Stair Calculator with Diagram: Rise, Run and Stringers

Type the total rise from finished floor to finished floor and get the riser count and exact height, the treads and total run, the stringer length and angle, the board to buy, and a side-profile drawing — with a pass/fail check on riser height, tread depth, width, headroom and stringer throat.

Measure the total rise from the finished floor below to the finished floor above — not the ceiling height, which leaves out the joists and subfloor — and enter it in inches. The calculator divides it by your target riser, rounds to a whole number of risers and recalculates the exact height (every riser must be identical within ⅜ in), then lays out the treads, the total run, the stringer length and angle, how many stringers the width needs and what length of 2×12 to buy. The code table turns red where a dimension misses IRC (or IBC, if you pick it), and the drawing redraws every time you change a number. Enter the run available if the stair has to land before a wall: the tool shortens the treads to the code minimum before it tells you it won’t fit.

How many steps for a 9-ft ceiling?

A 9-ft ceiling is about a 118-inch total rise once you add the 10 in of joists, subfloor and flooring above it, and 118 in takes 16 risers of 7 3/8 in (118 ÷ 16 = 7.375) with 15 treads. At a 10½-in tread that is a total run of 13 ft 1 1/2 in, a 35° stair, and stringers 16 ft 4 13/16 in long cut from 18-ft 2×12s. An 8-ft ceiling (about 106 in rise) takes 14 risers of 7 9/16 in; a 10-ft ceiling (about 130 in) takes 18 risers of about 7¼ in (7.22).

The riser count always comes from the total rise, never from the ceiling height alone. A 9-ft ceiling with 2×10 joists, ¾-in subfloor and ¾-in hardwood is 108 + 9¼ + 1½ = 118¾ in; with 11⅞-in I-joists it is 121⅝ in. An inch of error in the rise becomes a 1/16-in error on every riser — which is inside tolerance — but 3 in of error puts one riser out by ⅜ in and fails inspection. Measure at the stairwell with a laser or a plumb line, and if the two floors are not level to each other, measure at the exact point where the top and bottom of the stair will land.

Rise and run: how to calculate stairs

1. Measure the total rise finished floor to finished floor, in inches.
2. Divide by a target riser — 7 to 7½ in for a comfortable house stair, 7¾ in maximum under the IRC. 118 ÷ 7.25 = 16.3, so 16 risers.
3. Divide the rise by that whole number for the actual riser: 118 ÷ 16 = 7.375 in. If it comes out above 7¾ in, add a riser and divide again (118 ÷ 15 = 7.87 fails; 16 passes).
4. Treads = risers − 1. The top floor is the last “tread.” 16 risers, 15 treads.
5. Pick a tread depth measured nosing to nosing — 10 in is the IRC minimum, 10–11 in the usual comfort range. Add the nosing (¾–1¼ in) to get the tread board width: a 10½-in run with a 1-in nosing uses an 11½-in board.
6. Total run = treads × tread depth. 15 × 10.5 = 157.5 in, or 13 ft 1½ in, plus a landing of at least 36 in at the bottom.
7. Stringer length = √(run² + rise²): √(157.5² + 118²) = 16 ft 4 13/16 in. Buy the next stock length up with a foot to spare — an 18-ft 2×12.

If the run has to fit a wall, work backward: available run ÷ treads = the tread depth you can afford, and if that is under 10 in (11 in commercial) the stair needs a landing, a turn or a steeper allowance from the building department. The tool does this when you enter a run limit.

Two things the table shows. First, every foot of extra rise costs about 1.4 ft of extra run: a 12-ft floor-to-floor stair needs 17 ft of floor plus a landing, which is why tall-ceiling houses have switchback stairs. Second, the riser count changes in jumps: 108 in is 15 risers at 7.2 in, 112 in is also 15 risers but at 7.47 in — same treads, same run, steeper step. When you can choose the floor-to-floor (an addition, a deck), choose a rise that divides to a comfortable riser rather than one that lands just over a breakpoint.

Stair size standards: IRC, IBC and ADA

Houses follow the International Residential Code, section R311.7: risers no more than 7¾ in, treads at least 10 in deep, at least 36 in wide, 6 ft 8 in of headroom and a handrail at 34–38 in. Commercial buildings and apartment common stairs follow the IBC (section 1011), which is stricter: 7-in maximum risers, 11-in treads and 44-in width. Accessible routes under the ADA Standards use the IBC geometry plus handrails on both sides with extensions and closed risers. The table lists the numbers that decide whether a stair passes.

The limits that trip people up are headroom and width. Headroom is measured vertically from the nosing line (a line touching every tread nose) to the ceiling or beam above, and a stair that passes everywhere else fails at the header of the floor opening when the opening is too short — the calculator takes your available headroom but cannot see the opening, so measure it: the floor opening must be at least (80 in + the floor thickness) ÷ tan(angle) long past the top riser, about 11 ft for a 35° stair with a 10-in floor system. Width is measured above the handrail height; a 36-in stair with one handrail projecting 4½ in leaves 31½ in clear, which is the minimum.

