Stair Stringer Calculator
Calculate rise, run, stringer length and tread/riser dimensions for any straight stair - to IRC code.
The stair stringer calculator returns rise, run, stringer length and IRC code compliance for any straight stair. Enter the total rise (the distance from the lower floor to the upper floor) and the calculator divides it into equal risers within IRC limits (max 7.75 inches per riser). Treads default to 11 inches (matching a 2x12 board minus the nosing) but can be set to 10-11.25 inches. The stringer length is the diagonal cut length from the top to the bottom of the stair - this is the lumber length you need to buy, typically 2x12 pressure-treated for deck stairs or kiln-dried 2x12 for interior. The 2R + T ergonomic check confirms the stair feels right - 24 to 25 inches is comfortable.
Explore all our interior finishes calculator tools, or browse the full construction & home improvement diy calculators.
Frequently asked questions
Stringer length = sqrt(total_rise^2 + total_run^2). For a standard 9 ft floor (108 inch total rise) with 14 risers at 7.7 inches each and 13 treads at 11 inches each, total run = 143 inches, stringer = sqrt(108^2 + 143^2) = 179 inches = about 15 ft. Buy a 16 ft 2x12 to allow for cuts and the top/bottom plumb cuts. The longest stock 2x12 is typically 16 ft.
Maximum riser height: 7.75 inches. Minimum tread depth: 10 inches. Maximum variation between risers in a single stair: 3/8 inch (riser-to-riser must be consistent). Minimum stair width: 36 inches. Minimum headroom: 6 ft 8 inches. Handrails required on stairs with 4+ risers, between 34-38 inches above the nosing.
For a 9 ft floor (108 inch total rise) targeting 7.5 inch risers: 108 / 7.5 = 14.4, round to 14 risers. Actual riser = 108 / 14 = 7.71 inches (within IRC max of 7.75). 14 risers means 13 treads. With 11 inch treads, total run = 143 inches = 11.9 ft. You need 11.9 ft of horizontal space for the stair plus landing space top and bottom.
2R + T = 24-25 inches is the ergonomic comfort range, where R is riser height and T is tread depth. A 7 inch riser pairs with 11 inch tread (7+7+11 = 25). A 7.75 inch riser pairs with 9.5 inch tread (24.75) but 9.5 inch fails IRC minimum tread depth. The rule of thumb keeps stairs feeling like a natural walking stride - too shallow and you trip; too steep and you have to climb consciously.
2x12 is standard for any stair up to about 14 risers. Cuts notch about 6-7 inches into the 11.25 inch board depth, leaving the minimum 5 inches of solid wood (the throat) required by IRC. For longer stairs, sister two 2x12s together or use 2x14 (less common). Pressure-treated for exterior deck stairs; kiln-dried Doug fir or SPF for interior. Always use 3 stringers minimum for a 36 inch wide stair.