STAIR STRINGER
STAIR STRINGER
L = √(rise² + run²)
in
in
RESULT
—
FILL IN ABOVE
IRC R311.7: max riser 7-3/4", min tread 10". This is an estimate — verify with a licensed structural engineer or local building inspector. Not a substitute for engineered drawings.
About this calculator
This stair stringer calculator gives you the four numbers a framer or DIY builder needs before cutting a stringer: unit riser height, unit tread depth, total horizontal run, and the diagonal stringer length. Enter the total rise (finished floor to finished floor), the number of risers you want, and your tread depth, and the calculator returns each value plus an IRC R311.7 code check (max riser 7-3/4", min tread 10", target 2R + T = 24"–25" for comfortable stride). The stringer length is the Pythagorean diagonal — buy a 2x12 at least one foot longer to leave room for the seat cut at the bottom and the plumb cut at the top. ESTIMATE ONLY — verify with your local building department before cutting.
Common questions
What is the IRC code for stair rise and run?
Per IRC R311.7.5: maximum riser height is 7-3/4 inches and minimum tread depth is 10 inches measured nose to nose. The largest riser cannot vary more than 3/8 inch from the smallest within a flight. The "rule of comfort" target for a usable stair is 2R + T between 24 and 25 inches, where R is unit rise and T is unit run. This calculator flags violations on each side of those limits.
How long of a 2x12 do I need for a stair stringer?
Take the diagonal stringer length the calculator returns, round up to the nearest standard length, and add at least 12 inches for the seat cut at the bottom and the plumb cut at the top. A 9-foot rise with a 14-tread layout typically needs a 16-foot 2x12 even though the diagonal is only ~14 ft. Always lay out the cuts on paper before going to the lumberyard. This sizing is an estimate — confirm with your local building inspector before cutting.
Should I use 2 or 3 stringers?
For typical 36-inch-wide residential stairs, 3 stringers are recommended (one per edge plus a middle stringer). For wider stairs (over 36"), add an extra stringer per 16 inches of width. 2x12 is the standard size; 2x10 is allowed on shorter runs but leaves only ~3-1/2" of throat after the cuts, which gets fragile. Always verify spacing and stringer size with your inspector and engineered drawings.