How to Calculate Stairs: Rise, Run, and Stringers

Before you can lay out stairs you need four terms. Total rise is the full vertical height from the lower finished floor to the upper finished floor. A riser is the vertical face of a single step, and its height is the total rise divided by the number of steps. The tread is the horizontal surface you stand on, and its depth is called the run. The stringer is the angled board that supports the whole thing.

Every safe stair is a balance between these numbers. Too tall a riser and the climb is exhausting and easy to trip on. Too shallow a run and your foot does not fit. The goal is a set of steps that all feel identical, because uneven steps are the single most common cause of stair falls.

Everything starts with the total rise, so measure it carefully from finished floor to finished floor. To find the number of steps, divide the total rise by a comfortable target riser height, often around seven inches, and round to the nearest whole number. You cannot have a fraction of a step, so rounding is required.

Once you have a whole number of risers, divide the total rise by that number to get the exact riser height. Dividing evenly is what guarantees every step is the same height. A total rise of 105 inches over 15 risers gives exactly seven inches per riser, which is why the whole-number step count comes first.

The calculator handles the rounding and the geometry so every step comes out equal.

Building codes set limits so stairs are safe for everyone. A widely used residential guideline caps riser height at about 7.75 inches and requires a tread run of at least 10 inches, though your local code is what governs your project. Codes also require that risers within a single flight stay very close in height, typically within about three eighths of an inch of each other.

Beyond the hard limits there is a comfort rule carpenters use: twice the riser height plus the run should land between roughly 24 and 25 inches. Stairs that satisfy this relationship feel natural to climb because the trade-off between how high and how far each step moves matches a normal stride.

A staircase that is comfortable to climb still has to fit the room. The total run, which is the number of treads times the run of each, tells you how much floor the stairs will consume. If that length overruns the space, you may need a steeper design, a landing, or a turn, each of which changes the layout.

Do not forget headroom. Codes require a minimum clear vertical space above the stairs, often around six feet eight inches, so anyone on the stairs does not hit the ceiling or floor above. Confirm the riser height, run, total run, and headroom all work together before cutting your stringers, since a mistake in the layout is expensive to fix once boards are cut.