How to use this stair stringer calculator
- Measure the total rise from the landing surface to the top of the finished floor or deck.
- Keep the default maximum riser of 7¾ in (the US residential code limit) unless your local code differs.
- Enter the tread depth: at least 10 in per the IRC.
- Buy stringer lumber at least 1–2 ft longer than the stringer length shown.
Example
For a deck 42 inches above the ground, the calculator gives 6 risers of 7 in (17.8 cm) each, 5 treads and a total run of 50 in (127 cm). The stringer board must be at least 5 ft 5.3 in (1.66 m) long, so a 2×12 cut from an 8 ft or longer board works.
What to know before you buy
The comfortable stair rule
A common rule of thumb for comfortable stairs is that two risers plus one tread should equal 24–25 inches. For example, 7 inch risers with 10–11 inch treads feel natural. Very short risers with deep treads feel awkward, and steep stairs are harder to climb safely.
Code requirements
In the US, the International Residential Code allows a maximum riser of 7¾ inches and a minimum tread depth of 10 inches, with at least 6 ft 8 in of headroom. Stairs with four or more risers need a handrail. Local codes can differ, so check with your building department.
Stair layout by total rise (max riser 7¾ in, 10 in treads)
| Total rise | Number of risers (steps) | Riser height | Number of treads | Total run |
|---|---|---|---|---|
| 18 in | 3 | 6 in (15.2 cm) | 2 | 20 in (50.8 cm) |
| 24 in | 4 | 6 in (15.2 cm) | 3 | 30 in (76.2 cm) |
| 30 in | 4 | 7.5 in (19.1 cm) | 3 | 30 in (76.2 cm) |
| 36 in | 5 | 7.2 in (18.3 cm) | 4 | 40 in (101.6 cm) |
| 48 in | 7 | 6.86 in (17.4 cm) | 6 | 60 in (152.4 cm) |
| 60 in | 8 | 7.5 in (19.1 cm) | 7 | 70 in (177.8 cm) |
| 96 in (8 ft) | 13 | 7.38 in (18.8 cm) | 12 | 120 in (304.8 cm) |
| 108 in (9 ft) | 14 | 7.71 in (19.6 cm) | 13 | 130 in (330.2 cm) |
Formula
Risers = total rise ÷ maximum riser, rounded up. Riser height = total rise ÷ risers. Treads = risers − 1. Stringer length = √(rise² + run²).
Tips from the pros
- Measure the total rise from the landing surface to the top of the finished deck or floor, including decking or flooring.
- Keep all risers equal: the IRC allows a maximum difference of 3/8 inch between steps.
- Use pressure-treated 2×12 lumber for outdoor stringers and space them no more than 16 in apart for wood treads.
Common mistakes to avoid
- Forgetting to drop the stringer by the thickness of one tread so the bottom step is the same height as the rest.
- Measuring the rise on sloped ground at the wrong spot; measure where the bottom step will land.
Frequently asked questions
What is the standard riser height?
Between 7 and 7¾ inches for homes in the US; many builders aim for about 7½ inches.
Why is there one fewer tread than risers?
The top step is the deck or floor itself, so the last riser leads directly onto it.
Is this calculator a substitute for building code?
No. Always check your local building code for riser, tread, headroom and railing requirements.
How many steps do I need for a 3 ft rise?
A 36 inch rise needs 5 risers of about 7.2 inches each and 4 treads.
How long should a stair stringer be?
Calculate it as the diagonal of the total rise and total run (√(rise² + run²)), then add extra for the cuts at the top and bottom.