Stair Stringer Calculator

Enter the total rise of your stairs. The calculator sets riser count, riser height, tread run, and stringer board length, then checks the result against IRC stair limits.

Total rise: measure it first

Total rise is the vertical distance from the lower finished surface to the upper finished surface, ground to deck, or floor to floor. Measure straight down with a level and tape, not along a slope. Every other number in this calculator comes from that one measurement.

How to calculate stair stringer measurements

Start from total rise and a target riser height. Building convention holds risers between 7 and 7.75 inches for a comfortable climb, and 7.25 inches is a common starting target. Divide, round to a whole number of risers, then divide again to spread the rise evenly:

The stringer hangs from the deck rim at the top, so it needs one fewer tread than riser. Multiply treads by the run of each step to get the total horizontal distance the stairs cover:

The stringer itself is the hypotenuse of a right triangle formed by total rise and total run:

What is a stair stringer?

A stringer is the diagonal board that carries a set of stairs, cut with a sawtooth profile so a flat tread can sit on each notch. Deck and porch stairs typically use two or three stringers cut from 2×12 lumber, spaced under the width of the tread boards so the treads don't sag underfoot. Get the rise and run wrong on the layout and every subsequent cut compounds the error, which is why professional carpenters lay out one stringer completely, test-fit it against the framing, and only then trace it onto the rest of the stock.

Riser height by total rise

These use the 7.25-inch target riser height this calculator defaults to. Your actual numbers shift with the target you enter.

The 24-inch row shows why the check matters. Rounding 24 ÷ 7.25 gives 3 risers, and 3 risers spread over 24 inches works out to a flat 8 inches each, over the code maximum. Enter a lower target riser height, like 6.5 inches, and the calculator rounds up to 4 risers at 6 inches each instead.

Laying out stringers with a framing square

Set a framing square on the edge of the 2×12 with the riser height marked on the tongue, the short arm, and the tread run marked on the blade, the long arm. Line up both marks against the top edge of the board and trace along the outside of the square. That single trace is one riser and one tread. Clip a pair of stair gauges to the tongue and blade at your riser and run marks so every repeat lands in exactly the same spot, then slide the square down the board and trace again for each remaining step. Repeat until you've marked every riser and tread the calculator gives you, then cut along the lines with a circular saw and finish the inside corners with a handsaw or jigsaw.

Before you cut, drop the bottom of the stringer by the thickness of your tread board. Every riser above the bottom one already accounts for a tread sitting on top of the notch, but the bottom of the stringer rests directly on the pad or footing with nothing under it. Trim that first cut down by one tread thickness and all the risers come out equal once the treads are installed.

Figure the board footage for the full set of stringers, treads, and any decking with the board foot calculator, or add up the running length of decking boards you need with the linear foot calculator.

IRC stair code limits

The 2021 International Residential Code sets minimums and maximums for stair geometry in section R311.7.5.1. This calculator checks your riser height and tread run against the limits below and prints a flag if either one fails.

The IRC doesn't set a fixed minimum stringer count. Three stringers under a 36-inch-wide stair is common practice, but actual spacing depends on your decking's span rating and local amendments to the IRC. Check with your building department before you finalize a design that will be inspected.

Frequently asked questions