Fiat currency doesn't really have a set amount anymore. Millions of companies and governments have borrowed money and promised to exchange future goods and service in return. These debts are held by banks who issue deposits. This gives a backing to the USD or GBP. Stocks are valued at the present value of their future dividends.

Obviously though, most cryptos don't have backing assets or dividends so it's quite hard to value them. One way might be to think of the value of a bitcoin is that you can use it to put bytes on to the blockchain. Might write a post on this method.