Stair Stringer Calculator

Q: How do I calculate the number of stairs from the total rise?

Divide the total rise (the vertical distance from the lower floor to the upper floor) by a comfortable target riser height of about 7 to 7.5 inches, then round to the nearest whole number. That gives the number of risers. The actual riser height is the total rise divided by that number, and the number of treads is always one less than the number of risers.

Q: What is the maximum riser height and minimum tread depth?

Under the International Residential Code, the maximum riser height is 7 and 3/4 inches and the minimum tread depth is 10 inches. The variation between the tallest and shortest riser in a single flight must not exceed 3/8 inch. Local codes can differ, so always confirm with your building department.

Q: How long does a stair stringer need to be?

The minimum stringer board length is the diagonal of the staircase, found with the Pythagorean theorem: the square root of the total rise squared plus the total run squared. Buy a board longer than that figure to leave material for the top and bottom cuts.

Enter your total rise to get the step count, riser height, tread depth, total run, stringer length, and stair angle, checked against common code limits.

Your staircase

Total rise (floor to floor)

Target riser height (in) (sets the step count)

Tread depth / run per step (in)

Stair width (in) (optional)

Stringer spacing (in)

How to lay out a stair stringer

Every stair calculation starts from one measurement: the total rise, the vertical distance from the surface of the lower floor to the surface of the upper floor or landing. Everything else follows from it.

Risers and treads

Divide the total rise by a comfortable target riser height, around 7 to 7.5 inches, and round to the nearest whole number to get the number of risers. The actual riser height is the total rise divided by that count, so every step comes out equal. The number of treads is always one less than the number of risers, because the top landing acts as the final step.

Run, stringer length, and angle

Total run is the number of treads times the tread depth. The stringer board itself follows the diagonal of the staircase, so its minimum length is the square root of the total rise squared plus the total run squared. The stair angle is the arctangent of rise over run, and a comfortable flight usually lands between 30 and 37 degrees.

Code limits worth knowing

The International Residential Code caps riser height at 7 and 3/4 inches and sets a minimum tread depth of 10 inches, and it requires that the tallest and shortest riser in a flight differ by no more than 3/8 inch. This calculator flags whether your riser and tread fall within those limits, but local codes can be stricter, so confirm the final numbers before you cut.

Stair stringer FAQ

How do I calculate the number of stairs from the total rise?

Divide the total rise (floor to floor) by a comfortable target riser of about 7 to 7.5 inches and round to the nearest whole number for the riser count. Actual riser height is the total rise divided by that number, and treads are always one fewer than risers.

What is the maximum riser height and minimum tread depth?

Under the IRC, maximum riser height is 7 and 3/4 inches and minimum tread depth is 10 inches, with no more than 3/8 inch variation between risers in a flight. Local codes can differ, so confirm with your building department.

How long does a stair stringer need to be?

The minimum board length is the diagonal of the staircase: the square root of the total rise squared plus the total run squared. Buy longer than that to leave material for the top and bottom cuts.