Stair Stringer Calculator
Cut a stringer wrong and every step in the flight is off, so the math has to be right before the saw comes out. Enter your total rise, the vertical distance from the bottom landing to the top, below. The calculator figures the number of risers, the exact riser height, tread count, total run, and the stringer length you need to cut, and flags whether your numbers land inside typical code limits.
Cut a stringer right the first time
Cut one stringer wrong and every step in the flight is off, so the math has to be right before the saw comes out. Enter your total rise, the floor-to-floor or deck-to-grade height, and the tool gives you the riser count, the exact riser height, the tread depth, the total run, and the stringer length, and it checks each one against residential stair code.
The stair math
Start with total rise and divide by a comfortable riser, around 7 inches, to get a riser count. Round to a whole number, then divide the total rise by that count for the exact riser height. Treads always come to one less than the risers, because the top tread is the landing. Multiply the tread count by your tread depth for the total run, and the stringer length is the square root of the total rise squared plus the total run squared.
The code limits that keep stairs legal and safe
Residential stairs in the US follow the IRC, and inspectors check these numbers. The calculator flags any that fall outside the limits so you catch it before you cut.
| What | Limit |
|---|---|
| Max riser height | 7.75 in |
| Min tread depth | 10 in |
| Min headroom | 6 ft 8 in |
| Min stair width | 36 in |
| Max riser variation | 3/8 in across the flight |
The 7-11 rule and comfort
A stair feels right when the riser is about 7 inches and the tread about 11. That pairing puts the stair around 30 to 37 degrees, the range a body climbs without thinking. You can push to a 7.75-inch riser and a 10-inch tread and still pass code, but it climbs steeper. Stay near 7 and 11 when the run is available.
A real example
Say the total rise from grade to the deck is 44 inches. Divide by a 7-inch target and you get 6.3, so round to 7 risers. Each riser is 44 divided by 7, about 6.29 inches, well under the 7.75 max. Seven risers means six treads. At 11 inches each, the total run is 66 inches, and the stringer length is the square root of 44 squared plus 66 squared, about 79 inches. Every number passes, so you cut with confidence.
How many stringers, and the lumber
Stringers carry the treads, so they get spaced about 12 to 16 inches apart. A standard 36-inch stair runs three: one on each side and one down the middle. They're cut from 2x12 stock, and you leave at least 3.5 to 5 inches of uncut board behind the cuts so the stringer keeps its strength. Measure the stringer length, add a little, and buy 2x12s to suit.
Why the first step is shorter
When you frame stairs, the bottom riser gets cut down by the thickness of one tread. Without that cut, the first step ends up taller than the rest once the treads go on, which is both a trip hazard and a code fail on riser variation. Account for the tread thickness at the bottom and every finished step lands equal.
The code numbers every stringer must hit
The IRC caps risers at 7¾ inches and floors treads at 10 inches for homes, with no more than 3/8-inch variation between the tallest and shortest riser in the run. That variation rule is the one that trips DIY stairs: a first or last step an inch off is exactly what ankles notice at 6 a.m. The calculator divides your total rise into equal risers inside the legal window so the odd inch never lands in one step.
Quick answers
What size lumber for stringers? 2x12, no substitutes, with at least 3½ inches of uncut depth remaining at the deepest notch. Three stringers minimum for 36-inch-wide stairs, four for composite treads.
How many steps for 3 feet of height? Thirty-six inches of rise divides into five risers of 7.2 inches, which means four treads. Rise count is always one more than tread count.
Can stairs land on dirt? No. The bottom lands on a pad or footing, and the top hangs from proper hardware, not nails through the back. Deck stairs also usually need a permit and inspection like the deck itself.
Common questions
Start with total rise, divide by about 7 inches and round for the riser count, then divide total rise by that count for exact riser height. Treads are one fewer than risers. The calculator runs the whole sequence and gives you the stringer length to cut.
Common US residential limits are a maximum riser around 7.75 inches and a minimum tread depth near 10 inches, but local codes vary. The tool flags when your numbers fall outside the usual range so you can check before building.
Divide the total rise by a comfortable riser height near 7 inches and round. A 44-inch rise lands at 6 steps. Enter your exact rise above for the precise count.
Rise is the total vertical height the stairs climb; run is the total horizontal distance they cover. Each individual step has its own riser height and tread depth. The calculator gives you all four numbers.
Estimates for planning. Verify against your supplier's units and your local building codes before ordering or building. For anything structural, follow your engineer or local code.