No national insurance contributions are payable on a weekly wage of �91 or less (employees are treated as having paid notional contributions on earnings between �79 and �91 a week but the employee themself does not actually pay anything). On amounts over and above �91, contributions are payable at a rate of 11%, dropping down to 1% on earnings over �610 per week (unlikely for a 16 year old!) So to work out contributions payable, deduct �91 from the weekly wage and work out 11% of the balance.