How do I calculate stairs?
Start with the total rise: the height from the landing spot at the bottom to the top of the finished floor or deck. Divide it by the tallest riser you'll allow and round up. That's the number of risers. Divide the total rise by that number to get the actual riser height.
Risers = total rise ÷ max riser, rounded up. Treads = risers − 1. Total run = treads × tread depth
A deck 38 in above the ground: 38 ÷ 7.75 = 4.9, so 5 risers. 38 ÷ 5 = 7.6 in, or about 7 5/8 in each. There are 4 treads because the deck itself is the top step. At 10 in per tread the stairs run 40 in out from the deck. The stringer diagonal is √(38² + 40²) = 55.2 in, or 4 ft 7 3/16 in, so an 8 ft 2×12 is long enough.
Stair rules in the IRC
| Item | IRC limit |
|---|---|
| Riser height | 7-3/4 in maximum |
| Tread depth | 10 in minimum, nosing to nosing |
| Variation between steps | 3/8 in maximum, riser to riser and tread to tread |
| Stair width | 36 in minimum |
| Headroom | 6 ft 8 in minimum |
| Handrail | Required on flights with 4 or more risers |
These are the model code limits from IRC section R311.7. Your city may have adopted a different version or added its own rules, so check before you build.
Deck stair calculator
Measure the rise from the top of the deck boards straight down to where the bottom step will land, not to the ground at the house. If the ground slopes away, the rise is larger at the bottom of the stairs. Measure there, recalculate, and check again.
Deck stairs usually hang from the rim joist, with the top tread one step below the deck surface. Most deck stairs use 2×12 stringers with two 5/4×6 or 2×6 boards per tread.
Rise and run stair calculator
The rise and run you pick set how steep the stairs feel. A common comfort rule is that one riser plus one tread should add up to 17 to 18 in. A 7 5/8 in riser with a 10 in tread comes to 17.6 in. Lower risers with deeper treads make a gentler stair but take more room.
How many stringers do I need?
Stringers are spaced across the width of the stair. Divide the width by your maximum spacing, round up and add one. A 36 in stair with stringers no more than 16 in apart needs 4. At 18 in, it needs 3. Composite treads often need stringers 12 in apart or closer. Check the tread manufacturer's guide.
How to lay out stair stringers
Set stair gauges on a framing square at the riser height on one arm and the tread depth on the other. Mark each step along the 2×12, sliding the square down the board. Then take the thickness of one tread off the bottom of the stringer so the first step comes out the same height as the rest. Cut the first stringer, test it in place, and use it as the pattern for the others.
Pricing a stair job
Deck builders often price stairs per step or per riser, on top of the deck. Switch to contractor mode to add your labor and markup, add the deck from the deck calculator, and send both as one estimate.
Questions
What is the maximum riser height for stairs?
The IRC allows a maximum riser height of 7-3/4 inches and a minimum tread depth of 10 inches. Check which code your city has adopted.
How many steps do I need for a 38 inch rise?
5 risers of about 7 5/8 inches, with 4 treads if the deck is the top step.
How long should a stair stringer be?
At least the diagonal of the total rise and total run. For a 38 inch rise with a 40 inch run that's about 55 inches, so an 8 ft 2x12 works.
Do deck stairs need a handrail?
Under the IRC, a stair with 4 or more risers needs a handrail. Check your local code for guard and handrail rules.