Stair Calculator

Stairs are unforgiving of bad maths. Risers that vary by even a fraction of an inch are a trip hazard, and a stair that is too steep or too shallow is uncomfortable and may fail inspection. This calculator takes the one measurement that governs everything — the total rise from finished floor to finished floor — and divides it into equal, comfortable steps, then gives you the tread run and stringer length to lay them out.

How a stair is calculated

The key principle is that every riser must be the same height. You pick a target, divide the total rise by it, round to a whole number of risers, then divide the rise back by that whole number to get the exact, equal riser height. That recalculated height is what you build to.

Measuring the total rise

• Measure from finished floor to finished floor, not subfloor to subfloor — the finished surfaces are what people step on.
• Include the upper floor thickness if the stair lands on a different level’s finished surface.
• Be precise: an error here is multiplied across every step, so measure twice.

Code limits to respect

These vary by jurisdiction; always confirm your local building code before cutting stringers.

A worked example

A staircase with a 108 in total rise, target riser 7.5 in, tread depth 10.5 in:

• Risers = 108 ÷ 7.5 = 14.4 → 14 risers
• Actual riser height = 108 ÷ 14 = 7.71 in (all equal)
• Treads = 14 − 1 = 13
• Total run = 13 × 10.5 = 136.5 in
• Stringer = √(108² + 136.5²) = about 174 in

From numbers to cut stringers

With the riser height and tread depth set, you mark a framing square on the stringer stock, stepping off each rise and run, then cut. Remember to account for the thickness of the tread material when marking the bottom riser, so the finished steps end up equal. The stringer length above tells you the minimum board length to buy — the related board feet calculator helps price the lumber.

Landings, railings and headroom

Long flights may need an intermediate landing, which resets the rise calculation for each section. Every stair also needs a graspable handrail at the code-specified height, and enough headroom that taller users do not duck. Plan these alongside the step geometry, not after, since a landing changes how many risers each run contains.

Why equal risers matter so much

The human body climbs stairs on autopilot, expecting every step to be identical. A single riser that is even a fraction of an inch taller or shorter than the rest is a genuine trip hazard, because the foot lands where the brain expects the step to be and finds it is not. This is why building codes limit the variation across a flight to a small tolerance, and why the calculator divides the total rise into equal risers rather than letting you set an arbitrary height. Measure the total rise precisely, because any error there is divided across every step and can push the variation out of tolerance.

Stringers, treads and the cut list

The stringers are the sawtooth boards that carry the steps, cut from wide stock using the riser and run dimensions stepped off with a framing square. The stringer length the calculator gives is the minimum board length to buy, with extra for the cuts. Treads and risers are then fixed to the stringers. Remember to account for the thickness of the tread material when marking the bottom step, so the finished risers come out equal once the treads are on — forgetting this is a classic error that leaves the first and last steps slightly off. The related board feet calculator helps price the stringer, tread and riser stock.

Headroom, width and landings

A comfortable stair needs more than good step geometry. Headroom — the vertical clearance above the stairs — must let the tallest user climb without ducking, typically at least six feet eight inches by code. Stair width affects how it feels and whether furniture can pass. Long flights often require an intermediate landing, both for safety and because codes limit the vertical rise of a single run; a landing resets the rise calculation for each section. Plan these before cutting stringers, since adding a landing partway through changes how many risers each run contains.

Railings and finishing

Every stair needs a graspable handrail at the code-specified height, continuous along the flight, and guards on open sides to prevent falls. These are life-safety requirements, not optional trim. Open-riser stairs have gaps limited so a small child cannot pass through. Once the structure is right, the finish — carpet, hardwood treads, paint — goes on, and the nosing (the slight overhang at the front of each tread) is finished to be visible and slip-resistant. Get the geometry right first with equal risers and comfortable treads, then build the railings and finishes to match, and the stair will be safe and pleasant to use for its whole life.

Building to code and inspection

Stairs are heavily governed by building code because they are a common site of serious falls, and a stair that fails inspection must be rebuilt — expensive and disheartening. The key limits are maximum riser height, minimum tread depth, the tiny allowable variation between risers, minimum headroom, handrail height and graspability, and guard requirements on open sides. These vary by jurisdiction, so confirm the local figures before cutting stringers. The calculator produces equal risers and a comfortable tread within typical limits, but it is your responsibility to check the specific code that applies. For an interior stair in a permitted project, expect an inspection of the framing and the finished stair. Getting the geometry right the first time — equal risers above all — is far cheaper than rebuilding, so measure the total rise precisely, divide it into equal steps, and verify every dimension against code before you commit saw to stringer.