Stair Calculator

Calculate stair rise, run, step count, stringer length and total horizontal run from a floor-to-floor height.

What this calculator measures

Stairs are governed by geometry and by code limits on rise and run. This calculator turns a single floor-to-floor height into a complete stair layout: how many steps, how tall each step is, how deep each tread is, how long the stringer must be and how much horizontal space the staircase consumes. Getting these numbers right before cutting lumber prevents the two classic errors — a staircase that does not fit the opening and a last step that is a different height from the rest.

The tool treats the stair as a right triangle: the floor-to-floor height is the vertical leg, the total run is the horizontal leg, and the stringer is the hypotenuse. Every intermediate value is shown so the layout can be checked against a framing square and against the local code limits for rise and run.

How the calculation works

The calculator starts from the floor-to-floor height and a target rise per step (commonly 7 to 7.5 in). It divides the height by the target rise and rounds up to a whole number of steps so no step exceeds the target. It then divides the height evenly by that step count, giving an actual rise that is slightly less than the target. Total run is the number of steps multiplied by the chosen tread depth (run). Stringer length is the hypotenuse of the right triangle formed by the height and the total run.

Because the rise is distributed evenly, every step — including the top and bottom — is identical, which is the single most important safety property of a staircase. The horizontal space required is the total run plus any landing or nosing allowance you add on site.

Worked example

Using a floor-to-floor height of 108 in (9 ft), a target rise of 7.5 in and a 10 in tread depth, the calculator returns:

Fifteen steps at 7.20 in each reach exactly 108 in. A 10 in tread gives 150 in of total run, about 12.5 ft of horizontal space. The stringer must be at least 15.4 ft long before cuts. If the available opening is only 10 ft, the tread depth or the number of steps must change — and the calculator makes that trade visible.

Reading the result: the step count and actual rise are the numbers to check against code; the total run is the number to check against the available floor space; the stringer length is the number to check against stock lumber length.

Inputs, field by field

• Floor-to-floor height (in): Measure from finished floor to finished floor, including any subfloor or finish thickness that will be present at both levels.
• Target rise per step (in): The desired riser height. Residential stairs commonly target 7 to 7.5 in; many codes cap rise at 7.75 in. A lower target means more steps and a longer staircase.
• Tread depth / run (in): The horizontal depth of each step, usually 10 to 11 in. Deeper treads make a more comfortable stair but consume more horizontal space.

Why the inputs matter

Each field changes both the comfort and the legality of the staircase. Height sets the problem; target rise sets the step count; tread depth sets the horizontal footprint. Because rise and run are coupled through the available space, changing one usually forces a change in the other.

Units and conversion checks

All dimensions are in inches, with feet shown alongside for the totals. Consistency matters: a height entered in feet while the rise is in inches will produce a nonsense step count. Convert before entering, or enter everything in inches.

Common mistakes with this calculation

• Using the rough framing height instead of the finished floor-to-finished floor height, which leaves the top or bottom step the wrong height.
• Rounding the step count down instead of up, producing a rise that exceeds the target and possibly code.
• Forgetting that the top tread is the finished floor, so the number of risers is one more than the number of treads in some conventions — check your local convention.
• Cutting stringers to the calculated length without adding for the bottom cut and the top bearing.
• Ignoring headroom: the stringer length and run must also clear the code minimum headroom over the stair.

Ordering and materials

Stair framing consumes a surprising amount of lumber: stringers (often three for a 36 in wide stair), treads, risers and a landing if required. Order stringers in the next standard length above the calculated length and check that the lumber grade is suitable for structural use. Treads and risers are usually ordered by the linear foot with a waste allowance for miters and defects.

What this calculator does not decide

This tool does not determine code compliance, required headroom, handrail or guard requirements, structural capacity of the stringers, or whether a landing is required. Those depend on the applicable building code, occupancy and design loads. Where regulated construction is involved, the project documents and qualified professionals take precedence.

Field verification checklist

Before cutting, re-measure the floor-to-floor height in two places, confirm the finished floor thicknesses at both levels, check the step count and rise against the applicable code, verify the total run fits the available opening, and confirm headroom over the entire stair run.

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Practical workflow

Measure the finished floor-to-finished floor height, choose a target rise and tread depth, calculate, check the step count and rise against code, verify the total run fits the opening, then size and order the stringers and treads.