Key takeaways
- Riser count = total rise ÷ target riser, rounded to a whole number.
- The actual riser height is the total rise divided back by that count — so every step is equal.
- Treads are always one fewer than the risers; total run = treads × tread depth.
- IRC residential limits: riser ≤ 7¾″, tread ≥ 10″, all within 3/8″ of each other.
How to lay out stairs
Stair rise is the vertical height of each step and run is the horizontal tread depth; the number of risers = total rise ÷ target riser height. Start from the total rise — the finished-floor to finished-floor height — and divide it into equal risers near the comfortable 7–7.75″ range, then round to whole risers and solve the rest of the flight from there.
Equal risers matter most — an uneven last step is both a code fail and a trip hazard. The stringer length is the diagonal you'll lay out and cut from the framing board.
Worked example: a 108″ rise
For a 108″ total rise with a 7.5″ target riser and 10.5″ treads: 108 ÷ 7.5 = 14.4, which rounds to 14 risers. The actual riser height is 108 ÷ 14 = 7.71″ — under the 7¾″ limit. That leaves 14 − 1 = 13 treads, a total run of 13 × 10.5 = 136.5″, and a stringer of √(108² + 136.5²) = 174.1″.
IRC residential stair limits
| Dimension | IRC residential limit |
|---|---|
| Max riser height | 7¾ in |
| Min tread depth | 10 in |
| Min headroom | 6 ft 8 in |
| Max riser/tread variation | 3/8 in |
Sizing the stringer board
Cut stringers from 2×12 stock so enough wood remains below the deepest notch. The stringer length above is the diagonal you mark with a framing square; add length for the top and bottom connections. To convert that board run into lumber volume for ordering, run the dimensions through the board foot calculator.
Frequently asked questions
How do I calculate stairs?
Total rise ÷ ~7.5″, rounded, gives the riser count; actual riser = rise ÷ count; treads = risers − 1.
What's the code for rise and run?
IRC R311.7: riser ≤ 7¾″, tread ≥ 10″, headroom ≥ 6′8″, and all risers/treads within 3/8″ of each other. Confirm locally.
How many steps for an 8 foot rise?
96″ ÷ 7.5″ = 12.8 → 13 risers, so each riser is 96 ÷ 13 = 7.38″, with 12 treads.
How do I find the stringer length?
Stringer ≈ √(total rise² + total run²). A 108″ rise and 136.5″ run give about 174.1″.
How many treads do I need?
One fewer than the risers — the top floor or landing is the last step. 14 risers means 13 treads.
Why must all risers be equal?
Uneven risers cause falls and fail inspection; the IRC caps the variation in a flight at 3/8″, so split the rise into equal parts.
Stair limit figures are from the International Residential Code, Section R311.7 — IRC R311.7. Always verify against your locally adopted code.
Last reviewed June 2026