Deck Stair Calculator
Riser height, tread depth and stringer length from your deck height, solved so every riser is equal and inside code, with the stringer throat checked before you cut.
Enter your values
Results update instantly as you type.
Everything is worked out in your browser, so the numbers never leave your device. The shareable link reopens the calculator with exactly these values.
Results
Side elevation, drawn to scale from your rise and tread.
Working
- ideal risers = 42 / 7 in = 6, rounded to 6
- riser = 42 / 6 = 7 in, against a 7.75 in maximum
- treads = 6 - 1 (top tread is the deck) = 5
- run = 5 x 11 = 55 in
- stringer = sqrt(55^2 + 42^2) = 69.2 in
- throat = 11.3 - (7 x 11) / sqrt(7^2 + 11^2) = 5.34 in
- 2R + T = 2 x 7 + 11.8 = 25.75 in
Quick answer
Divide the total rise by seven inches and round to a whole number: that is your riser count. Divide the rise by that count and every riser comes out equal, which is what the code actually cares about. Tread depth is then your choice above the ten inch minimum, and the run is treads times tread depth.
risers = round(total rise / 7); riser = total rise / risers; run = (risers - 1) x tread
Worked example
A deck 42 in above the ground, 11 in treads cut from 2 x 12 stringers.
Working
- ideal risers = 42 / 7 in = 6, rounded to 6
- riser = 42 / 6 = 7 in, against a 7.75 in maximum
- treads = 6 - 1 (top tread is the deck) = 5
- run = 5 x 11 = 55 in
- stringer = sqrt(55^2 + 42^2) = 69.2 in
- throat = 11.3 - (7 x 11) / sqrt(7^2 + 11^2) = 5.34 in
- 2R + T = 2 x 7 + 11.8 = 25.75 in
Six risers at exactly 7 in, which is as comfortable as stairs get. Note the run: 55 in of ground is needed in front of the deck before the bottom riser, and that is before the 36 in landing the code wants beyond it.
Deck stair layout by deck height
Riser counts chosen to land as close to 7 in as possible while staying under the 7-3/4 in maximum.
| Total rise | Risers | Riser height | Treads | Run at 11 in treads | Stringer |
|---|---|---|---|---|---|
| 2 ft | 4 | 6 in | 3 | 2 ft 9 in | 3 ft 5 in |
| 2 ft 6 in | 4 | 7-1/2 in | 3 | 2 ft 9 in | 3 ft 8.5 in |
| 3 ft | 5 | 7-3/16 in | 4 | 3 ft 8 in | 4 ft 9 in |
| 3 ft 6 in | 6 | 7 in | 5 | 4 ft 7 in | 5 ft 9 in |
| 4 ft | 7 | 6-7/8 in | 6 | 5 ft 6 in | 6 ft 9.5 in |
| 5 ft | 9 | 6-11/16 in | 8 | 7 ft 4 in | 8 ft 10.5 in |
| 6 ft | 10 | 7-3/16 in | 9 | 8 ft 3 in | 10 ft 2.5 in |
| 8 ft | 14 | 6-7/8 in | 13 | 11 ft 11 in | 14 ft 4 in |
Run is the horizontal distance the stairs eat. Add a landing at least 36 in deep beyond the bottom riser.
At a glance
| What it does | Solves riser and tread to code, then gives run, stringer length and throat |
|---|---|
| Inputs needed | Total rise, tread depth, nosing, stair width, stringer stock |
| Main output | Riser count and riser height |
| Formula | riser = total rise / round(total rise / 7); run = treads x tread depth |
| Category | Home & DIY |
| Cost | Free, no sign-up, no download |
How to use the Deck Stair Calculator
- Measure the total rise from the finished ground or landing surface to the top of the decking, not to the frame.
- Set the tread depth. Two 2 x 6 deck boards with a gap give about 11 in, which is the common answer.
- Check the riser height that comes back sits comfortably under 7-3/4 in, and adjust the tread if 2R + T is outside 24 to 25 in.
- Check the throat figure before buying stringer stock. A steep stair on 2 x 10 will fail it.
- Remember to cut the bottom of the stringer down by one tread thickness so the finished risers come out equal.
Frequently asked questions
What is the maximum riser height for deck stairs?
7-3/4 in under the IRC, with a minimum tread depth of 10 in. Just as important, the tallest and shortest riser in a flight may differ by no more than 3/8 in, which is what most failed inspections come down to.
How do I calculate the number of stairs for my deck?
Divide the total rise by seven inches and round to the nearest whole number, then divide the rise by that count to get the actual riser. Rounding gives you equal risers automatically, which is what matters.
Why is my bottom step shorter than the others?
Because the stringer was not dropped. Once treads are laid on the cut stringer, every riser gains one tread thickness except the bottom one, which sits on the ground. Cut the stringer bottom down by exactly the tread thickness before installing.
What is the stringer throat and why does it matter?
The material left behind the notch, measured perpendicular to the stringer edge. Five inches is the usual minimum. A steep stair notched from 2 x 10 stock can leave under four inches, at which point the stringer is a row of weak points rather than a beam.
How many stringers do I need?
At 16 in centres, so three for a 36 in stair and four for a 48 in one. Composite decking usually wants 12 in centres on stairs, which adds one, because it sags between supports more than timber.
Do deck stairs need a handrail?
Once there are four or more risers. It must be graspable, 34 to 38 in above the nosing line, and continuous for the full flight.
Riser, tread, handrail and variation limits follow the International Residential Code, section R311.7. Your local code may amend them and deck stairs are usually an inspected item, so confirm the figures with your building department before cutting a full set of stringers.