The stair calculator above takes one measurement you can get with a tape — the total floor-to-floor rise — and turns it into the numbers you actually need to cut a flight: how many risers there are, exactly how tall each one is, how much floor length the stairs will consume, how long the stringer stock has to be, what angle the flight sits at, and whether the headroom under the upper floor clears the code minimum. It then checks each of those results against the riser, tread, width and headroom limits published in the 2021 International Residential Code.
Arb Digital builds free calculators for trades and homeowners who are tired of doing this arithmetic twice on the back of an offcut. Stairs punish rounding more than almost any other part of a build: get the riser height wrong by an eighth of an inch and the error multiplies by the number of steps, which is exactly the failure mode the code's uniformity rule exists to catch. Everything here is a planning estimate — your local building department, and the amendments it has adopted, govern the finished design.
What This Stair Calculator Does
The calculator solves the standard straight-flight geometry problem. You give it the vertical distance between two finished floors and the riser height and tread depth you would like to build to. It divides the rise by your desired riser to get a raw step count, rounds that to a whole number of risers — because you cannot build a fraction of a step — and then divides the rise back by that whole count to give the exact riser height every step must actually be.
From there it derives the horizontal run, the diagonal stringer length, the pitch angle, and the headroom at the tightest point under the framed floor opening. Each result carries a pass or fail against a specific code limit.
It handles both imperial and metric input. The code figures themselves are written in inches and feet, so in metric mode the tool converts each limit rather than substituting a different national standard: 7 3/4 in becomes 196.9 mm, 10 in becomes 254 mm, 36 in becomes 914.4 mm and 6 ft 8 in becomes 2032 mm.
How to Use It
- Measure the total rise. Finished floor below to finished floor above. If the upper floor covering is not down yet, add its thickness — a 3/4 in hardwood layer added after the stairs are cut will shorten your top riser by exactly that amount.
- Set a target riser height. Seven inches is a comfortable starting point for a house. The calculator will round the step count and hand back the true riser height that results.
- Enter your tread depth. This is the nosing-to-nosing run of one step, not the width of the tread board, which overlaps the step below by the nosing projection.
- Enter stair width, floor thickness and the stairwell opening length. These drive the width check and the headroom check. Floor thickness means the whole assembly — joist, subfloor and ceiling finish.
- Choose the top tread arrangement and read the results grid. Any failed check appears in the code-check note directly beneath it.
The Formula / How It's Calculated
Five formulas do all the work.
Riser count = round(total rise ÷ desired riser height). Rounding, not truncating: a 108 in rise at a 7 in target gives 15.43, which rounds to 15 risers. Truncating to 15 would be the same answer here, but a rise of 110 in gives 15.71, which must round to 16 or the resulting riser height would exceed your target by a wide margin.
Exact riser height = total rise ÷ riser count. This is the number you cut to. 108 ÷ 15 = 7.2 in. Every riser in the flight is 7.2 in, and this is the value the code's maximum-riser and uniformity rules are measured against — not your original 7 in wish.
Total run = (riser count − 1) × tread depth when the upper floor itself serves as the top tread, which is the normal arrangement for a flight landing directly on a floor. There is always one more riser than there are treads, because the last riser lifts you onto the floor above and that floor is the surface you step onto. If instead the flight ends on a separate top tread set back from the floor edge — for example where the stair lands on an intermediate platform — the run becomes riser count × tread depth. The tool exposes both through the top-tread selector, because getting this wrong shifts the whole flight by one full tread and is one of the most common layout errors.
Stringer length = √(rise² + run²). The stringer is the hypotenuse of the triangle formed by the total rise and the total run. For a 108 in rise and a 140 in run, that is √(11,664 + 19,600) = √31,264 = 176.8 in. Buy stock longer than that: the figure is the diagonal of the cut line, and you still need material beyond it at the top hanger and the bottom kick plate.
Angle = atan(riser ÷ tread), in degrees. 7.2 over 10 gives 35.75 degrees. Anything much past 37 degrees starts to feel steep underfoot even when it passes every numeric limit.
The IRC Limits This Tool Checks
Four numbers from the 2021 International Residential Code drive the pass and fail flags, and all four can be read on the ICC's published text of IRC Chapter 3.
- IRC R311.7.5.1 — maximum riser height 7 3/4 in (196.9 mm). Measured vertically between the leading edges of adjacent treads. A 7.2 in riser passes comfortably; a rise of 108 in divided into 13 risers would give 8.31 in and fail.
- IRC R311.7.5.2 — minimum tread depth 10 in (254 mm). Measured horizontally between the vertical planes of the foremost projection of adjacent treads, at right angles to the nosing.
- IRC R311.7.5.1 — uniformity tolerance 3/8 in (9.5 mm). The greatest riser height in a flight must not exceed the smallest by more than 3/8 in. The same tolerance applies to tread depth under R311.7.5.2. This is why the exact riser height matters more than the target.
- IRC R311.7.1 — minimum stair width 36 in (914 mm) above the permitted handrail height, and IRC R311.7.2 — minimum headroom 6 ft 8 in (2032 mm), measured vertically from a sloped line connecting the tread nosings to the ceiling or obstruction above.
Local jurisdictions amend these figures. Several adopt a 7 in maximum riser and an 11 in minimum tread for residential work, and commercial or workplace stairs sit under an entirely different rule set — OSHA 1910.25 permits a maximum riser of 9.5 in and requires a minimum tread depth of 9.5 in for general-industry stairs, which is steeper than anything the IRC allows in a home. Never carry a number from one rule set into a project governed by the other.
