Stair Stringer Calculator
Riser count, riser height, run, and stringers from total rise.
Vertical distance from lower surface to upper surface.
10 in minimum per code; 10.5 to 11 is comfortable.
You need
6 risers
Equal risers at or under the 7 3/4 in code maximum.
- Exact riser height
- 7.50 in
- Treads to cut
- 5 treads
- Total run
- 4.38 ft
- Stringers (16 in oc)
- 4 stringers
- Stringer length (approx)
- 5.8 ft
Estimates only. Verify quantities before ordering.
How this is calculated
Risers = total rise / 7.75 in, rounded up. Riser height = total rise / risers. Treads = risers - 1 (the top surface is the deck or floor). Total run = treads x tread run. Stringers assume 16 in on center across the stair width. Stringer length is the diagonal of rise and run.
Measure the total rise (the vertical distance from the lower landing surface to the upper floor or deck surface) and this calculator lays out the stair: how many steps, the exact height of each riser, the total horizontal run, and how many 2x12 stringers to cut. It divides the rise by the code maximum riser of 7 3/4 inches and rounds up, which yields equal risers at or under the limit.
Equal risers are the entire game in stair building. Code allows no more than 3/8 inch variation between the tallest and shortest riser on a flight, because uneven steps trip people. That is why you calculate from the total rise and divide evenly rather than framing steps one at a time.
Stair layouts for common deck heights
Rows use a 10.5 in tread run and a 36 in wide stair.
| Total rise | Risers | Riser height | Total run |
|---|---|---|---|
| 24 in (low deck) | 4 risers | 6.00 in | 2.63 ft |
| 36 in | 5 risers | 7.20 in | 3.50 ft |
| 45 in | 6 risers | 7.50 in | 4.38 ft |
| 60 in | 8 risers | 7.50 in | 6.13 ft |
| 108 in (full story) | 14 risers | 7.71 in | 11.38 ft |
The code numbers that govern the cut
Residential stairs are tightly regulated because falls are common and consequential. The IRC caps riser height at 7 and three quarter inches and requires a minimum tread depth of 10 inches, with minimum headroom of 6 feet 8 inches measured vertically from the tread nosing. Local amendments exist, so confirm before cutting, but these are the figures most of the country works to.
The rule that catches people is uniformity: the difference between the tallest and shortest riser in a flight cannot exceed three eighths of an inch, and the same applies to tread depth. Your foot calibrates to the first two steps and expects every one after to match, which is why an odd step at the top or bottom is the classic trip hazard. This is also why the bottom riser must account for the thickness of the tread material, or it ends up taller than all the others.
Total rise divided by an assumed riser height gives a fractional number of steps; round to a whole number and divide back to get the actual riser. Always work from the finished floor at the top to the finished floor at the bottom, including any flooring, decking, or concrete not yet poured, because measuring to a subfloor and adding the finish later is what produces that mismatched bottom step.
Cutting stringers and what stock to buy
Stringers are cut from 2x12 stock, and nothing smaller is adequate, because the notches remove most of the board and what remains, the throat, is what carries the load. Aim to leave at least 5 inches of uncut material behind the deepest notch. Buy the straightest boards on the rack and sight down each one, since a crowned stringer transfers its bow into every tread.
Lay out with a framing square and stair gauges clamped at your rise and run figures, mark the whole flight, then cut. Stop the circular saw just short of each inside corner and finish with a handsaw rather than overcutting, because a saw kerf running past the corner weakens the throat exactly where the stress concentrates. Cut one stringer, test fit it in place, and only then use it as a template for the rest.
Space stringers no more than 16 inches on center for most decking and 12 inches for thinner composite treads, which are less stiff than they look. Check the decking manufacturer specification rather than assuming, since composite stair span requirements are frequently tighter than for the deck field.
Attaching the top and bottom
The top of a stringer hangs off the deck rim or a header and needs a real connection: a properly sized hanger or a ledger with the stringer notched over it, not just screws driven through the face of the rim into the end grain. End grain holds fasteners poorly and this is a joint that fails suddenly rather than gradually.
At the bottom, stringers should land on a concrete pad or footing rather than on soil or a patio block, both because wood in contact with wet ground rots and because a settling base changes the bottom riser height. Any stringer touching concrete must be pressure treated, and a physical connection at the base, an angle bracket anchored into the pad, stops the flight from kicking outward under load.
Guards are required on most exterior stairs above a certain drop, and handrail dimensions and graspability are specified in code alongside the stair geometry. Plan the post locations while the stringers are being laid out, because retrofitting a structurally adequate guard post to a finished flight is far more work than setting it during the build.
Common questions
What is the maximum riser height by code?
The IRC allows risers up to 7 3/4 inches and treads at least 10 inches deep, with no more than 3/8 inch variation between risers in a flight. Some jurisdictions are stricter. Comfortable stairs usually land around a 7 inch riser with a 10 1/2 to 11 inch run.
How do I measure total rise correctly?
Measure vertically from the finished surface you will stand on at the bottom to the finished surface at the top, at the exact spot the stair will land. Sloped ground matters: a landing spot 6 feet out can be inches lower or higher than at the deck, and the stair must be figured at its landing point.
What lumber do stringers use?
2x12s, because cutting a 7 in riser and 10 1/2 in tread notch out of anything narrower leaves too little uncut wood. The rule of thumb is at least 5 inches of solid material behind the notches, sometimes 3.5 in throat is cited minimal; verify locally. Use pressure-treated 2x12 outdoors and crown all stringers the same way.
How many stringers does a stair need?
Space stringers no more than 16 inches on center for 2x lumber treads, so a 36 inch wide stair takes three and a 48 inch stair takes four. Composite treads usually require closer spacing, often 12 inches or less, so check the decking manufacturer's span chart.