Stair Stringer Calculator

A stair stringer calculator gives you the length of stringer to cut, the number of risers and treads, the exact riser height, and how many stringers the stair needs — everything to lay out and order the carriage boards. The stringer is the sawtooth board the steps sit on, and getting its length and notch layout right is the whole job.

Your measurements

Result

Enter the total rise (finished floor to finished floor) and your tread run, and it sizes even risers, then gives the diagonal board length so you buy stock long enough.

How it is calculated

Risers = total rise ÷ ~7.5 in, rounded; riser height = total rise ÷ risers; treads = risers − 1; total run = treads × tread run. Stringer length = √(total rise² + total run²). Stringers = stair width ÷ spacing + 1. Codes limit riser height and tread depth and require uniform risers — follow local code.

Steps

Measure the total rise from finished floor to finished floor.
Enter your tread run (10 in is common).
Enter the stair width and stringer spacing (16 in OC typical).
Read the stringer length, riser count and riser height.
Buy boards a little longer than the stringer length for the cuts.

Worked example

A 44 in total rise with 10 in treads gives 6 risers at 7.33 in and 5 treads — a stringer about 5.5 ft long, with 3 stringers for a 36 in wide stair.

Common questions

How long should a stair stringer be?

The diagonal √(rise² + run²). For a 44 in rise and 50 in total run, that's about 5.4 ft — buy stock a bit longer for the cuts.

How many stringers do I need?

One every 16 inches across the width, plus one — so a 36 in stair needs three. Wider treads or heavy loads want closer spacing.

What is a comfortable riser height?

Around 7 to 7.75 inches, kept uniform within 3/8 inch. The calculator divides the total rise into even risers.

Do I cut the stringer height for flooring?

Yes — drop the bottom of the stringer by one tread thickness so the first and last steps match once flooring and treads are on.