It depends on your contract and your length of service. Typical figures: one week during probation, one month for most office roles, three months for senior or specialist positions, and six months or more for executives. There's also a statutory minimum: one week after a month of service, scaling up to twelve weeks (employer to employee) at twelve years of service.
UK notice periods come from two sources: the contractual figure (whatever your employment contract says) and the statutory minimum (set out in the Employment Rights Act 1996). The longer of the two applies. Contractual notice almost always exceeds statutory notice for any role above the most junior level, so most employees only need to look at their contract to find the relevant figure.
Statutory minimum notice from the employer to the employee scales with length of service. Less than one month of service: no statutory notice. One month to under two years: one week. Two to twelve years: one week per complete year (so three years means three weeks, eight years means eight weeks). Twelve years or more: twelve weeks, capped. The figures from the employee to the employer are different: one week from one month of service, regardless of length.
Contractual notice is set by the employment contract you signed. Typical patterns by seniority: junior roles often have one to two weeks during probation, rising to one month afterwards. Mid-level professionals tend to have one to three months. Senior managers and specialist roles often have three months. Director-level and C-suite roles commonly have six months, occasionally twelve.
Different sectors have different norms. Financial services and consulting often have three-to-six month notice periods even at moderate seniority. Technology companies often have shorter notices (one month even at senior level) reflecting faster talent movement. Public sector and academic roles often have longer notices tied to the academic year or operational cycle. Healthcare follows NHS-specific notice structures.
Notice during probation is almost always shorter than after probation. Common patterns: one week during probation rising to one month afterwards, two weeks during probation rising to three months, or the same notice both during and after (less common). The contract states each figure separately. The shorter probation notice is a feature designed to make the trial period easy to end on both sides.
Your notice period doesn't reset if you change role within the same employer (a promotion, an internal transfer, a restructure). Continuous service runs from the first day of employment with that employer (or with a predecessor employer in some TUPE transfer situations). So if you've been at the same company for ten years across three roles, your length of service for statutory and benefit purposes is ten years.
Both the employer and the employee can be released from notice by mutual agreement, by PILON (if the contract allows), or by garden leave (employee remains employed, just not at work). What can't happen is a unilateral shortening; the employer can't shorten your notice without paying you for the difference, and you can't shorten yours without negotiating it.
Practical recommendation: find your notice clause and read it carefully. Note the difference between the probation figure and the post-probation figure. Confirm which one currently applies. Check whether there's a PILON clause and a garden leave clause. The notice period calculator on this site handles the date arithmetic for any notice length, including bank holidays and weekend rollback.
General information about UK employment law. For your specific situation, contact ACAS or an employment-law solicitor.