Stair Making Calculator: Planning a Build from Rise to Runner

The four numbers every stair build starts with

Before you cut a single stringer, a stair build comes down to four numbers: total rise (floor to floor height), total run (horizontal distance available), riser height (height of each step), and tread depth (also called going, the horizontal depth of each step). A stair making calculator takes your total rise and total run, then solves for the number of steps and the size of each riser and tread that will fit within your space while meeting code.

Step one: measure total rise and total run

Total rise is measured plumb from the finished floor below to the finished floor above. Total run is the horizontal distance available for the staircase, which is often the limiting factor in tight spaces. For our worked example, total rise is 105 in and total run is 130 in, giving a rough overall pitch angle once we divide the two.

Step two: solve for riser count and height

Divide the total rise by a target riser height near 7.5 in: 105 / 7.5 = 14 risers exactly, which is unusually tidy but real designs rarely land this cleanly on the first try. If the division does not come out even, round to the nearest whole number of risers and recalculate: for example, 105 / 14.3 risers would round to 14, giving 105 / 14 = 7.5 in per riser, right at the edge of comfort and just under the IRC's 7.75 in cap.

Step three: solve for tread depth

The number of treads is always one less than the number of risers, because the top riser lands on the upper floor itself rather than a tread. With 14 risers, you have 13 treads. Divide total run by 13: 130 / 13 = 10 in per tread, a comfortable, standard depth. Check this against the comfort formula 2R + T: 2(7.5) + 10 = 25, right at the top of the ideal 24-25 in range, confirming this is a well-proportioned staircase.

Step four: lay out the stringer

With riser height and tread depth fixed, mark your stringer using a framing square set to 7.5 in on the tongue and 10 in on the body, stepping it along the board 13 times for the treads and marking 14 risers. Remember that the bottom riser must be shortened by the thickness of the tread material (since the first tread sits on the floor, not on another tread) and the top of the stringer must land flush with the upper floor framing. This is the step where DIY builders most often make a small, cascading measurement error, so double-check every mark before cutting.

UK Building Regulations for a self-build stair

Under Approved Document K, a private stair must keep risers between 150-220 mm and use the 2R + G rule with a result between 550-700 mm. Our worked metric example of 19 cm risers and 25 cm treads gives 2(19) + 25 = 63 cm, comfortably within range. Pitch (the angle of the staircase) must also not exceed 42 degrees for a private stair, which you can check by taking the arctangent of total rise divided by total run.

Sizing a runner for your new staircase

Once your stair is built, sizing a runner is the last calculation. Using our 13-tread example: 10 in tread + 7.5 in riser + 1 in nosing wrap = 18.5 in per step, times 13 steps = 240.5 in (20 ft), plus a finishing allowance of about 24 in brings it to 264.5 in (22 ft), and with a 10% waste margin, order roughly 24 ft. In metric: 25 cm + 19 cm + 2.5 cm = 46.5 cm per step, times 13 = 604.5 cm, plus 60 cm finishing = 664.5 cm, plus 10% waste, order about 7.3 m. A stair runner length calculator will do this arithmetic instantly once your final tread and riser numbers are locked in from the build.

• Measure total rise and total run before choosing a riser count.
• Divide rise by a trial riser height, round to a whole number, and recalculate exactly.
• Confirm tread depth against the 2R + T comfort formula.
• Cut and test-fit one stringer before cutting the rest.
• Run final tread and riser numbers through a stair runner calculator for the finish.

When space is tighter than ideal

If your available total run is shorter than a comfortable stair demands, you have a few options: reduce tread depth slightly (down to a 9-9.5 in minimum under most codes, though this feels steep), add a winder or landing to change direction and reclaim run, or consider a steeper but still code-compliant pitch. Whatever you choose, always recheck your final numbers against your local code's minimum tread depth and maximum riser height before finalizing the stringer layout.

Sizing the stringer timber for this build

With a 105 in total rise and 130 in run cut into 14 risers at 7.5 in and 13 treads at 10 in, the notch depth removed from each stringer board is significant enough that 2x12 construction-grade lumber is the right default, since it leaves roughly 5 in of throat above the deepest notch corner, comfortably above the 3.5-5 in minimum most codes expect. For a 36 in wide flight, three stringers spaced 16 in on centre carry the load; if you widen the stair beyond 36 in, add a fourth stringer or tighten spacing to 12 in on centre rather than relying on just three boards across a wider walking surface.

Formula recap for a repeatable build process

The full build sequence reduces to four formulas you can reuse on any project: riser count = round(total rise / target riser height); riser height = total rise / riser count; tread count = riser count - 1; tread depth = total run / tread count. Run these in order every time and you will never end up with an odd, non-code riser height. For a second example, a 96 in rise over 110 in run: 96 / 7.3 = 13.15, round to 13 risers; 96 / 13 = 7.38 in per riser; 12 treads; 110 / 12 = 9.17 in per tread, which fails the 10 in IRC minimum, telling you immediately that this run needs a landing or a wider footprint before the layout will pass.

• Riser count = round(total rise / target riser height, usually 7-7.5 in or 175-190 mm).
• Riser height = total rise / final riser count.
• Tread count is always riser count minus one.
• Tread depth = total run / tread count, then check against the 10 in / 220 mm minimum.
• If tread depth fails, add a landing, widen the footprint, or accept a steeper (but still legal) pitch.

Whatever you decide, get the numbers first: the stair runner calculator doubles as a stair runner length calculator, adding nosing wrap, landings, winders, pattern repeat and waste so you order the right length the first time.

Still deciding on material, width or pattern? Our hub on stair runners compares weaves, widths, backings and fitting styles side by side, then hands the numbers straight to the calculator.