I am buying a new road bike. I have looked online for calculators but I am getting different answers on which frame size I should get. I've got 53 cm on a calculator that only considers inseam height and 56 cm on another one that also considers riders height so I don't know which one is correct because I don't want to end up with a bike that is either too big or too small for me.
IMO Frame size is more than just measurements. A proper bike fitting pairs your riding style & level with the frames own handling characteristics. (defined more by tubing and angles than measurement alone)
BUT... not everyone's out to get a custom bike frame made or go research their frames butted tubing or headtube angles... But I always suggest visiting a place where they know how to "fit bikes" (for racing, touring, etc) and getting advice from a bike fitter.
Don't forget to buy a water bottle and power bar while you're there, chances are they don't charge you for the advice. #smallbusiness
As I have written in another thread, the seat tube length (a.k.a. "frame size") is not the most important measure for how a bicycle fits you. Bicycle fit is mostly about putting the contact points between rider and bike (saddle, handlebars and pedals) into the correct positions with regard to the rider's body.
Where these points should be is a matter of body geometry, riding style and personal preferences - and not an easy question to answer (although there are some general guidelines that work pretty well for many riders). As the starting point should not be the bike but the rider, it is especially hard to answer bike fit questions over the internet without seeing you sit on a bike. Guidance from someone who knows about bicycle fitting is a good investment.
In order to determine if your contact points could be positioned correctly on a certain frame, the measures stack and reach are more important than the seat tube length. Stack is the vertical distance between the bottom bracket and the top of the head tube, and reach is the horizontal distance between these two points. These two measures (plus handlebars, spacers and stem geometry) determine basically where the hands will be in relation to the feet. When these two points are fixed, the saddle can pretty easily be positioned correctly by using an appropriate seat post (length and setback).
tl;dr: Try to find out what stack and reach you need, either by consulting a bike fitter or by measuring a bike that fits you very well. Compare frames by their stack and reach measurements instead of the seat tube length to see if they could fit you.
I have found that the one from Competitive Cyclist is pretty good: http://www.competitivecyclist.com/Store/catalog/fitCalculatorBike.jsp
It takes into account the lengths of your legs and arms, so prepare for some measuring. Also, if you have no idea what you are looking for, do not buy blind but try the bikes in person.