StringerPro

Stair Stringer Math, Done Right

A stair stringer is the diagonal 2x12 (or 2x10) that holds the treads. Cutting one wrong is expensive — a single 16-foot 2x12 #1 SYP runs $35 to $55, and most stair builds need three. The rise needs to be uniform (within 3/8 inch by IRC R311.7), the tread depth needs to clear the minimum, the throat needs to be wide enough that the stringer isn't structurally compromised, and the bottom riser needs to be shorter than the others by exactly one tread thickness so the finished stair walks evenly. Every one of those rules has a way to fail. StringerPro keeps you out of trouble.

Why Most Stair Calculators Get the Bottom Step Wrong

The single most common DIY stair mistake is forgetting that the bottom riser has to be cut shorter by the tread thickness. If your treads are 1 inch thick and your unit rise is 7-1/4 inches, the bottom riser on the stringer needs to be cut at 6-1/4 inches — so that once the tread sits on top of the bottom step, the walking rise from the floor up to the first tread is the same 7-1/4 inches as every other step. Skip this and your bottom step is 8-1/4 inches tall while every other step is 7-1/4 inches. The 1-inch difference is enough to trip people, fail an inspection, and force a recut. StringerPro handles this automatically using whatever tread thickness you specify.

IRC R311.7 in Plain English

The 2018 International Residential Code section R311.7 sets the rules for residential stairs in most of the United States. Maximum riser height is 7-3/4 inches. Minimum tread depth is 10 inches. The variation between the tallest and shortest riser, or the deepest and shallowest tread, is 3/8 inch — meaning every step has to be uniform within that tolerance. The minimum throat for a notched 2x12 stringer (the wood remaining below the deepest notch) is 3-1/2 inches, otherwise the stringer is structurally weakened. StringerPro shows a green / yellow / red badge for each of these rules in real time as you type — change a number and watch the badges update instantly.

Fraction-Native Because Lumber Is

2x12 lumber isn't 2 inches by 12 inches — it's 1-1/2 by 11-1/4. Stair tread depth in finished form is usually 11-1/4 inches with a 1-inch nosing for an effective 10-1/4. Total rise from a finished basement floor to the underside of the upstairs subfloor is something like 108-3/8 inches. None of these numbers fit cleanly into a decimal calculator. StringerPro's input screen is fraction-native — feet + whole inches + a sixteenths picker that defaults to the most common stops (0, 1/4, 1/2, 3/4) and lets you scroll for the in-between values. It feels like a Construction Master calculator built for stairs.

The Framing-Square Cut Sheet

Before stair calculators, carpenters laid out stringers with a framing square — the 16-inch and 24-inch L-shaped tool every framer carries. You'd set the rise dimension on one leg and the run dimension on the other, walk the square down the top edge of the 2x12 marking each notch, and then cut. StringerPro's cut sheet is the modern equivalent: a step-by-step list of every framing-square heel position along the top edge, with the exact rise leg and run leg for each step. Plus the seat cut at the bottom (where the stringer meets the floor or pad) and the plumb cut at the top (where it meets the upper floor framing). Print it, take it to the cut station, work down the list.

The 1:1 Print Template

For builders who want to skip the math entirely, Premium unlocks a 1:1 PDF print template. The side view of the stringer is tiled across letter or A4 pages at full scale, with crop marks on every corner. Print, tape together, lay the template on your 2x12, trace the outline, and cut to the line. Used carefully, it eliminates every measurement transfer error that creeps in when you're reading dimensions off a phone screen with one hand and marking lumber with the other.

Material Estimator for Quotes

If you build stairs for a living, you spend half your time figuring out how much material a job needs and what it'll cost. The material estimator takes stringer count, stair width, and lumber unit prices and gives you total linear footage, board feet, and a job total including fasteners. Useful for handing a customer a quote without needing a separate spreadsheet, or for grabbing the right amount of lumber on the way to the jobsite without a second trip.

Deck Stair Mode

Exterior deck stairs follow IRC R311.7.5 with a few key differences from interior stairs: nosing dimensions match the deck board standard (typically 5/4 or 2x decking with a 1-inch overhang), the seat cut at the bottom usually lands on a concrete pad instead of a finished floor, and the maximum riser height stays the same but tread depth is often increased to 11 inches for safety in wet conditions. Switch from the main calculator with one tap and the inputs and code rules update accordingly.

Built for One Job, Done Exceptionally Well

There are construction calculator apps that try to do everything — stringer layout, drywall takeoff, paint coverage, concrete volume, lumber pricing, framing math. They're useful for the rare problem you encounter once a year, and they always feel like a museum of half-finished features when you actually try to lay out a stair. StringerPro does one thing — turn rise and run into a code-checked, ready-to-cut stringer layout — and does it better than any general-purpose calculator. If you build stairs more than once or twice a year, that's worth one $5.99 in-app purchase.

Privacy and Offline Use

StringerPro stores every saved job locally on your device. No account, no cloud sync, no analytics on your job data. The app works completely offline, which matters because the basement of a remodel rarely has signal. The free tier includes a banner ad; Premium removes it.