There are some tools out there to help design PVC projects. One is Google SketchUp. I use it all the time to make mine, and Formufit has downloadable components of all their PVC fittings on their website (formufit.com) in their Downloads section, and you can also find them on the Google 3D Warehouse. If you don’t know how to use SketchUp, look it up on Instructables or YouTube. It’s marvelous.
When designing PVC projects that involve fittings/connectors, there are a few things to keep in mind:
Include the fitting: This is obvious, but I have to say it. The length of any pipe that is attached to a fitting must include the fitting. Duh, right? Well, I’ve made a mistake or two on this one. If you make a project that is a square box and you use fittings on the ends, and you want it to be 2’ square, you need to cut the pipe shorter than 2’, as the fittings on each end will take up extra length themselves. So if your fittings take up an extra 1” at each end, the pipe needs to be cut to 1’-10”, to accommodate the fittings.
The distance that the pipe goes into the fitting is equal to its diameter (normally): Sound complex? It isn’t. To embellish on this, you need to also remember that the pipe usually goes into the fitting a certain amount (so you can cement or screw it), and is different for the pipe size. There is a detent inside of each fitting that stops the pipe from going too far in. For instance, a 3/4” pipe will go into a 3/4” fitting about 3/4” of an inch. A 1-1/4” pipe will go into a 1-1/4” fitting about 1-1/4”, and so on and so forth. Make sense? So if you are building something you need to make sure that you account not only for the fitting length, but the amount of pipe that actually goes into the fitting itself. Try it out; you’ll see what I mean.
Use the right pipe: If you have something that is going to bear weight, be it a person, or a structure, make sure you use at least 1-1/4” pipe or above. 1” or lower is not going to hold much weight and will flex. And you will fall on your bottom.