How to lay out a stair stringer
- Measure the total rise from the landing spot to the top of the finished deck.
- Enter the largest riser you will accept and the tread depth you want. The calculator finds the fewest equal risers that stay under your maximum.
- Set stair gauges on a framing square at the riser height (tongue) and tread depth (blade) shown in the results.
- Walk the square down the board, marking each riser and tread, then mark the top plumb cut and the bottom level cut.
- Cut the tread thickness off the bottom so the first step matches the rest, then cut one stringer and use it as a pattern.
How the layout is calculated
Risers = total rise ÷ maximum riser, rounded up
Riser height = total rise ÷ risers
Treads = risers − 1 (the deck is the top step)
Total run = treads × tread depth
Notch depth = riser × tread ÷ √(riser² + tread²)
Throat = stringer board width − notch depth
Worked example
A deck 30 inches above the patio with a 7 3/4 inch maximum riser needs 30 ÷ 7.75 = 3.87, rounded up to 4 risers of 7 1/2". With 10 1/2 inch treads there are 3 treads and a total run of 2' 7 1/2". Each notch is 6 1/8" deep, which leaves a throat of 5 1/8" on a 2 x 12. The board needs to be at least 3' 7 1/16" long, so an 8 foot 2 x 12 is enough.
Stair rules to know before you cut
Under the 2021 International Residential Code (section R311.7):
- Risers may be at most 7 3/4 inches high, and treads at least 10 inches deep.
- Within one flight, the largest riser or tread may not exceed the smallest by more than 3/8 inch. Equal risers from this calculator keep you inside that.
- Treads shallower than 11 inches need a nosing that projects 3/4 to 1 1/4 inches.
- Stairs need at least 36 inches of clear width and 6 feet 8 inches of headroom.
Many areas adopt the IRC with local changes, and deck stairs also need guards and handrails once they reach certain heights. Check with your building department before you build.
Frequently asked questions
How do I measure the total rise for stairs?
Measure straight down from the top of the finished deck or floor to the spot where the bottom step will land. If the ground slopes, measure at the landing point, not next to the house, because that is where the stairs end.
How many steps do I need for a 36 inch rise?
Divide the rise by the largest riser you will accept and round up. With the 7 3/4 inch IRC maximum, 36 ÷ 7.75 = 4.65, so you need 5 risers of 7 3/16" and 4 treads cut into the stringer.
What are the code limits for stair risers and treads?
The 2021 International Residential Code allows risers up to 7 3/4 inches and requires treads of at least 10 inches. The tallest riser in a flight may not exceed the shortest by more than 3/8 inch, and the same limit applies to tread depth. Your local code may differ, so check with your building department.
Why is the bottom of the stringer cut short?
Treads sit on top of each cut, which raises every step by the tread thickness. The deck surface at the top does not move, so without a correction the bottom step would be taller than the rest. Cutting the tread thickness off the bottom of the stringer evens them out.
How many stringers do I need?
Divide the stair width by the maximum span of your tread material and add one. A 36 inch stair with treads that can span 16 inches needs 4 stringers. Check the tread manufacturer, because composite treads often span less than wood.
What is the throat of a stringer?
The throat is the solid wood left between the inside corner of each notch and the bottom edge of the board. It carries the load, so a deeper notch or a narrower board weakens the stringer.