Stair Stringer Calculator

Risers, treads, stringers and lumber for a staircase.

A staircase is pure geometry: your total rise (floor to floor) sets the number of equal risers, and the tread depth sets the run. Get the riser-and-tread math right and the build is straightforward; get it wrong and every step feels off — or fails inspection.

DifficultyIntermediate–Advanced

TimeA day or two for a straight run of deck or basement stairs

SkillsCareful layout with a framing square; accurate, repeatable cuts

PermitsStairs are code-scrutinized (IRC R311.7) and usually part of a deck or remodel permit. Risers and treads must be uniform — the difference between the largest and smallest can’t exceed 3⁄8".

Tools you'll need

Framing square with stair gauges
Circular saw + handsaw for the corners
Tape measure and level
Speed square and pencil
Drill/driver
Stair gauge or jig

Step by step

Measure the total riseMeasure the exact vertical distance from the lower floor to the upper floor (or deck surface). This total rise drives everything — enter it in the calculator to get the riser count and exact riser height.
Set risers and treadsThe calculator divides the rise into equal risers ≤ 7¾" each (IRC max) and pairs them with your tread run (≥ 10"). Aim for the “comfort” rule: riser + tread ≈ 17–18".
Lay out the stringerMark the rise and run on a 2×12 with a framing square and stair gauges, step the pattern down the board, then drop the stringer by one tread thickness so the first and last steps come out equal.
Cut and test-fit one stringerCut the first stringer carefully (finish the inside corners with a handsaw to avoid overcutting), set it in place, and confirm every riser is level and equal before using it as the template for the rest.
Install stringers, then treads and risersHang the stringers ~16" on-center (3 for a 36" stair), fasten securely top and bottom, then add riser boards and treads. Finish with a code-compliant handrail (34–38" high) and guards.

Common mistakes

Unequal riser heights

Risers must be uniform within 3⁄8" — a stray step height is a trip hazard and an automatic inspection fail. Divide the total rise evenly; don’t “eyeball” the last step.

Forgetting to drop the stringer

Subtract one tread thickness from the bottom of the stringer so the first step isn’t taller than the rest once treads are on.

Tread run too shallow

Keep the tread run at least 10" so a foot fits. Steep, shallow stairs are dangerous and out of code.

No handrail or undersized guards

Stairs need a graspable handrail 34–38" above the nosing, and open sides need guards — plan for them before you build.

Safety

Stairs are a fall hazard the moment they exist — add a temporary rail and don’t leave open stringers unattended.
Wear eye protection when cutting; finish corner cuts with a handsaw to avoid kickback from overcutting.

Frequently asked questions

What is the maximum riser height?

Under the IRC, 7¾" maximum, and all risers must be within 3⁄8" of each other. The calculator always sizes risers to stay at or below 7¾".

What is the minimum tread depth?

10" minimum run (the horizontal walking surface, not counting nosing). Deeper treads (11") feel more comfortable on long flights.

How do I figure how many steps I need?

Divide the total floor-to-floor rise by ~7" and round to a whole number of risers; the calculator does this and gives the exact equal riser height. Treads = risers − 1.

How many stringers does a staircase need?

Three for a typical 36"-wide stair (about 16" on-center). Wider stairs or thinner treads need more so the treads don’t flex.