How to make a stair railing
Reduce it to inches. 15 feet is 180 inches, plus 7 &amp; 3/16 inches

Total distance 187.1875 inches. Divide by 6 (4" space and 2" slat) 31.1979166666667

Per my understanding of stair code (LESS than 4", not 4") use 32 2" slats. Or 3" or 4" slats, for that matter.

Alternatively, divide by 32 (proposed number of slats) and get a unit (slat and space) of 5.849609375 ; or divide by 31 and get a unit of 6.0383064516129