Turnover is not income: the four layers
The balance of your business account is the worst dashboard there is. It goes up when a client pays, down when a deadline lands, and it never tells you which of the two movements was actually yours.
Between what you invoice and what you really keep, there are four layers. None of them is optional.
1. VAT was never yours
You collect it for the state. It passes through your account, sits there for a few weeks, and leaves. A quarter that looks excellent because the VAT has not gone out yet is not a good quarter: it is a quarter whose price has not been paid yet.
So the first discipline is to look at your turnover excluding VAT, always.
2. Business expenses
What you spend in order to work is deducted from what you will be taxed on. A software subscription, equipment, professional insurance, travel: every justified expense lowers the base.
That is not a discount on the cost, it is a discount on the tax on that cost — the difference matters: spending to spend is still spending.
3. Social contributions
They are computed on your professional income and fund your cover: pension, healthcare, incapacity. They are not a tax, even though they leave the same account, and they are themselves deductible from the taxable base.
Their peculiarity is the lag: they are often paid on income that is not the current year's, then reconciled. It is the lag that surprises people, not the amount.
4. Income tax
It comes last, on what survives the three layers above. It is also the most progressive layer: the first euro you earn and the last one do not cost the same.
What it means day to day
You do not need the rates by heart — they change, and they depend on your country and your situation. You need to know that at any moment, part of your balance already belongs to someone else.
The only figure that counts is the one left after all four layers. Everything else is an optical illusion.