The code is usually made up of several numbers and a letter. In general this represents the amount (divided by ten) of income you can receive before you pay tax. Therefore if you are entitled to ٤,475 before paying tax then your tax code will be 647L. It follows that most codes are numbers although some are just letters, for example BR (Basic Rate) which tells the employer to deduct tax on all income at the basic rate, or NT (No Tax) usually for non-resident individuals who are not liable to UK tax on their earnings. If you are entitled to the basic Personal Allowance then L is added to the number. If you are entitled to a higher allowance due to age then P (65 to 74 ) or Y (over75 ) are added.
You will be allocated a K code if your deductions are more than your allowances. If you're starting your first job your employer will give you a P46 to complete and your employer will work out the tax you owe. When HMRC deal with your P46 they will revise your tax code and the tax paid will be adjusted accordingly. BR means your pay from that employment will all be taxed at the basic rate, normally because you have a second job or pension. D0 signifies that all your income is taxed at the higher rate, again usually because you have another source of income.
If you earn more than £100,000, your tax code also allows for the income-related reduction to the Personal Allowance. If you earn more than £150,000 from your main source of income, tax is deducted at 50 per cent as appropriate. It may be that your tax may well need to be adjusted when your self assessment tax return is submitted so that gift aid or pension contributions are treated properly. Whatever your circumstance, if you are unsure whether or not you are on the right tax code, you should first contact your payroll department. If you have an accountant, they can check whether it is correct. Most of the time, codes are fairly accurate but it's always worth checking to make sure.