One of the most common first-time stair mistakes is cutting every notch equal and ending up with an uneven first and last step. Here is why you subtract a tread thickness from the bottom, exactly how much, and how mounting choice changes the cut.
When you lay out a stair stringer with a framing square, every notch is identical — same riser, same tread. That seems correct, but it ignores one thing: the tread boards sit on top of the notches. Once you install the treads, every step gains the thickness of the tread material. The catch is at the two ends of the flight.
The bottom step rests on the floor with a full tread thickness added on top of the notch, so it finishes one tread-thickness taller than the rest. The top step, meanwhile, finishes flush against the upper floor and ends up one tread-thickness shorter. The result is a flight where the first and last risers are off by the tread thickness — exactly the kind of uneven riser an inspector measures for. The IRC allows no more than 3/8 inch of variation between the tallest and shortest riser in a flight, and a forgotten drop blows straight past it.
The fix, called dropping the stringer, is to cut exactly one tread thickness off the bottom level foot of the stringer before you set it. That single cut shifts the whole stringer down by the tread thickness, so once the treads go on, every finished riser — bottom, middle, and top — is equal.
So the drop is not a fudge factor — it is precisely the finished thickness of whatever tread material you are installing. Measure your actual tread stock (nominal "1 inch" decking is often 1 inch finished; a 2x is about 1.5 inches) and drop by that exact amount.
Say your layout riser is 7.5 inches and you are using 1-inch treads. You cut every notch at 7.5 inches, then trim 1 inch off the bottom foot. After installation: the bottom step rises 6.5 inches of stringer plus 1 inch of tread = 7.5 inches finished. Every interior step is 7.5 inches of stringer notch (the tread on top is offset by the tread you stepped off below). The top step finishes flush with the upper floor at 7.5 inches. All equal — which is the whole point.
How the stringer meets the upper floor decides the top of the cut, and it is the other half of getting the end risers right.
Decide the mount before cutting, because it changes the top riser calculation. Getting the mount and the bottom drop right together is what makes the first and last steps match the middle ones.
The stringer calculator accounts for the tread-thickness drop and the mounting type, and the straight stair calculator reports the finished riser so you can confirm uniformity before cutting. Both keep the first and last steps equal to the rest automatically.
Stair Stringer Layout with a Framing Square · How to Build Deck Stairs (Step by Step) · Rise and Run Explained