Stair Calculator: Risers, Run, Stringer, and Code

How the stair calculator works

Total rise is the only fixed measurement. It runs finished floor to finished floor and never bends to suit a nicer riser number.

Divide total rise by the riser height you want and round to a whole riser. Then divide total rise by that count again. The second division is the one that matters: it gives the height you cut and makes every riser identical. Rounding the height itself and eating the difference on the last step fails inspection.

Worked example: 109 inches of total rise

Nine foot ceilings with a joist and floor assembly land near 109 in. At a 7.5 in target, 109 over 7.5 is 14.53, rounding to 15 risers. Divide back: 109 over 15 is 7.27 in per riser, under the 7 3/4 in cap.

Fifteen risers means 14 treads. At 10.5 in each the run is 147 in, or 12 ft 3 in. The stringer is the square root of 109 squared plus 147 squared: 183 in, so a 16 ft 2×12 cuts one.

SimplyWise Cost Estimator turns that framing into a priced, itemized estimate from one photo.

Common floor to floor heights

Target riser 7.5 in, tread 10.5 in.

Watch the jump from 108 to 109 in. One inch adds a whole riser and another 10.5 in of run.

What the stair code actually says

These limits come from the residential code stair sections, published in free municipal handouts. A riser tops out at 7 3/4 in. A tread runs 10 in minimum, measured between the leading edges of adjacent treads. Neither may beat the smallest by more than 3/8 in.

The stair is 36 in wide minimum. Headroom is 6 ft 8 in, measured down from the sloped line touching each nosing. That one catches remodels, where the opening above is already framed.

Nosing gets skipped. On solid risers it projects 3/4 in to 1-1/4 in, and at 11 in treads it is not required. It adds nothing to the run, so subtract it from tread depth when you mark the stringer.

Cutting the stringer

For exterior stairs the American Wood Council deck guide is prescriptive: stringers are 2×12 minimum, a cut stringer stair needs three, and spacing tops out at 18 in on center. A 60 in stair takes five.

The same guide requires an intermediate landing once stairway height passes 12 ft on an exterior stair. Interior stairs get more room: the code section on vertical rise allows 151 in between floors or landings in the current edition, 147 in in older ones. The calculator holds the stricter 12 ft line, so a 150 in rise reads as a landing job.

Lay the stairs out in this order

1. Measure total rise on finished surfaces

   Floor to floor, finish included. Forgotten 3/4 in hardwood spreads across every riser.

2. Set the count, then the height

   Round to a whole riser, then divide the rise back by it. Never round the height itself.

3. Check the run against the room

   Run is treads times tread depth. A deeper tread makes a tight run worse.

4. Confirm headroom before you cut

   Measure 6 ft 8 in down from the sloped line touching each nosing. The opening above is costly to move.