Stair Calculator

Set the step count and get riser height, run, stringer and pitch, checked against your limits.

Your stair

The opening

Measure the whole vertical distance, including the thickness of the finished floor at the top. Getting this wrong by the depth of a floor covering is the classic way to end up with one riser that is out of tolerance.
Tread depth is measured from the leading edge of one tread to the leading edge of the next, so the nosing overhang does not add to it. A 2x12 cut to 11.25 inches with a 1 inch nosing is a 10.25 inch tread.

The limits you build to

The model IRC allows 7.75 inches for a dwelling. States amend it: Michigan permits 8.25. Check what your jurisdiction has adopted rather than assuming the model figure, because this is the number that sets the step count.
The model IRC minimum is 10 inches; Michigan allows 9. It is only used to check the tread depth above, so set it to whatever your jurisdiction enforces.

How it is framed

The IRC minimum is 36 inches of clear width. Measure between the finished surfaces, not the framing, if there is drywall or a skirt board to come.
Sixteen inches suits most interior flights and 2x treads. Wider spacing bounces, and composite or thin treads usually want 12.
Prices
Price the stock length shown in the results rather than the cut length, since that is what you carry out of the yard.
Priced per linear foot of the board you will cut treads from. Risers, skirts and fasteners are not counted here.
Riser height
7.71 in
Risers 14 risers
Treads 13 treads
Total run 130 in
Angle
37.6 degrees
Run in feet
10.83 ft

The flight, to scale

Cutting the stringer

Stringer length 169.01 in
Stock to buy 16 ft
Stringers to cut 3 stringers
Tread material 39 ft

Code checks

Against your limits Riser and tread both sit inside the limits above
Landing Under the 147 in limit for one flight

What it costs

Material cost $252
Cost per step $18.00

Every figure here is a planning estimate, not a professional takeoff. Yields, waste factors and code requirements vary by product, supplier and jurisdiction, so confirm quantities against the manufacturer’s data and your local building department before you order.

Generated by Mathterial

A stair is the one job on this site where you do not get to choose the answer. The rise is whatever the building gives you, and every riser in the flight has to be the same height, so the only decision is how many steps to divide that rise into. Everything else follows: the riser height, the run, the stringer, the angle. This stair calculator starts from the step count for that reason, rather than asking you to guess a riser height first.

Why it starts with the step count

The whole stair from one division

risers = total rise ÷ maximum riser, rounded up

Then riser height = total rise ÷ risers, and the flight is uniform by construction

Rounding up is what keeps you legal. Divide a 108 inch rise by a 7.75 inch maximum and you get 13.9, so the flight needs 14 risers, each 7.71 inches. Round down to 13 and every riser is 8.31 inches, which is over the limit in most of the country. Sharing the rise equally between whole steps is also how the uniformity rule is satisfied without trying: the code allows the tallest riser to exceed the shortest by 3/8 inch, and a stair cut from one division has no variation at all.

One more riser than tread

A flight of 14 risers has 13 treads, because the floor you arrive at is the last tread. That is the single most common mistake in stair arithmetic, and it is worth 10 inches of run, which is often the difference between a stair that fits the opening and one that does not.

It also changes the angle. The pitch of a stair is one riser over one tread, so 7.71 over 10 is 37.6 degrees. Take the whole opening instead, 108 over 130, and you get 39.7, which is not the angle of anything on the drawing.

The limits are inputs, not constants

The model IRC puts the maximum riser at 7.75 inches and the minimum tread at 10, with headroom of 6 feet 8 inches and a hard stop at 147 inches of rise for a single flight. States amend those numbers: Michigan allows a 8.25 inch riser and a 9 inch tread. This calculator asks for the limits rather than assuming them, and tells you whether the geometry sits inside the ones you entered. That is an arithmetic check against your own figures, not a code compliance opinion, and your building department is the authority.

What the stringer needs

The stringer spans the whole opening, so its cut length is the diagonal of the total rise and the total run: 108 by 130 inches is 169 inches, or 14.1 feet, which means 16 foot stock because lumber comes in even lengths. Cutting a 14 foot board for a 14.1 foot stringer is a mistake you make exactly once.

Stair calculator FAQ

How many stairs do I need for a 9 foot rise?

Fourteen risers and thirteen treads. A 108 inch rise divided by a 7.75 inch maximum is 13.9, which rounds up to 14 steps of 7.71 inches each, and the run works out at 130 inches on a 10 inch tread.

How long does a stair stringer need to be?

The diagonal of the total rise and the total run. For a 108 inch rise and a 130 inch run that is 169 inches, or 14.1 feet, so you buy 16 foot stock because lumber comes in even lengths.

What is the maximum riser height for stairs?

The model IRC allows 7.75 inches in a dwelling, with a minimum 10 inch tread, and the tallest riser in a flight may not exceed the shortest by more than 3/8 inch. States amend these figures, so check what your jurisdiction has adopted.

How many stringers do I need?

Three for a 36 inch stair at 16 inch centers: one at each edge and one in the middle. Wider flights, thinner treads or composite material want them closer together.