Decks, Patios, Porches and Docks - Stair Stringer Confussion

I want to add another set of steps to my existing deck but calculating the stringer size is confusing me. The top of the deck where I want to put the steps is 51" off the ground. I want the 1st step even with the deck surface. How long of a stringer do I need to make a comfortable climb. My interior steps have a 7" rise and I think that would transfer nicely. Would it be better for me to buy 3 precut stringers or make my own & if you suggest make my own then I would need the builders square figures for the rise and return markings. I built the deck without difficulty several years ago, you would think making the additional steps would be no big deal, but I just can't get pasted the math...

Thanks,

Doug

ok you only need 1 more measurment to figure out how to do this how long is your run?you gave the rise

Buy the premade step stringers. They will all be the same, and be treated, and are cheaper then you can buy the lumber to build them. To get the right ones, measure the distance up and down from where you want to start to the bottom, and divide by 7". This will tell you how many steps in your stringer you want. Get three of them. One for each end and one for the middle.