Stair calculator

How to calculate stairs from the total rise

Every stair starts with one number: the total rise, the height from the finished surface at the bottom to the finished surface at the top. Everything else follows from it. Here is the math with the values the calculator opens with: a 36 inch rise, risers up to 7½ inches, 10½ inch treads and a 36 inch wide stair.

1. 1

   Count the risers

   Divide the rise by the riser height you are aiming for and round up. 36 ÷ 7.5 = 4.8, so 5 risers.

2. 2

   Find the exact riser height

   Divide the rise by the number of risers: 36 ÷ 5 = 7.2 inches, a hair under 7 3/16 on the tape. Every riser must match, so use this number, not the target.

3. 3

   Treads are one fewer than risers

   The top floor or deck is the last step, so 5 risers need 4 treads. At 10½ inches each, the stairs run 42 inches, or 3 ft 6 in, out from the top.

4. 4

   Stringer length and angle

   The stringer is the long side of a right triangle: √(42² + 36²) = 55.3 inches, about 4 ft 7 in. Buy the next board size that leaves room for the end cuts: an 8 ft 2x12. The angle is the arctangent of 7.2 ÷ 10.5, or 34.4 degrees.

5. 5

   Check comfort and code

   Two risers plus one tread is 2 x 7.2 + 10.5 = 24.9 inches, inside the comfortable 24 to 25 inch band. The riser is under 7¾ inches and the tread is over 10, so it meets the IRC. With 5 risers it also needs a handrail.

For a 36 inch wide stair the calculator lists 3 stringers from 8 ft 2x12s, three 12 ft 2x6s cut into eight tread pieces (four 36 inch pieces will not fit in a 12 ft board once you allow for saw cuts), connectors, screws and a concrete pad at the bottom. At the default prices that comes to $217.

Building code limits for stairs

Most US towns build to the International Residential Code. These are its main rules for stairs in and around a house (section R311.7, plus R312 for guards). Your town may have amended them, and decks often follow the American Wood Council's DCA 6 guide too.

The 3/8 inch rule catches the most common mistake: a bottom step that ends up taller or shorter than the rest because the landing was poured later, or the floor finish was added after the stringers were cut. Measure the rise to the finished surfaces, and if a slab or flooring is still to come, account for it now.

Steps for common heights

Risers up to 7½ inches and 10½ inch treads. Tap a height to load it into the calculator.

The 108 inch row is a typical basement or second-floor stair: 8 ft walls plus floor framing. Indoors, the stringers are usually fastened to a wall or framed with a mid-span support, and the long run is why stairs take so much floor space. Plan the opening above for 6 ft 8 in of headroom over every nosing.

Laying out and cutting a stringer

• Use stair gauges. Clamp a pair of stair gauges (small brass buttons) on a framing square at the riser height and tread run. Every triangle you trace will then be identical.
• Drop the bottom by one tread thickness. Once treads go on, the first step would be taller than the rest. Cut the thickness of the tread (1½ inches for a 2x6) off the bottom of the stringer so the first riser comes out right.
• Keep enough wood behind the cuts. The uncut strip of a 2x12 below the notches, called the throat, carries the load. Deck guides call for at least 5 inches. Finish the inside corners with a handsaw instead of overcutting with a circular saw.
• Cut one, test it, then copy it. Set the first stringer in place, check it with a level, then use it as the template for the others.
• Mind the span. The DCA 6 deck guide limits a cut 2x12 stringer to about 6 ft of horizontal span. Longer runs need a post and beam under the middle or solid stringers with tread hangers.
• Space the stringers. No more than about 18 inches apart: 3 stringers for a 36 inch stair, 4 for 48 inches. Composite treads often need closer spacing; check the maker's rules.

What stairs cost

Material for simple treated-wood deck stairs is modest: the default 5-riser stair comes to about $220. Inch Calculator's cost guide puts a contractor-built set of eight 48 inch wide deck steps with four stringers at $1,380 to $2,520 installed, which works out to roughly $170 to $315 per step, with labor at 40% to 60% of the bill. Railings add about $40 to $60 per linear foot for wood or metal. Finished interior stairs with hardwood treads, a skirt board and balusters cost far more and are usually a carpenter's job.

Deck and porch stairs are a fair DIY project if you can cut an accurate stringer. Hire out stairs that need a landing, a winder or a turn, interior stairs that change the floor framing, and anything where the inspector wants plans. A bad stair is a tripping hazard every day, so if your measurements don't come out even, slow down and recheck before cutting.

Common questions

Sources and assumptions

Riser, tread and stringer numbers are plain geometry. Code limits come from the model code most US towns adopt; prices are October 2026 list prices and can be edited.

• Stair rules (riser, tread, nosing, width, headroom, flight rise, handrails): 2021 International Residential Code, section R311.7; guards over 30 in: section R312.1. Local amendments can be stricter.
• Deck stair framing (stringers no more than 18 in apart, cut stringers spanning no more than about 6 ft, a throat of at least 5 in): the American Wood Council's DCA 6 Prescriptive Residential Wood Deck Construction Guide, also written into rules such as Wisconsin's deck stair code.
• Lumber prices: October 2026 list prices at Menards: 2x12x12 treated $42.95 and 2x6x12 $19.48. Stringer boards of other lengths are priced at the same cost per foot.
• Installed cost ($1,380 to $2,520 for eight 48 in steps, labor 40% to 60%, railing $40 to $60 per foot): Inch Calculator, deck stairs cost guide. The page was last revised in 2023, so treat the range as a floor.
• Fixed prices (estimates): a stringer connector and structural screws at $8 per stringer, exterior screws at $10 a box per 90 screws, 1x8 treated riser boards at $16 per 12 ft, and 80 lb concrete bags at $6.48 with 0.6 cu ft yield, as in our concrete calculator.
• Assumptions: stringer boards are bought at least 12 in longer than the stringer for end cuts, in 8 to 20 ft lengths; treads are two 2x6 boards with a ¼ in gap, an 11¼ in tread that leaves a ¾ in nosing on a 10½ in run (three boards when the run is over 11¼ in); the comfort rule (2R + T of 24 to 25 in) is a design guideline, not code.