Stair Calculator

How this stair calculator works

The stair calculator has two modes. Compare Step Counts takes the total rise (floor to floor height) and either the run of one step or the total run, then lists every layout whose rise falls between 5 and 10 inches. Design One Staircase works out a single staircase from a fixed rise or a fixed number of steps, with optional tread thickness, a headroom check, and the stringer mount.

For example, a total rise of 108 inches (9 ft) with 10-inch treads can be built with 13 steps and 14 risers of 7.71 in. The total run is 10 ft 10 in, and each stringer is 13 ft 8.19 in long and climbs 8 ft 4.29 in at an angle of 37.65°.

How to calculate stairs: rise and run

The rise is the height of one step, measured from the top of one tread to the top of the next. The run is the depth of one tread. A stair rise and run calculator, or stair riser calculator, divides the total rise into equal risers, and there is one more riser than treads because the last riser ends at the upper floor:

Rise = total rise ÷ risers
Steps = risers − 1
Total run = steps × run
Angle = arctan(rise ÷ run)

With a fixed rise, the calculator uses as many full rises as fit and makes the first step from what is left over. A 108-inch total rise with a 7.5-inch rise gives 14 full rises and a 3-inch first step, so a fixed number of steps is usually the better choice when you want every step the same height.

Stair stringer calculator

The stringer is the sloped board that carries the treads. Its length is the hypotenuse of a right triangle formed by the total run and the stringer height:

Stringer length = √(total run² + stringer height²)

With a standard mount, the stringer stops one rise below the upper floor, so its height is the total rise minus one rise. A flush mount runs the stringer up level with the upper floor, which adds one step and makes the stringer height the full total rise. If the treads sit on top of the stringer, the tread thickness comes off the stringer height and off the first step cut, so the first step keeps its intended height once the tread is on.

Stair headroom and floor openings

Turn on the headroom check to enter the length of the opening in the upper floor, the floor's thickness, and the headroom you need. The calculator measures the clearance from the tread below the edge of the opening up to the underside of the floor, and if it falls short, how long the opening needs to be. Most US building codes require at least 6 ft 8 in of headroom.

Frequently asked questions

How many steps do I need?

Divide the total rise by the rise you want and round up to get the number of risers; the number of steps is one less. For a 108-inch rise and a code maximum of 7¾ inches, 108 ÷ 7.75 = 13.9, so you need 14 risers of 7.71 in and 13 steps.

What are the building code limits for stairs?

The International Residential Code, which most US states adopt, allows a riser of up to 7¾ inches and requires treads at least 10 inches deep and at least 6 ft 8 in of headroom. Risers within one flight may differ by no more than ⅜ inch. Local codes can differ, so check with your building department.

Can I use this as a deck stair calculator?

Yes. Measure the total rise from the ground where the stairs will land up to the top of the deck surface, then use Compare Step Counts or Design One Staircase as you would for indoor stairs. Remember that the ground may slope, so measure at the spot where the bottom step will sit.

What is the difference between total run and run?

The run is the depth of one tread. The total run is the horizontal distance the stairs cover from the first riser to the last, which is the run times the number of steps. If your space limits how far the stairs can extend, enter the total run and the calculator finds the tread depth for each step count.

Limitations

The calculator assumes a straight flight of equal steps with no landings, winders, or nosing overhang, and it measures headroom from the top of the tread below the opening rather than from the sloped line along the tread nosings that codes use. Lumber sizes, local building codes, and site conditions all affect the final layout, so confirm the measurements before you cut stringers.