Hi Rob, If you're thinking of using these types of railings that I referred to, you really don't have to calculate anything short of the distance between the end post and the top post.
Since the bars between posts are all the same length, it's fairly simple to just measure out the distance you need to go between the post, then buying what you need to assemble it. I should also note that the cross bar will adjust to any slope, as the ends of the crossbar is hinged, so you can swivel it up and down to adjust for the degree of slope you need. I think this is great, especially if you're dealing with some uneven steps, as is the case on most older homes. Since I was pouring new concrete steps in the front, I was able to slope it just right, but aside from that, no muss, no fuss. You might want to check with Lowes and Home Depot and see if they can advise you on how to measure it. Victor Co-moderator Blind Movie Buffs List Guidedogs List
