Why do I have one more riser than tread?

The upper floor is the last tread. A stringer climbing to a landing has the same number of risers as steps but one less tread, because you step off onto the finished floor rather than onto a tread you cut.

Why do stair stringers have to be 2x12?

After notching, the remaining material measured perpendicular from the notch corner to the bottom edge is called the throat, and it should stay at or above five inches. At a 7.5 inch rise and 10 inch run a 2x12 leaves 5.25 inches of throat. A 2x10 leaves only 3.25 inches, which is not enough.

How much do I cut off the bottom of a stair stringer?

Cut the bottom down by the thickness of the finished tread material. Without that adjustment the first step is one tread thickness taller than every other step, which fails the three eighths inch uniformity rule.

What is the maximum riser height for stairs?

Residential stairs under the IRC allow a maximum riser of 7.75 inches with a minimum tread depth of 10 inches. Commercial stairs under the IBC are tighter at 7 inches maximum rise and 11 inches minimum tread. Risers within one flight may not vary by more than 3/8 inch.

Stair Stringer Calculator — Rise, Run, Cut Layout & Code Check

Laying it out

Set the framing square to the tongue and blade numbers above using stair gauges, then step off the sawtooth along the crown edge of the 2×12.
Step off the full riser count. The last mark should land at the top of the board — if it drifts, your gauges moved.
Cut the notches with a circular saw, stopping short of the inside corner, and finish every corner with a handsaw. Overcutting the corner is what cracks stringers under load.
Take the bottom cut adjustment off the bottom of the stringer, not the top.
Use the first stringer as the template for the rest. Never lay out each one separately.

The bottom cut, and why the first step is wrong

A stringer stepped off straight from the square puts the notches at equal spacing, but the finished treads sit on top of those notches. Everywhere in the middle of the flight that cancels out. At the bottom it does not, because the stringer sits on the slab and there is no tread underneath. The first step ends up one tread thickness taller than the rest.

The fix is to cut the bottom of the stringer down by the tread thickness. At the top the same logic runs the other way — the last riser is short by one tread thickness unless the upper floor is already finished when you measured, which is why total rise is always finished to finished.

Throat depth

The throat is what is left of the board measured square from the inside corner of a notch to the bottom edge. It is the actual structural section of the stringer, and it is the reason cut stringers are 2×12 and not 2×10.

Stock	Throat at 7½ / 10
2×10 — 9¼″	3¼″	too thin
2×12 — 11¼″	5¼″	standard
2×14 — 13¼″	7¼″	long spans

Five inches is the working minimum most inspectors and framing references expect. Deeper runs eat throat faster than taller risers do, so a 12″ tread on a 2×12 gets tight.

Code limits

	IRC	IBC
Max riser	7¾″	7″
Min tread	10″	11″
Min headroom	6′-8″	6′-8″
Min width	36″	44″
Riser variation	⅜″	⅜″

The ⅜″ uniformity rule is the one that fails inspections. It is measured between the tallest and shortest riser in a single flight, and a mis-cut bottom step blows it on its own.

Questions

Why is there one more riser than tread?

Because the upper floor is the last tread. You step off onto finished floor rather than onto something you cut.

What is a comfortable rise and run?

Two risers plus one tread should land between 24″ and 25″. A 7½″ rise with a 10″ run hits 25″ exactly, which is why it shows up in every framing book.

Can I use a 2×10 if I bolt on a support?

A solid-stock stringer with a cleat or a housed stringer changes the analysis completely, and at that point you are outside rule-of-thumb territory. Ask an engineer rather than guessing at it.