National Insurance is payable quarterly. Employee's NI is deducted from each payslip and, together with employer's NI, is paid to HMRC by the company each quarter.
Although directors are classed as employees and pay National Insurance, the method of calculation is slightly different compared to other employees as it is based on what is called the annual earnings method.
An employee of a company that is not a director will have their NI calculated based on what they earn in each pay period, i.e. weekly or monthly. The annual earnings method is designed to give greater flexibility to directors as they are more likely to have irregular earnings.
The rates and thresholds for directors and employees are the same therefore they would both pay the same amount of NI if they received a consistent tax efficient salary split evenly across the tax year. The difference for a director is that they do not start paying NI until they reach the earnings threshold where NI becomes payable.
However, if a director is appointed part way through a tax year, the national insurance thresholds are pro-rated based on the number of weeks remaining in the tax year.
It is normal at the start of a tax year for a director to not be paying NI. It is only when their earnings to date, in that employment, for a tax year reaches the threshold that national insurance will start to be deducted.