I am using the Kicad trace width calculator tool for finding the width required for a 50 ohm trace.
The obtained value of trace width is 0.355mm. Assume this trace width is not enough for the required current.
I checked many other online tools but I did not see an option to put the current specification in anywhere.
Why is it like that?
For example, I need a 50 ohm trace on a two layer PCB which carries 5A current. How do I find the required width using these tools?
Any calculator makes assumptions. A calculator that required you to explicitly restate things like: "I won't be pouring molten metal on it during operation" would not save you enough time to become popular. PCB calculators usually assume that your stackup (materials, layer count and order, thicknesses) is already fixed; you supply these values, but it won't recommend changing them because that isn't the typical use case.
When calculating current capacity, it is a little more likely that you will be able to use thicker copper, but the normal case is still likely to be "Given copper of this thickness, how much current can it carry?" or "Given copper of this thickness and width, how much current can it carry according to legally required safety standards?" or "Given copper of this thickness and width, and given this current, how much temperature rise should I plan for?"
If you care about both impedance and current capacity in the same trace (and normally you don't!), then first calculate the minimum size with a current-carrying calculator, and then go to a different (impedance) calculator to see if that works. If it doesn't work, then you need to change your stackup somehow -- typically by using a thicker substrate, or at least clipping out the local ground plane so that it has to use a more distant reference. The impedance calculator can tell you if this new stackup works, but it won't usually suggest changing the stackup (or adding cooling), because those options are "beyond its paygrade".
That's due to conflicting requirements.
You can either use some PCB material and stack-up where a certain width of some transmission line type gives you exactly 50 ohm impedance you want at some high frequency.
Or you can define a current you with the copper thickness you want and get track width you need to pass that current with some defined drop along the defined length.
But the width of a track that has 50 ohm impedance and track that can pass 5A with your requirements will have a different width based on calculations, so you can't have both.
You would at least try to change the type of transmission line, or impedance, or PCB material, or PCB stackup, or current carryin ability, to converge on a result that fills all the things you want - impedance and current.
Perhaps there is a misunderstanding and you really don't need a 5A 50 ohm trace.
