I found a great online tool for calculating deck spans on the American Wood Council website. As you can see from the image below, you can enter all the relevent specs and get the maximum span. Click on the image to go to the tool.
This tool is great in the case of deconstructing and reusing lumber: You are in the position of having materials that drive the design process, instead of the normal situation where design leads to material selection.
In my case, I want to use the 2x4 studs in the current shed walls and rafters as the floor joists. They are too short to be used as those on the new shed. Also, I want to keep the shed as close to the ground as possible, since its already on a slope. The 2x4's will go across a series of beams (see last shed post).
The wood I have is stamped stud-grade Douglas fir, and the calculator gives a 6 ft max span for 12-inch centers. I am going 24 inches and doubling up, but it's the same structure (this is a shed, so I am not getting too precise).
Based on this, I am going to need to put a center beam in the floor, giving a 5 ft span - well within limits.
To get rid of the center beam (and posts and cost), I would need to go to 2x6 #2 lumber on 16 inch spacing. This may end up being a better option if I make the shed 11x18 or 10x20 instead of 12x16. I would prefer 12x16, but I have to fit it in between the required property set-back and a plum tree I want to keep in place. If I go that route, I will likely reuse the lumber for rafters. That will be this week's challenge.