How to use the stair calculator
- Measure the total rise. Measure from the finished floor at the bottom to the finished floor (or deck surface) at the top. If the ground at the bottom slopes, measure where the stairs will actually land.
- Pick a tread depth. 10 in is the code minimum. Deck stairs often use about 11 in (two 2×6 boards); with deeper treads, slightly shorter risers keep the stride comfortable.
- Read the results. The calculator divides the rise into equal risers no taller than your maximum, then works out the run, the stringer length, the headroom and how many stringers to cut. Turn the 3D view to check the result.
- Lay out the stringer. Clamp stair gauges to a framing square at the riser and tread numbers, trace each notch along the board, and trim the tread thickness off the bottom. Print the stringer layout to take with you.
Residential stair rules
These are the main limits in the 2021 International Residential Code. Many US cities adopt it with local changes.
| Rule | Limit | IRC section |
|---|---|---|
| Riser height | 7 3/4 in maximum | R311.7.5.1 |
| Tread depth | 10 in minimum | R311.7.5.2 |
| Stair width | 36 in minimum | R311.7.1 |
| Headroom | 6 ft 8 in minimum | R311.7.2 |
| Rise of one flight | 151 in maximum between landings | R311.7.3 |
Common stair sizes
Worked out with a 7 3/4 in maximum riser and 10 in treads.
| Total rise | Risers | Riser height | Treads | Total run |
|---|---|---|---|---|
| 30 in (deck) | 4 | 7 1/2 in | 3 | 2 ft 6 in |
| 96 in (8 ft) | 13 | 7 3/8 in | 12 | 10 ft |
| 108 in (9 ft) | 14 | 7 11/16 in | 13 | 10 ft 10 in |
| 120 in (10 ft) | 16 | 7 1/2 in | 15 | 12 ft 6 in |
How the numbers are worked out
The number of risers is the total rise divided by the maximum riser height, rounded up. Every riser then gets the same height: total rise ÷ risers. There is one fewer tread than risers, because the top floor acts as the last step. Total run is treads × tread depth, and the stringer length is the diagonal of the rise and run. Headroom is checked at the edge of the ceiling opening: the clear height from the stair's nosing line up to the underside of the upper floor.
Questions people ask
How many steps do I need for an 8 foot rise?
An 8 ft (96 in) rise needs 13 risers of about 7 3/8 in each, with 12 treads. Dividing 96 by the 7 3/4 in code maximum gives 12.4, which rounds up to 13 risers.
What is the maximum riser height and minimum tread depth?
Under the International Residential Code (R311.7.5), risers can be at most 7 3/4 in tall and treads must be at least 10 in deep, measured without the nosing. Your local code may differ, so check with your building department.
How much headroom do stairs need?
The IRC requires at least 6 ft 8 in of clear headroom (R311.7.2), measured straight up from the sloped line connecting the tread nosings. Enter your ceiling opening length to check it.
Why is the first riser cut shorter than the others?
The tread on top of each step adds its thickness to every riser except the first, which sits on the floor. Trimming the tread thickness off the bottom of the stringer makes every finished step the same height.
How long should my stair stringer board be?
The stringer runs along the diagonal from the floor to the top landing. The calculator adds one extra step of length for the top and bottom cuts and suggests the next standard lumber length.
How many stringers do I need?
The calculator divides the stair width by your maximum stringer spacing and adds one, with a minimum of two. Tread material sets how far it can span, so check the tread manufacturer’s rating.