Most wooden stringers are cut from 2x12 lumber, which leaves enough wood below the notches for strength.
How the stair calculator works
Stairs start with one measurement: the total rise from the finished lower floor to the finished upper floor. That height is divided by your maximum riser height and rounded up, which gives the number of risers. The total rise is then divided by that number so every riser is exactly the same height. A flight with uneven steps is a trip hazard, so codes only allow a tiny variation.
When the top step is the upper floor itself, there is one fewer tread than risers. The total run is the number of treads multiplied by the tread depth, and the stringer length is the diagonal of the total rise and total run. StairCalc rounds the stringer up to the next standard lumber length so you know which board to buy.
Laying out a stringer
Set stair gauges on a framing square at the riser height on the tongue and the tread depth on the blade. Mark the first notch at the top of the board, slide the square along and repeat for every step. Cut the bottom of the stringer short by the thickness of one tread so the first step is not taller than the rest. Cut one stringer, test it in place and use it as a template for the others.
Comfort rules
A long-standing rule says two risers plus one tread should be about 24 to 25 inches, or 600 to 640 mm, which matches a normal walking stride. The UK uses a wider band of 550 to 700 mm. A steep, short-tread stair saves floor space but feels harder to climb, especially going down. Everything is calculated in your browser.
Frequently asked questions
How long should a stair stringer be?
The square root of total rise squared plus total run squared, then buy the next longer lumber size.
How many stringers do I need?
Usually one every 16 inches across the stair width, so three for a 36 inch stair.
What size board for stringers?
2x12 is standard; it leaves about 5 inches of wood below each notch.
Is StairCalc free?
Yes, with no sign-up or ads.
Are my measurements saved?
Yes, in this browser only.