How long does a stair stringer need to be?
Stringer length is the hypotenuse of the rise and run triangle: √(total rise² + total run²). A 57-inch total rise divides into 8 risers of 7⅛ inches, which gives 7 treads. At 10½ inches each that is a 73.5-inch run, so the stringer measures √(57² + 73.5²) = 93 inches, or 7 feet 9 inches. Buy a 10-footer — you lose material at both the plumb cut and the level cut, and a 8-foot board leaves you nothing.
How many you need is set by the tread material, not the stair. Two-by decking spans 24 inches, 5/4 board wants 16 inches, and almost every composite tread manufacturer requires 12 inches on centre for stairs even when the decking itself spans 16. Divide the stair width by that spacing and add one.
The number nobody checks is what is left of the board after you cut the notches. On a 2×12 with a 7-inch riser and an 11-inch tread, the notch removes about 5.9 inches measured square to the edge, leaving 5.35 inches. That clears the 5-inch minimum in AWC DCA 6. Push the tread to 12 inches and the throat drops below 5 and the stringer is no longer code-compliant.
How the calculation works
Risers = total rise ÷ target riser, rounded to a whole number, because you cannot build a fraction of a step. Actual riser = total rise ÷ that whole number, and every riser must be within 3/8 inch of every other one — IRC R311.7.5.1.
Treads = risers − 1 whenever the top step lands flush with the deck surface, which is the normal detail. Total run = treads × tread depth.
The uncut depth is the part most calculators skip. The notch cuts into the board by rise × run ÷ √(rise² + run²), measured perpendicular to the board edge — that is the altitude of the cut triangle, not the riser height. Subtract it from the board width (11¼ in for a 2×12, 9¼ in for a 2×10) to get what is left carrying the load.
Comfort is checked with the Blondel rule: 2 × riser + tread should fall between 24 and 25 inches. Outside that band a stair meets code and still feels wrong, which is what people mean when they say a staircase is "steep" or "shuffly".
Typical stair stringer cost, mid-2026
| Material | Unit | Typical range |
|---|---|---|
| 2×12 pressure-treated, 8 ft | each | $18.00 – $38.00 |
| 2×12 pressure-treated, 12 ft | each | $28.00 – $60.00 |
| 2×10 pressure-treated, 8 ft | each | $14.00 – $30.00 |
| Pre-cut stringer (3–7 step)Fixed 7½ × 10½ geometry | each | $20.00 – $55.00 |
| Stair tread, 2×6 PT | per lf | $1.80 – $4.00 |
| Composite stair tread | per lf | $5.00 – $12.00 |
| Stringer hangers / angle | each | $4.00 – $12.00 |
| Labour to cut and hang | per stringer | $45.00 – $120 |
Pre-cut stringers are only worth it when your rise happens to divide evenly by their fixed riser height. It almost never does, and shimming the bottom to make it fit is how you end up with one short step at the top.
What goes wrong on this job
- Measure total rise to finished grade, not to dirt. A 4-inch landing pad you have not poured yet is most of a riser, and the top step is the one people trip on.
- Cut one stringer, then use it as the template for the rest. Marking each one independently from the framing square guarantees they will differ by an eighth somewhere.
- Drop the bottom riser by the thickness of the tread stock. Skip this and every riser is right except the first, which is now a full tread thicker than the others and fails the 3/8-inch uniformity rule.
- If the throat comes out under 5 inches, do not shave it and hope. Switch to solid stringers with cleats or steel angle — a notched 2×12 with a 4-inch throat is a hinge, and it fails in the middle of the flight.
- Three stringers is the practical minimum on any stair a person uses, even a 36-inch one. Two flex, and the bounce is what makes a set of stairs feel unsafe long before it is.
Frequently asked questions
How do I calculate stringer length?
Take the square root of (total rise squared + total run squared). For a 57-inch rise and a 73.5-inch run that is √(3249 + 5402) = 93 inches, or 7 ft 9 in. Buy the next even board length up so you have material for the plumb and level cuts.
How many stringers do I need for a 4 ft wide stair?
Four for 2× decking at 24 inches on centre, four for 5/4 at 16 inches, and five for composite at 12 inches. Divide the width by the spacing and add one.
What size board is a stair stringer?
A 2×12 in almost every case. Its 11¼-inch width is what allows a normal 7-inch riser and 11-inch tread to be notched and still leave the 5 inches of uncut material that AWC DCA 6 requires. A 2×10 only works with shallow treads.
How far apart should stair stringers be?
Up to 24 inches on centre for 2× treads, 16 inches for 5/4 board, and 12 inches for most composite treads. Check the tread manufacturer — composite stair spans are always tighter than their deck spans.
Why is my bottom step shorter than the others?
The bottom riser has to be cut down by the thickness of the tread material, because the first tread sits on top of the stringer while every other one sits inside a notch. Forget it and the first step ends up an inch and a half taller than the rest.