As an employer it is vital to know which National insurance (NI) category to put your employee onto, this will dictate how much NI both you and the employee pay on their earings.
The below table shows the specific National insurance category letters and the worker groups that fall into them.
| Category letter | Employee group |
| A | All employees apart from those in groups B, C, J, H, M and Z in this table |
| B | Married women and widows entitled to pay reduced National Insurance |
| C | Employees over the State Pension age |
| J | Employees who can defer National Insurance because they’re already paying it in another job |
| H | Apprentice under 25 |
| M | Employees under 21 |
| Z | Employees under 21 who can defer National Insurance because they’re already paying it in another job |
Category letter X
Employers use category letter X for employees who don’t have to pay National Insurance, for example because they’re under 16.
Mariners and deep sea fishermen
HMRC has detailed information on National Insurance for mariners and deep sea fishermen.