You are building a set of steps from the yard up to a deck that sits 3 ft off the ground. That short flight is where most people cut a stringer too short or make the risers too tall, so the stairs feel like a ladder on the way up and a cliff on the way down. Here is how the numbers land before you touch a 2x12.
Measure the rise
The total rise is the vertical distance from the finished ground to the deck surface, not from a dirt patch that will settle. For this deck it is 36 inches. That single number drives everything else. If the grade slopes, measure at the low side and plan to dig the footing down so the stairs always start on concrete, never on bare dirt.
Set the riser count
A comfortable step rises about 7.25 inches. Divide 36 inches by 7.25 and you land near 5 steps. You cannot build a fraction of a riser, so round to 5 and reset the math. Each riser comes out to 7.2 inches. That sits squarely inside the code band of 7 to 7.75 inches, so the stair meets inspection and feels normal underfoot. Split 36 inches over 4 risers instead and each step jumps to 9 inches, which is a ladder. Split over 6 and each drops to 6 inches, which makes the run awkward.
Lay out the treads
The tread depth is the flat part you step on, and 10 inches is the residential minimum. Since one more riser exists than treads, 5 risers mean 4 treads. Four treads at 10 inches each give a horizontal run of 40 inches off the face of the deck. That tells you how far out from the deck the bottom of the stair lands, so you can place the footing before you cut anything.
Cut the stringer
The stringer is the diagonal 2x12 board that carries the steps. Its length is the hypotenuse of the rise and the run, which works out to about 4 ft 6 in for this 36 by 40 inch flight. Stock 2x12 boards come in whole-foot lengths, so buy 6 ft stock and trim the ends square. A stringer cut too short leaves you scarfing in a piece, which never holds.
Space the supports
Three stringers carry a stair up to 36 inches wide, nailed or screwed 16 inches on center. That is the common residential setup. If the stair is wider than 36 inches or will see traffic like a rental deck, go to four stringers. Three stringers mean three 6 ft 2x12 boards, plus a couple of 2x6 tread boards to span between them.
See a bigger flight
For a full interior flight between two floors, the rise climbs to 108 inches. Split that over 15 risers at 7.2 inches each, with 14 treads at 10 inches, and the diagonal stringer runs near 14.7 ft. That is why you buy 16 ft 2x12 stock for a house stair rather than the 6 ft length a deck step needs. The same rules apply, just on a longer board.
A set of stairs feels right when the riser lands near 7.2 inches, the tread hits 10 inches deep, and the stringer is cut from stock a foot longer than the diagonal. Measure the rise twice, lay out once, and the stairs will outlast the deck.
Stair Stringer Calculator
Walk a flight up to a deck that sits 3 ft high, set the riser to a code-compliant 7.2 in, and size the diagonal stringer and the 2x12 stock it comes from.
How to use
- Measure the total rise from grade to deck in inches
- Divide the rise by a 7.25 in target riser
- Round the riser count and reset each riser to match
- Set tread depth to 10 inches
- Read the diagonal stringer length
- Decide on 3 stringers for a 36 in wide stair
Supported formats and limits
- Riser must stay between 7 and 7.75 inches
- Tread must be at least 10 inches deep
- Does not include a landing or a handrail
- Stock length rounded up to the foot
Examples
36 in rise with 10 in tread gives 5 risers at 7.2 in, 4 treads, and about a 4 ft 6 in stringer on 3 supports.Common errors
- Total rise must be a positive number
- Tread depth must sit between 9 and 14 inches
- Stair width must be under 6 ft for the 3 stringer rule
Privacy and browser support
All calculations run locally in your browser. No measurements are uploaded.
FAQ
How many risers fit a 3 ft tall stair
Three feet is 36 inches. Split over 5 risers, each step lands at 7.2 inches, which is inside the 7 to 7.75 inch code range.
What riser height feels right under foot
Target 7.25 inches per step. Anything over 7.75 inches trips people up, and anything under 7 inches makes the run long and awkward.
How long does a stringer need to be
For 5 risers you cut 4 treads. At 10 inches each, the run is 40 inches, and the diagonal stringer works out to about 4 ft 6 in, so buy 6 ft 2x12 stock.
How many stringers do I need
Three stringers for a stair up to 36 inches wide. Go to four stringers if the stair is wider than 36 inches or will see heavy use.
How does a full floor-to-floor flight compare
A 108 inch rise splits into 15 risers at 7.2 in each, 14 treads, and a stringer near 14.7 ft, so you buy 16 ft 2x12 stock.
Do risers and treads always match in count
No. One more riser than treads, because the deck surface itself acts as the last tread. Five risers means four cut treads.