Stair Stringer Calculator
Riser, tread and stringer layout with code checks — including the bottom cut that makes the difference between an even flight and a violation.
The cut everyone forgets
The bottom of the stringer has to be cut down by the thickness of the tread. Mark a stringer as a row of equal triangles and the notches are perfectly spaced relative to each other — but the treads sit on top of them. So the first step measures a full tread thickness too tall. And because the top of the flight is fixed by the upper floor, the last step comes out a tread thickness too short. One omission, two errors, pointing in opposite directions.
Which makes it a code violation rather than a blemish. Both the residential and the commercial codes cap the difference between the tallest and shortest riser at ⅜″. Skipping the drop puts twice the tread thickness between the first riser and the last — 1½″ on ¾″ treads, four times the limit; 3″ on 2× treads, eight times it. Uneven risers are the most reliable way there is to make someone fall on a staircase, which is exactly why the tolerance is written that tight.
And a standard stair cannot be cut from a 2×10. A 7½ × 10 notch removes 6″ measured square to the board — not 7½″, because the cut is perpendicular to the stringer rather than vertical. A 2×10 is 9¼″ wide, so it keeps 3¼″ of throat, under the 3½″ minimum. The same notch in a 2×12 leaves 5¼″. It is worth knowing before the timber is bought rather than after.
- There is always one more riser than tread. The upper floor is itself the last tread, which is why the counts differ by one and why the run is shorter than people expect.
- The riser count is forced up, not rounded. One riser fewer means every riser exceeds the maximum, so there is no judgement call — the arithmetic decides it.
- Flooring laid on the lower landing afterwards reduces the drop, one for one, because it raises the floor back up. If it is thicker than the treads, the stringer needs packing up instead.
- Blondel's rule: 2 × riser + tread should land near 25″. It captures the way a stride shortens as you climb, and it is why a stair can satisfy every individual code limit and still walk badly. A 7¾″ riser on a 10″ tread — both exactly at the residential limits — comes to 25½″, at the steep edge of comfortable. Compliance is a floor, not a target.
- Measure the total rise at both ends of the opening. Floors are rarely as level as they look, and any discrepancy lands on a single riser rather than spreading itself out politely.
How to use
- Measure the total rise finished floor to finished floor, at both ends of the opening.
- Pick the code that applies and the tread depth you want to build to.
- Read the drop figure — it is the cut that keeps the first and last risers even.
- Check the throat remaining in your stringer stock before buying the timber.
Frequently asked questions
Why do you cut the bottom of a stair stringer shorter?
Because the treads sit on top of the notches. Mark a stringer as a row of equal triangles and the notches are perfectly spaced relative to each other, but every horizontal surface gains a tread thickness — so the first step measures a full tread thickness too tall. Cutting that thickness off the bottom brings the first step back to the same height as all the others. It is usually called dropping the stringer, and it is the step people miss.
What happens if I forget to drop the stringer?
You get two errors from one omission, and they point in opposite directions. The first riser comes out a tread thickness too tall, and because the top of the flight is fixed by the upper floor, the last riser comes out a tread thickness too short. So the spread between the tallest and the shortest riser is twice the tread thickness — an inch and a half on three-quarter inch treads, three inches on 2x treads.
How much can risers vary in height?
Three eighths of an inch between the tallest and the shortest, in both the residential and the commercial codes. That is a tight tolerance for carpentry, and deliberately so: people climb stairs by feel rather than by looking, and an unexpected riser height is the most reliable way there is to make someone fall. Forgetting the stringer drop misses the limit by four to eight times depending on the tread material.
What are the maximum riser and minimum tread?
Under the residential code, a 7¾ inch maximum riser and a 10 inch minimum tread. Under the commercial code it is 7 and 11 — which is where the familiar "seven and eleven" rule comes from. The commercial numbers make a noticeably easier stair, and they are worth considering even on a house, though they cost close to three extra feet of floor length on a typical storey.
How many risers do I need?
Total rise divided by the maximum riser, rounded up — and it really is forced up rather than rounded, because one riser fewer means every riser in the flight exceeds the limit. A 9 foot floor-to-floor is 108 inches, which needs 14 risers of 7.714 inches under the residential code. There is always exactly one more riser than tread, because the upper floor is itself the last tread.
Can I cut a stair stringer from a 2x10?
Not for a standard stair. A 7½ by 10 inch notch removes 6 inches measured square to the board — not 7½, because the cut is perpendicular to the stringer rather than vertical. A 2x10 is 9¼ inches wide, so it keeps only 3¼ inches of throat, under the 3½ inch minimum usually required. The same notch in a 2x12 leaves 5¼ inches. It is worth working out before the timber is bought.
What is the throat of a stringer?
The material left behind the deepest point of the notch, measured square to the board rather than vertically. It is what actually carries the stair, since the sawtooth removes a surprising amount of the depth. Three and a half inches is the usual minimum, though some jurisdictions ask for five, and an engineered stringer or a housed stringer changes the question entirely.
Does flooring on the bottom landing change the stringer drop?
Yes, one for one. Tile or wood laid on the lower landing after the stringer is set raises the floor by its thickness, so the drop reduces by exactly that much. If the finish is thicker than the treads — thick tile against thin treads, say — the drop goes negative and the stringer needs packing up rather than cutting down. Either way it lands entirely on the first riser, so it is worth settling before you cut.
What is Blondel’s rule?
Twice the riser plus the tread should come to about 25 inches. It encodes the way a stride shortens as you climb, and it is the one measure of a stair that is about walking rather than about compliance. It matters because a stair can meet every individual code limit and still feel wrong: a 7¾ inch riser on a 10 inch tread, both exactly at the residential limits, comes to 25½ — the steep edge of comfortable. Code compliance is a floor, not a target.
How much floor space does a staircase need?
More than people expect, and it is the number that decides whether the stair fits at all. A 9 foot floor-to-floor with 10 inch treads needs 13 treads, which is nearly 11 feet of run — before any landing at either end. Under the commercial code the same rise takes 13¾ feet. Working this out first often changes the plan, because the run cannot be shortened without steepening the stair past the limits.
Where do I measure the total rise from?
Finished floor to finished floor, and measure it at both ends of the opening rather than once. Floors are rarely as level as they look, and any discrepancy does not spread itself politely across the flight — it lands on a single riser, usually the first or last, which is exactly where the uniformity rule is least forgiving.
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