Stair Stringer Calculator
Enter the total rise from the ground to the top of the deck or floor. You get equal riser heights, the number of treads, a to-scale stringer diagram, the exact marks to lay out, and a check against the common IRC code limits.
- Tested formulas
- Feet or metric
- Printable shopping list
2 What to buy
Updates as you typeThese dimensions meet the common IRC limits. Always confirm with your local building department.
- ✓ Riser height 7.50" ≤ 7.75" IRC maximum
- ✓ Tread depth 10.5" ≥ 10" IRC minimum
- ✓ Comfort rule (2 × riser + tread = 25.5") is within the 24–25½" sweet spot
- ✓ Remaining stringer throat 5.15" ≥ 5" commonly recommended (some inspectors accept 3½")
- ✓ Stair width 36" ≥ 36" IRC minimum
| Riser height (exact) | 7.5" · 7 1/2" · 19.05 cm |
|---|---|
| Tread depth (run per step) | 10 1/2" · 26.7 cm |
| Bottom riser cut (drop for tread) | 6 1/2". Trim the bottom of the stringer so the first step equals the rest once treads are on |
| Step diagonal (layout increment) | 12 7/8" |
| Stringer length needed | 4' 8 3/8" → buy 8 ft boards |
| Throat remaining | 5 1/8" |
| Handrail | Required (4 or more risers) |
Layout marks along the stringer
Measure from the bottom end of the board along its top edge. Each mark is where a riser line meets the edge.
| Step 1 | 1' 7/8" (32.8 cm) |
|---|---|
| Step 2 | 2' 1 13/16" (65.5 cm) |
| Step 3 | 3' 2 11/16" (98.3 cm) |
| Step 4 | 4' 3 5/8" (131.1 cm) |
Shopping list
Quick answer
Divide the total rise by about 7.5 inches and round up to get the number of risers; treads are one fewer. A 30-inch rise gives 4 risers of 7 1/2" and 3 treads of 10 1/2", cut from 8 ft 2×12s. The IRC allows risers up to 7¾" and treads of at least 10".
Stairs for common heights
| Total rise | Risers | Treads | Total run | Stringer |
|---|---|---|---|---|
| 1' 6" | 3 × 6" | 2 | 1' 9" | 8 ft 2×12 |
| 2' 0" | 4 × 6" | 3 | 2' 7 1/2" | 8 ft 2×12 |
| 2' 6" | 4 × 7 1/2" | 3 | 2' 7 1/2" | 8 ft 2×12 |
| 3' 0" | 5 × 7 3/16" | 4 | 3' 6" | 8 ft 2×12 |
| 3' 6" | 6 × 7" | 5 | 4' 4 1/2" | 8 ft 2×12 |
| 4' 0" | 7 × 6 7/8" | 6 | 5' 3" | 10 ft 2×12 |
| 5' 0" | 8 × 7 1/2" | 7 | 6' 1 1/2" | 10 ft 2×12 |
| 6' 0" | 10 × 7 3/16" | 9 | 7' 10 1/2" | 12 ft 2×12 |
| 8' 0" | 13 × 7 3/8" | 12 | 10' 6" | 16 ft 2×12 |
| 9' 0" | 15 × 7 3/16" | 14 | 12' 3" | 18 ft 2×12 |
Target riser 7½", tread 10½", 1" treads, deck or floor as the top step. Use the calculator above for your exact size.
How stair stringer math works
- Number of risers = total rise ÷ your target riser height, rounded up.
- Exact riser height = total rise ÷ number of risers. Every step must be equal; the IRC allows no more than ⅜" difference between the tallest and shortest riser.
- Treads = risers − 1 when the deck itself is the top step, which is the usual case.
- Total run = treads × tread depth. This tells you where the stairs land.
- Stringer length comes from the Pythagorean theorem: √(rise² + run²), plus a little extra for the cuts.
Worked example: a 30" deck
- 30 ÷ 7.5 = 4 risers of exactly 7 1/2"
- 3 treads at 10 1/2" = total run of 2' 7 1/2"
- Stringer length ≈ 4' 8 3/8", so 8 ft 2×12s; 3 stringers for a 36" wide stair
- Cut 1" off the bottom so the first step is 7 1/2" once the 1" treads are on (bottom riser cut = 6 1/2")
- 4 risers means a handrail is required (4 or more risers)
Laying out the stringer with a framing square
- Clip stair gauges (or tape) on a framing square at the riser height on the tongue (narrow arm) and the tread depth on the blade (wide arm).
- Lay the square on the 2×12 with both gauges touching the top edge and trace the notch. Slide it along to the next mark and repeat for every step. The layout marks table above gives the exact distance of each mark from the end, so small errors don't add up.
- Mark the bottom cut: shorten the first riser by the tread thickness.
- Cut with a circular saw, stopping short of the inside corners, and finish with a hand saw or jigsaw. Overcutting weakens the stringer.
- Use the first stringer as a template for the others after you've test-fitted it.
Code requirements (IRC R311.7)
| Rule | Limit |
|---|---|
| Maximum riser height | 7¾" |
| Minimum tread depth | 10" |
| Max difference between risers or treads | ⅜" |
| Minimum stair width | 36" |
| Handrail required | 4 or more risers, 34–38" high |
| Minimum headroom | 6' 8" |
Most US jurisdictions follow the IRC, but local amendments are common. Commercial stairs follow the IBC (7" max riser, 11" min tread). Always check with your building department before you build.
Frequently asked questions
How do I calculate the number of stairs I need?
Divide the total rise by about 7.5 inches and round up. That gives the number of risers. The number of treads is one less when the deck or floor is the top step. For a 30" rise: 30 ÷ 7.5 = 4 risers and 3 treads.
What size lumber should I use for stair stringers?
Use 2×12 pressure-treated lumber for outdoor stairs. After the notches are cut, a 2×12 keeps enough wood (the "throat") to carry the load. 2×10s only work for very short stairs with small risers.
How far apart should stair stringers be?
At most 18" on center for wood treads. Most composite decking needs 12" spacing on stairs, so check the manufacturer's instructions. A 36" wide stair usually gets 3 stringers, or 4 with composite treads.
Why do I cut the bottom of the stringer shorter?
The tread adds its thickness to every step except the first, which sits on the ground or slab. Cutting the bottom riser down by the tread thickness makes every finished step the same height.
How many stair stringers do I need?
Space stringers no more than 18 inches apart for wood treads (closer for most composite treads), with one at each side. A 36 inch wide stair needs 3 stringers and a 48 inch stair needs 4. The calculator counts them from your stair width.
Last reviewed 2026-10-21. See how we calculate for our sources and assumptions.