PCB microstrip width calculator
The impedance of your transmission line depends on a wide variety of factors. The main value you want to match is determined by your drivers/receivers. A lot of single-ended drivers shoot for 50Ω but you MUST look at the datasheet for information.

Once you have your target impedance, you need to consider your PCB dimensions. The position of the signal trace with respect to its reference plane greatly affects the impedance, as does the thickness and type of dielectric. You really need to consider the PCB stackup. Rough estimates will not work here. Furthermore, if there are any other traces nearby they will also have an effect on the impedance, as well as their shape and physical size. In my experience all of those calculators out there designed for just one trace are practically useless as they don't take surrounding traces into effect, and the calculated impedance can be off by 10Ω or more just due to surrounding traces.

I was recently introduced to a very powerful tool called HyperLynx which I have found to be extremely useful. It is very expensive ($18k USD) but there is a free trial that should help you out. I recommend the HyperLynx SI Virtual Lab for your project: https://www.mentor.com/pcb/product-eval/hyperlynx-si-virtual-lab

This software uses Maxwell's equations and a variety of other mathematical formulas to accurately calculate impedance. There is also a tool for checking the coupling between traces and for doing field analysis (to determine if crosstalk would be a problem or not). There is a bit of a learning curve but I highly recommend it. Here are a couple of screenshots:

Differential Impedance:


Single-ended Impedance:


Significant field collisions (bad design--will be too much crosstalk):


In short, do not put much faith in the single calculators you find online. They will often lead you astray, unless you only have one trace on your board, or surrounding traces are a centimeter away.