Stair building calculator

Stringers: for interior residential stairs, three 2x12 stringers are standard up to about 36 in width; wider stairs or heavier loads often call for a fourth stringer in the middle. Confirm spacing with local code or an engineer for anything unusual.

Treads: one board per step, cut to the stair width plus nosing overhang on each open end. Tread stock is commonly 1 1/16 to 1 1/2 in thick hardwood or engineered tread material.

Risers: one board per step (unless open-riser), typically 1/2 to 3/4 in plywood or MDF for a paint-grade stair, matched to the riser height.

Skirt boards: if the stair runs against a wall, a skirt board follows the stringer profile on that side, usually cut from 1x10 or 1x12 stock the length of the stringer.

Cutting and assembling stairs in the wrong order causes most of the rework. Lay out and cut one stringer first, test-fit it in the opening, then use it as a pattern for the rest.

1. Lay out riser and tread marks on the first stringer using a framing square and stair gauges set to your riser and tread numbers.
2. Cut the first stringer, then drop the bottom by one riser thickness so the first step matches the others once installed.
3. Test-fit the stringer in the actual opening before cutting the rest — check top riser height, run, and headroom.
4. Trace the fitted stringer onto the remaining stock and cut the matching stringers.
5. Install stringers level and square, fasten to the header and floor per your framing plan.
6. Install risers first (if closed), then treads over the risers, working bottom to top.
7. Add skirt boards and any trim last, once treads and risers are fastened.

Forgetting to drop the stringer is the single most common error — it leaves the bottom riser one full riser height taller than the rest, which is both a code problem and a trip hazard.

Getting the top riser wrong at the header is the second most common — measure to the finished floor level, not subfloor, and account for any flooring thickness that hasn't been installed yet at the time of framing.

Not accounting for finish flooring at the bottom has the same effect in reverse — if 3/4 in of flooring goes down after the stringers are set, the bottom riser ends up 3/4 in short unless it was planned for.

Cutting all stringers before test-fitting one is a fast way to waste a sheet of plywood if the opening doesn't match your assumptions.

Most jurisdictions inspect stairs at framing and again at final. Framing inspection commonly checks stringer count and fastening, riser and tread consistency, and headroom. Final inspection commonly checks handrail height, guard height and baluster spacing.

Because requirements vary by jurisdiction, treat any specific number here as a starting point for your own check with the local building department, not a guarantee of passing inspection.

A straight interior stair with standard framing is a reasonable DIY project for an experienced carpenter. A stair with a landing, winders, an open-riser design, or structural spans beyond standard 2x12 stringer capacity is worth having an engineer or licensed contractor review before you build.