgetting that the run projects 5 or 6 feet out and discovering the stairs do not fit the space. Building too few stringers and feeling the treads bounce. Skipping the landing on a tall straight run. And the quiet one, forgetting that the tread count is one less than the riser count, then coming up a board short. None of these are hard to avoid. They come down to getting the numbers right first, which is exactly what the calculator is for, and then checking the throat on the real board before you commit the cuts to all of them.