Calculate stair stringer length, the number of notches and the rise and run cut marks for laying out a stringer board.
The calculator divides the total rise into equal risers, works out the matching number of treads, the total horizontal run, the stringer length (the diagonal you cut from) and the stair angle. It also checks the “2 × riser + going” comfort rule used by builders.
The stringer is the inclined board that supports the treads and risers. Its length is the diagonal of the staircase, found from the total rise and total run using Pythagoras.
UK Building Regulations allow domestic risers of 150–220 mm with a going of at least 220 mm. In the US the IRC limits risers to 7¾" maximum with a minimum 10" tread. Around 190 mm / 7.5" is a comfortable target.
Twice the riser height plus the going (tread depth) should fall in a comfortable range – about 550–700 mm (24–27½") . The calculator flags whether your design meets it.
The top step lands on the upper floor, so a flight always has one more riser than it has treads. The calculator accounts for this automatically.
Buy timber a little longer than the calculated stringer length to allow for the top and bottom cuts and any fixing. Always mark and cut a test stringer before committing.
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