Okay, here goes . . .
Your Personal Allowance for 2010/11 is £6,300, which means you can earn £6,300 before you pay any tax (for most people it’s £6,475).
Let’s say, for example, that you earn £2,100 per month. You probably wouldn’t be very happy if you paid no tax at all for Months 1, 2 and 3, then suddenly your salary dropped by 20% for the rest of the tax year (once you’d hit the £6,300 PA limit).
The PAYE system smoothes out the tax, so that you receive the same net amount each month. In your case, you are entitled to earn £525 tax-free each month (approx £540 for others) ie £6300 / 12
However – and this is the clever bit – the PAYE system is self-correcting, so if you had no income for a period, then the “unused” chunks of Personal Allowance normally get transferred into subsequent months when you next get paid.
The way it usually works is that tax is calculated on how much you’ve earned so far in the tax year, less the accumulated chunks of PA.
So, in Month 2 (May), it would be:
Gross 2 months @ £2,100 = 4,200
Less PA 2 months @ £525 = 1,050
Taxable = 3,150
And in Month 10 (January) , it would be:
Gross 10 months @ £2,100 = 21,000
Less PA 10 months @ £525 = 5,250
Taxable = 15,750