How do you calculate stair risers?
Divide the total floor-to-floor rise by your preferred riser height and round to a whole number — risers must be equal, so the total rise then divides evenly by that count. A 108 in rise with a 7 in target gives 15.4, so you use 15 risers at exactly 7.2 in each. There is always one fewer tread than riser, because the upper floor forms the last step.
What is a comfortable stair rise and run?
US residential code (IRC) caps risers at 7.75 in and requires treads of at least 10 in. Beyond code, the classic comfort rule is 2 × riser + tread ≈ 24–25 in (about 61–63.5 cm). Stairs that satisfy that formula feel natural to climb; ones that miss it feel either cramped or stretched even when they are technically legal.
How do I find the stringer length?
The stringer is the diagonal board the steps sit on, so it is √(total rise² + total run²) — the Pythagorean theorem. For a 108 in rise and 147 in run that is about 182 in. Add extra for the connection at the top and any trimming at the base before you cut.
What angle should a staircase be?
Most comfortable residential stairs land between 30° and 37°. The angle is arctan(riser ÷ tread), so a 7 in riser with a 10.5 in tread gives about 33.7°. Steeper than 40° starts to feel like a ladder and usually breaks code for a main staircase.
How much can the steps vary in height?
Far less than most people assume. Building codes commonly limit the difference between the largest and smallest riser in a flight to around 3/8 of an inch, or roughly 10 mm, and similarly restrict tread depth variation. The reason is that people climb stairs on autopilot after the first two or three steps, with the foot placed by expectation rather than sight — so a single odd step is a trip hazard out of proportion to its size. Check the code that applies where you are, since limits differ.
Why does the bottom or top step end up the wrong height?
Almost always because the calculation used the wrong reference. Total rise must be measured between finished floor levels, not the bare subfloor at each end. If the upper floor gains tile and the lower gains carpet after the stringer is cut, every riser stays equal except the first and last, which absorb the difference — and those are exactly the two that must not be odd. Decide the floor finishes at both ends before cutting, and measure from where the surfaces will actually sit.