Why the Uniformity Rule Catches More Stairs Than the Maximum
Most flights that fail inspection fail on the 3/8 in uniformity tolerance rather than on maximum riser height, and almost always at the top or bottom step. A carpenter cuts the stringer to a perfect set of equal risers, then the flooring trade puts 3/4 in of hardwood on the upper floor and leaves the slab bare at the bottom. That is a 3/4 in variation — double the allowance — with no cutting error anywhere.
The fix is to decide the finished floor thickness at both levels before the stringer is cut, and to build the difference into the bottom riser deliberately. Carpenters call this dropping the stringer: the bottom riser is cut short by the thickness of the tread material so that once the treads are installed, every riser measures the same. Enter your total rise as a finished-floor-to-finished-floor number in this calculator and the arithmetic already accounts for it.
Headroom Is a Floor-Opening Problem, Not a Stair Problem
Headroom is the check that most often forces a redesign, and it is almost never fixed by changing the stairs. The measurement runs vertically from an imaginary sloped line touching every tread nosing up to whatever is above — usually the underside of the upper floor framing at the edge of the stairwell opening. The tightest point is at the inner edge of that opening, because that is where the stair is still low and the floor above is still present.
This calculator works out that clearance directly. It finds the horizontal position of the opening's inner edge along the run, computes how high the stair has climbed at that point, and subtracts it from the underside height of the upper floor. With a 108 in rise, a 12 in floor assembly and a 120 in opening on a 140 in run, the inner edge sits 20 in along the run, the stair has climbed 20 × 0.72 = 14.4 in, and the clear headroom is (108 − 12) − 14.4 = 81.6 in, which clears the 80 in minimum by 1.6 in. Shorten the opening to 108 in and headroom drops to 74.9 in — a fail, with the stairs themselves completely unchanged.
When headroom fails, the levers are: lengthen the floor opening, reduce the floor assembly depth, or shorten the tread depth so the flight climbs more steeply per foot of run.
Fitting a Flight Into a Space That Will Not Take It
Total run decides whether a stair fits a house. A 15-riser flight at a 10 in tread eats 140 in of floor length, plus landing space top and bottom. If your opening is 11 feet, no riser adjustment inside the code range saves a straight flight: dropping to 14 risers pushes the riser to 7.71 in, barely inside the limit, and still leaves a 130 in run.
The real options are a landing with a 90-degree turn, which trades floor length for width, or a U-shaped flight with a half landing. Our square footage calculator will size the landing zone quickly. Winders — tapered treads that turn without a landing — are permitted under IRC R311.7.5.2.1 but carry their own minimum tread depth measured at the walking line, and are outside what this straight-flight calculator models.
Arb Digital builds free, technically accurate tools like this one to earn organic traffic. If you want the same approach applied to your own site, start with the tools hub or get in touch.
Browse Free Tools Contact Arb DigitalCommon Mistakes to Avoid
- Measuring floor to ceiling instead of floor to floor — the rise must include the depth of the upper floor assembly, or every riser will come out short.
- Cutting the stringer before the finished floors are decided — a late flooring change is the single most common cause of a uniformity failure at the top or bottom step.
- Confusing tread depth with tread board width — the nosing projection overlaps the step below, so a 10 in run typically needs an 11 in board.
- Using the desired riser height instead of the exact one — the code is measured against what you actually built, not what you aimed for.
- Treating a passing headroom result as final — ductwork, a soffit, or a beam dropped below the joists after the stair is set will fail the same check on site.
Related Free Tools From Arb Digital
Stairs rarely arrive alone. Size the landing slab with the concrete calculator, frame the adjacent partition with the wall framing calculator, cover the treads with the flooring calculator, and price the finished work with the cost per square foot calculator.
Frequently Asked Questions
Fifteen risers at 7.2 inches each. Divide 108 by a comfortable target riser of 7 inches to get 15.43, round to 15 whole risers, then divide 108 by 15 to get the exact riser height of 7.2 inches. At a 10 inch tread the flight has 14 treads and a total run of 140 inches.
IRC R311.7.5.1 sets a maximum riser height of 7 3/4 inches, or 196.9 mm, measured vertically between the leading edges of adjacent treads. Many local jurisdictions amend this figure downward, so confirm with your building department before cutting.
Because the upper floor itself acts as the top tread. A flight always has one more riser than it has treads, since the final riser lifts you onto the floor above. If the stair instead ends on a separate tread set back from the floor edge, the run equals the riser count times the tread depth.
IRC R311.7.2 requires a minimum of 6 feet 8 inches, or 80 inches, measured vertically from a sloped line joining the tread nosings to the ceiling or obstruction above. The tightest point is normally at the inner edge of the framed floor opening.
It is the uniformity tolerance. Within a single flight, the greatest riser height must not exceed the smallest by more than 3/8 inch, and the same limit applies to tread depth. Most inspection failures are uniformity failures at the top or bottom step rather than maximum-height failures.
Take the square root of the total rise squared plus the total run squared. For a 108 inch rise and a 140 inch run that is the square root of 31,264, or 176.8 inches. Order stringer stock longer than the diagonal to allow for the top and bottom cuts.
Yes. Switch the units selector to millimetres and every input and output converts. The IRC limits are written in inches, so the tool converts each limit rather than substituting a different standard: 196.9 mm maximum riser, 254 mm minimum tread, 914.4 mm minimum width and 2032 mm minimum headroom.
Results from this stair calculator are planning estimates only and are not an approved design. Local amendments to the building code govern, and you must confirm every dimension with your local building department or a licensed design professional before construction.
