Stair Calculator

At 7.5 in desired riser: 108/7.5 = 14.4, round to 14 risers and 13 treads. Actual riser = 108/14 = 7.71 in. Within IRC 7.75 in maximum.

7.75 inches maximum per IRC R311.7. Minimum tread depth is 10 inches. The typical comfortable target is 7 to 7.5 inches.

Stringer length = √(total rise² + total run²). For 108 in rise with 14 risers and 13 treads at 10 in (130 in run): √(108² + 130²) = √(11664 + 16900) = √28564 = 169.0 in (14.1 ft). Buy 16 ft stringers.

A traditional rule: riser + tread = 17 to 18 inches. So with a 7-in riser, use a 10-11 in tread. Another rule: 2 x riser + tread = 24-25 inches.

Most people find 7 to 7.5 inches the most comfortable. Steeper than 7.75 in is tiring; shallower than 6 in requires awkward short steps. The "rule of comfort" is: 2 x rise + run = 24-25 inches. With a 7-in riser, aim for a 10-11 in tread.

A 9-ft ceiling with a 12-in floor structure gives a total rise of about 120 in. At 7 in per riser: 120/7 = 17.1, round to 17 risers. Actual riser = 120/17 = 7.06 in. 16 treads (one fewer than risers) at 10 in = 160 in total run (13 ft 4 in).

Risers are the vertical faces; treads are the horizontal surfaces you step on. A staircase always has one more riser than tread because the top riser leads to the upper floor level which is the last tread.

Use this calculator's stringer length result and add a few inches for waste (the plumb cut at the top and level cut at the bottom). Standard 2x12 lumber is available in 10, 12, 14, and 16-ft lengths - pick the next size above your calculated length.

Use at least three stringers for any stair 36 inches or wider; the center stringer prevents tread bounce. Stairs over 48 inches wide may need four, typically spaced about 16 inches on center. Use pressure-treated 2x12 stringers for exterior stairs, and interior SPF or fir 2x12 where spans stay under about 13 ft. Lay out and notch the stringers only after checking this calculator against your actual lumber.

A closed or housed stringer has the treads and risers routed into the side of the board for a finished look, common indoors and needing more carpentry skill. An open, cut, or sawtooth stringer has triangular notches cut from the top edge to receive the treads and risers, with the end grain exposed from the side. The open type is faster to build and common on exterior and basement stairs. Strength is the same when each is properly sized and supported.