How do you know the amount of tax you pay in using bitcoin as payment?

[sumangs]

I know that tax depends on earnings, but how do they tax it?

[Iranus]

It only depends on the country you live in.  In the UK:

-Capital gains tax is paid when converting to fiat
-VAT is paid when paying for goods and services (how well this is enforced is another matter)
-Mining and mining-related services are exempt from VAT.

You have to be careful about declaring it if you're dealing with large amounts.  The repercussions of not paying tax when cryptocurrencies are handled better by the authorities could be a lot worse than the taxes that you have to pay on them.