What makes a stair comfortable: the 2R + T rule

Code limits are maximums, not targets. The old carpenter’s rule is that twice the riser plus the tread should equal 24–25 in (2R + T), which matches an adult stride; the companion rules are R + T = 17–18 in and R × T = 72–75. A 7-in riser with an 11-in tread (2 × 7 + 11 = 25) is the classic comfortable stair and also the commercial standard; a 7¾-in riser with a 10-in tread (25½) is legal and noticeably steeper. Steps that break the rule in either direction cause trips: a short riser with a deep tread makes people shuffle, a tall riser with a shallow tread catches toes.

Pick the riser first from the rise (it has to divide evenly) and then choose the tread from this table, not the other way round. If the run is tight, keep the riser comfortable and accept a 10-in tread with a 1¼-in nosing — the nosing adds foot room without adding run. Exterior and deck stairs are usually built shallower (6–7-in risers, 11–12-in treads) because people carry things and the treads get wet.

Cutting stringers: layout, dropping the stringer and the throat

Lay out a stringer with a framing square and stair gauges clamped at the riser height on the tongue and the tread depth on the blade. Starting at one end of a 2×12, trace the triangle, slide the square along and trace the next, until you have the full count of risers; the sawtooth is the nosing line, and the top and bottom cuts are perpendicular (the top plumb cut sits against the header, the bottom level cut on the floor). Cut with a circular saw to the line and finish each inside corner with a handsaw — overcutting the corner weakens the throat.

Drop the stringer

With the stringer sitting on the floor and treads of thickness t added to every cut, the first step would be riser + t high and the last step riser − t. The fix is to cut t off the bottom of the stringer (“dropping” it) — for 1-in treads, the bottom riser cut is 6 3/8 in instead of 7 3/8 in on the 118-in example. If the bottom floor will get a finish floor of thickness f later, subtract (t − f) instead. The calculator’s “drop the stringer” row shows the exact first cut.

Throat depth and stringer count

What is left of the board after the notch — the throat — carries the load, and the usual rule (from the AWC’s deck guide DCA 6 and most inspectors) is at least 5 in of solid wood on a 2×12. A 7⅜ × 10½ step notches 6 in out of 11¼, leaving 5¼ in; a 7¾ × 11 step leaves 4.9 in, which is why 2×10 stringers almost never pass and why steep, deep stairs use LVL stock or an added 2×4 cleat. Space stringers no more than 16 in apart for 1-in to 1½-in wood treads (three stringers for a 36-in stair, four for 48 in) and 12 in apart for composite treads.

Deck and exterior stairs

Deck stairs follow the same R311.7 limits plus a few of their own. The total rise is deck surface to grade — and grade is rarely level, so set a landing pad (a 36-in-deep concrete pad or pavers on gravel) first and measure to it. Stringers must bear on the pad, not on dirt, and attach to the deck rim with a hanger or a ledger, not toe-nails; cut stringers of pressure-treated 2×12 are good to about 6 ft of span before they need a mid-span post or a 2×4 nailed along the underside. Treads of two 2×6s (11 in) with a ⅜-in gap suit an 11-in tread with no nosing, which the IRC allows at that depth; composite treads need stringers 12 in apart.

Guards are required on the open sides of any stair with more than 30 in of total rise, 34 in high at the nosing with openings a 4⅜-in sphere cannot pass, and a graspable handrail is required on one side of any flight of four or more risers. Draw the whole thing in the deck builder: it places the stair and landing in plan so the run lands where you want, and the lumber list adds the stringers and treads to the deck order.

When the stair will not fit: landings, winders and cost

A straight flight for a 118-in rise needs about 13 ft of run plus a 3-ft landing at each end — 19 ft of floor. When the plan does not have it, the options in order of cost are: steepen within code (10-in treads, 7¾-in risers cut the run to 12 ft 6 in); add a landing and turn (an L-stair needs a 36 × 36 in landing that counts as one tread and splits the run between two walls; a U-stair needs a 36 × 72 in landing and stacks the two runs); winders (pie-shaped treads allowed by R311.7.5.2.1 with at least 6 in at the narrow end and 10 in at the 12-in walk line) that replace the landing in an L-stair and save about 3 ft; or a spiral (26-in minimum clear width, 9½-in riser maximum, 6¾-in treads at the walk line 12 in from the narrow edge, 6 ft 6 in headroom) for a loft or secondary access only. Stair openings in the floor also drive framing cost: the header carries the cut joists and the opening must be about 11 ft long for a 35° stair with 6 ft 8 in headroom.

Costs in 2026: a site-built straight flight of cut stringers with pine treads and risers runs $1,500–$3,500 installed; oak treads and a finished balustrade $4,000–$10,000; a factory-built hardwood stair or a floating steel-stringer stair $8,000–$25,000; a basic deck stair $600–$1,500. The cost to build a house calculator puts a two-story stair in the context of the whole frame, and standard room sizes lists hallway and landing widths that work with a 36-in stair.