The amount of Income Tax that HMRC take from you each month depends upon how much you earn above the Personal Allowance.
Some of your annual income is tax free, because it falls under the Personal Allowance.
According to HMRC’s own guidance, the standard Personal Allowance is currently £12,500. The Personal Allowance is the amount of income you can earn each year without having to pay income tax. You pay income tax on any amount you earn above this Personal Allowance.
HMRC guidance states that you may be able to increase your Personal Allowance if you are married or in a legally-recognised civil partnership, provided your partner earns less than £12,500 per year. This is because Marriage Allowance allows you to transfer some of their unused tax-free Personal Allowance by claiming Marriage Allowance.HMRC Marriage Allowance eligibility criteria