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HMRC internal manual

National Insurance Manual

Earnings Periods: Employee earns a fee for each session they work: Payment made irregularly but treated as paid regularly

If an employer pays an employee a fee at irregular intervals for each session they workbut you can treat the payment as paid at regular intervals eg the interval of payment isagreed under the contact of service, the earnings period is the length of the regularinterval. If the treated interval between payments is less than a week, the earningsperiod is a week and the payments should be aggregated.