Stair Stringer Calculator — Length, Angle & Riser Height

Result

Theoretical values for estimating only. Confirm against the mill certificate or a stamped calculation before ordering or fabricating.

A stringer is a right triangle. Get the rise and the run right and the length and angle follow exactly — which is what you need before you order material or mark a cut.

How the stringer length is found

The stringer is the hypotenuse of a right triangle whose height is the total rise and whose base is the total run. Run = (number of risers − 1) × tread depth, because the top riser lands on the finished floor.

Riser height and why it must be equal

Riser height = total rise ÷ number of risers. Every riser on a flight must be identical; a single uneven riser is the most common cause of stair falls and is a code violation in every jurisdiction.

Code limits to check

Most codes require risers between 100 mm and 200 mm and treads of at least 250 mm, with a maximum variation of a few millimetres across a flight. The calculator flags a result outside these limits.

Cutting the stringer

Allow for the plumb cut at the top and the level cut at the bottom — roughly one stringer width at each end. The calculator adds two stringer widths to the theoretical length so you can order the right board.

Frequently asked questions

How do I calculate stair stringer length?

Square the total rise, square the total run, add them and take the square root. For a 2,800 mm rise and a 3,750 mm run the stringer is about 4,680 mm.

How many risers do I need?

Divide the total rise by a target riser height — typically 180 mm. For a 2,800 mm rise that is 15.6, so 16 risers at 175 mm each.

What is the maximum riser height allowed?

200 mm (about 7.75 in) under most building codes, with 190 mm common in commercial work. Anything above 200 mm is normally not permitted on a general-access stair.