This may be well known but it is a situation I have only just encountered. Where an employee's earnings fluctuate above and below the monthly primary threshhold over a tax year, it seems that the employer's total secondary contributions for the year are greater when taking the sum of the monthly contributions than if the employee's earnings are the same amount each month. For example, take a part time employee who earns £500 per month for 6 months and then £1,000 per month for the rest of the year. No Ers NI for the first 6 months as earnings are less than the £719 monthly limit. The Ers NI on £1,000 is 0.138 x (£1,000-£719) = £38.79 per month or £232.67 for the tax year. If the same employee earns £750 per month for 12 months the monthly Ers NI is £4.28 per month or £51.36 for the tax year. In each case the annual earnings are £9,000. As I say, I have not encountered this before so may be making a schoolboy error, but is that correct? I need to know which figure to use for budgeting purposes. Thanks.