Web calculator

Stair stringer calculator

Lay out a straight run of stairs from the total rise. The estimate shows riser count, riser height, total run and the diagonal stringer length.

Estimate · rounded to 1/16″

16 risers at 6 ¾″

Risers: 16
Riser height: 6 ¾″
Treads: 15
Total run: 12′ 6″
Stringer length: 15′ 4 13/16″Diagonal, before any cuts or drop.

IRC maximum riser 7 ¾″: 6 ¾″ riser
IRC minimum tread 10″: 10″ tread

Exact to 1/64″ in the app

Web estimates round lengths to 1/16″. Nothing you type is stored or sent anywhere. Span64 works to an exact 1/64″ on iPhone, iPad and Android.

The formula

The riser count is the total rise divided by your target riser, rounded up: 108″ ÷ 7″ = 15.4, so 16 risers. Dividing the rise back by that count and rounding up gives the actual riser height (108″ ÷ 16 = 6 3/4″). A straight run has one tread fewer than risers, so the total run is (risers − 1) × tread depth and the stringer is the hypotenuse of total run and total rise.

The pass and warn lines apply the IRC residential limits: a riser of 7 3/4″ maximum and a tread of 10″ minimum. Local codes can be stricter, so check yours before cutting.

The web calculator is an estimate rounded to 1/16″. Span64 runs the same job on exact 1/64″ arithmetic, keeps a Tape of every result and works with no signal.

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