Do you really need a bike fit? More often than not people have difficulty distinguishing the difference between a bike sizing and actually being fitted on a bike. Bike sizing tools and bike fitting tools will often be similar but the approach to how and why something is being done is very different. The easiest way to explain the difference is bike sizing is about the bike and then you, whereas a bike fit is about you and how you interface with the bicycle and its components.
A quick search on the net will show a huge discrepancy in how or why one would choose one size or model bike over another. In most cases, bike sizing is done using a protocol, bar width similar to the width of the client's shoulder, knee over the pedal axle, cleats in a neutral position, and setting seat height to a spot relative to 30 - 35 degrees from full extension of the legs. Using this concept a person of 5'10" could be on a bike ranging from a 51cm to 56cm. Bike fitting, on the other hand, is all about you as an individual and usually takes around 3 hours. The fit specialist and the cyclist will work 1 on 1 to understand your cycling past, any injuries that you may have and what your goals are. The fit process begins with a physical exam evaluating any asymmetry of the body, the flexibility of the individual and possible issues relating to the person's range of motion. The tools used in bike fitting create a process that has better data and is more efficient in the fitting process. A proper bike fit has more to do with the saddle, handlebars, brake levers and hoods, stem and, most importantly, shoes, cleats, and pedals.