The intent of the simple calculator below is to help you choose the best gearing for the kind of riding you do on a fixed or single speed. Generally, the main things to consider are terrain, what speed do you normally maintain on the flats, and at what approximate crank revolution (RPM) rate, and what your training goals (high RPM/anerobic conditioning, lower RPM/time trialing, etc.) are.
The idea is to insert different sizes and/or RPM rates, and see how that speed compares to what you're looking for. For example, the default setup on the 700C sized Wabi bikes is 46T front ring and 18T rear cog. This yields an 18.5 MPH speed at 90 RPM and allows for good acceleration and climbing. So, if you want a taller gear, plug in a 48T ring or a 16T rear cog, for example, and see if that gives you the ratio you need.