Stair Calculator – Riser Height, Tread Depth and Stringer Layout
A stair calculator turns one measurement, the total rise between two floors, into a complete stair design. This one works out how many steps you need, the exact riser height and tread depth, and checks each dimension against the IRC, the IBC or UK Approved Document K. It then draws the stair to scale and gives you the numbers needed to cut the stair stringers: framing-square settings, a step-by-step layout, the stringer length and a cut list.
Step 1: Measure the Total Rise
The total rise is the vertical distance from the finished lower floor to the finished upper floor. Include the flooring on both levels. If it isn't down yet, add its thickness to your measurement. A 1/2 in error in the rise is shared across every step, but an error in one step isn't. That is why all the math starts from this one number.
Step 2: Divide the Rise into Equal Risers
Every riser in a flight must be the same height, so the riser height is simply R = total rise ÷ number of risers. The fewest risers you can use is ⌈rise ÷ maximum riser⌉. For a 108 in rise under the IRC's 7 3/4 in limit that is ⌈108 ÷ 7.75⌉ = 14 risers of 7 11/16 in. Adding a riser makes each step lower and the stair longer. The calculator lists every workable count and recommends the one that passes every rule with a riser closest to your target height, usually around 7 in or 175 mm. With 15 risers, each one is 108 ÷ 15 = 7.2 in (7 3/16 in).
Step 3: Choose a Comfortable Tread Depth
Tread depth, called the going in the UK, is measured nosing to nosing. It is the horizontal distance you move per step, not the width of the tread board. A classic comfort rule links the two: 2R + T ≈ 25 in (or 600–640 mm). In Auto mode, the calculator uses T = 25 − 2R, keeps it within the code limits and rounds it to 1/8 in or 5 mm. For 7.2 in risers that gives a 10 5/8 in tread and a 34.1° stair. There is always one fewer tread than risers, because the top step is the upper floor itself.
Step 4: Check Headroom and the Stairwell Opening
Headroom is measured vertically from the nosing line, the sloped line through the front edges of the treads. The IRC requires 6 ft 8 in and UK rules 2.0 m. The upper floor must be cut back far enough that its joists never dip into this zone. For a straight flight, the opening length measured back from the top nosing is (headroom + floor thickness) × T ÷ R. With 10 in floor framing, 10 1/2 in treads and 7.2 in risers it is 90 × 10.5 ÷ 7.2 = 131 1/4 in. The drawing shades the headroom band so you can see exactly where the opening has to end.
Step 5: Lay Out and Cut the Stringers
Set a framing square with the riser on the tongue and the tread on the blade, then step it along the board once per tread. Before cutting, drop the stringer: trim the bottom by one tread thickness. Once the tread boards go on, the first step ends up the same height as all the others. Check the throat, the solid wood left below the notches. It equals board depth − R × T ÷ √(R² + T²). AWC's DCA 6 deck guide asks for at least 5 in. Use the step layout table to mark positions: each height and offset is rounded on its own, so small rounding errors don't add up over a long flight.
Landings, L-Shaped and U-Shaped Stairs
Codes limit how high a single flight may climb, so tall rises need a mid-flight landing. Turn one on and the calculator splits the stair, sizes each flight's stringers separately and redraws the headroom zone. An L-shaped or U-shaped stair can be designed the same way: treat each leg as a straight flight with the same riser height, and use the landing to turn. Winders and spiral stairs follow different rules and aren't covered here.