Your tax code determines the amount of tax you pay each pay period and for current tax year the standard tax code is 1250L.
Whatever your tax code the numbers denote the amount you can earn each year before you are liable to pay tax, you just need to add a zero to the end of code and this then means the 1250 code gives you a figure of £12,500 you can earn before you pay tax.
This amount is then split into the number of pay periods you will have in the tax year, 52 if weekly and 12 if monthly.
Weekly £12500/52 = £240.38
Monthly £12500/12 = £1041.67
Once you have reached the above earnings in each pay period you then pay tax at 20%.
If you’re annual earnings are above £50,001.00 then the tax rate increases to 40% on earnings above this.
If you have several jobs it could well be that one of your employers has the full allowance (£12,500.00) and the other is using a tax code of BR, BR denotes that all earnings in that employee pay tax at 20% as the full allowance is being used elsewhere.
Should both employers have the full tax code, you will then need to contact HMRC directly (0300 200 3300) to get one of these corrected, you will need your NI number to hand when contacting them.