Quick answer: Divide the total rise (floor to floor) by the maximum riser height and round up to get the number of risers. Divide the total rise by that number for the exact riser height. You need one fewer tread than risers. A 108 in rise needs 14 risers of 7.71 in and 13 treads.
Key Terms
- Total rise: vertical distance from the lower floor to the upper floor surface.
- Riser: the vertical part of each step.
- Tread / run: the horizontal depth of each step.
- Total run: horizontal length of the whole staircase.
- Stringer: the angled board that supports the treads.
US Code Limits (IRC)
| Dimension | Residential limit |
|---|---|
| Maximum riser height | 7¾ in |
| Minimum tread depth | 10 in |
| Minimum headroom | 6 ft 8 in |
| Minimum width | 36 in |
Check your local code, which may differ. In the UK, Approved Document K allows up to 220 mm risers on private stairs and requires 2 × rise + going between 550 and 700 mm.
Step-by-Step Example
- Total rise: 108 in (9 ft).
- 108 ÷ 7.75 = 13.9, so round up to 14 risers.
- Riser height: 108 ÷ 14 = 7.71 in.
- Treads: 14 − 1 = 13.
- Total run with 10 in treads: 13 × 10 = 130 in (10 ft 10 in).
- Stringer length: √(108² + 130²) = 169 in, about 14 ft 1 in, so buy 16 ft stringer stock.
Skip the math: enter the total rise and tread depth and it checks the code limits for you. Open the free Stair Calculator →
Comfort Rules of Thumb
Stairs feel most comfortable when riser height plus tread depth is about 17–18 in, or when 2 × riser + tread is about 24–25 in. A 7 in riser with an 11 in tread is a classic comfortable combination.
Outdoor and Deck Stairs
Deck stairs follow the same math. For poured steps, estimate concrete with the concrete steps calculator, and plan the deck itself with the deck calculator.
Common Mistakes
- Forgetting to include the finished floor thickness in the total rise.
- Making the top or bottom step a different height; all risers must be equal within ⅜ in.
- Not leaving enough headroom under the floor above.
Stair Dimensions for Common Floor Heights
| Total rise | Risers | Riser height | Treads | Run at 10 in |
|---|---|---|---|---|
| 24 in | 4 | 6.00 in | 3 | 30 in |
| 36 in | 5 | 7.20 in | 4 | 40 in |
| 48 in | 7 | 6.86 in | 6 | 60 in |
| 96 in | 13 | 7.38 in | 12 | 120 in |
| 102 in | 14 | 7.29 in | 13 | 130 in |
| 108 in | 14 | 7.71 in | 13 | 130 in |
| 114 in | 15 | 7.60 in | 14 | 140 in |
Laying Out a Stringer
- Set a framing square’s tongue at the riser height and blade at the tread depth along the edge of the board.
- Trace each step down the board.
- Subtract the tread thickness from the bottom riser so all steps finish equal.
- Cut the first stringer, test it, then use it as a template for the rest.
- Use at least three stringers for stairs 36 in wide, spaced no more than about 16 in apart for typical treads.
Handrails and Guards
Under the IRC, stairs with four or more risers need a handrail on at least one side, 34–38 in above the tread nosing. Open sides more than 30 in above the floor or grade need guards. Check local rules for baluster spacing and height.
Frequently Asked Questions
How many steps do I need for 8 ft?
A 96 in rise divided by 7.75 in gives 12.4, so 13 risers of about 7.4 in and 12 treads.
What is the standard stair rise and run?
A common comfortable layout is a 7–7¾ in rise with a 10–11 in tread.
How long is a stair stringer?
Use the Pythagorean theorem: √(total rise² + total run²). Add extra for cuts.
How many treads for 14 risers?
13 treads, because the top floor acts as the last step.
How many stringers do I need?
Usually three for a 36 in wide stair; wider stairs need more so treads are supported every 16 in or so.
Related Guides
- How Much Grass Seed Do I Need? Rates per 1,000 Sq Ft
- How Many Squares Is My Roof? How to Measure a Roof from the Ground
- Roof Pitch Explained: How to Measure It and Convert to Degrees
Try the free Stair Calculator or browse all home improvement calculators.