You're allowed to earn �6475 tax free in the current tax year.
That allowance equates to �124.52 per week.
Last week was Week 18 in the tax year.
18 x �124.52 comes to �2241.36.
For anyone who's earned more than that, so far in this tax year, here's what happens:
The system subtracts �2241.36 from their earnings. The rest is taxable at 20%, so the system calculates 20% of their 'taxable earnings'. (That shows how much tax they should have paid up until the end of Week 18).
The system then looks at how much tax they've already paid this year, and calculates the difference (which is how much tax they must now pay).
For anyone who's earned less than �2241.36, so far, in the current tax year, there will be no tax to pay. (Lucky you!).
You won't start paying tax until your total earnings exceed the number of weeks into the tax year multiplied by �124.52
Chris