Contributions plus compounding
Your pension grows from three things: your contributions, your employer's, and investment growth compounding year after year. Auto-enrolment sets a minimum of 5% from you and 3% from your employer — but the employer match is essentially free money, so it's worth at least capturing it in full.
Because of compounding, starting age matters enormously: the same monthly contribution a decade earlier can mean a far bigger pot.
Common questions
Under auto-enrolment, the minimum is 8% total — at least 5% from you and 3% from your employer, based on qualifying earnings.
No. This projects your workplace/personal pension pot only. The State Pension is separate and paid on top if you qualify.
Long-run diversified returns average around 7% before inflation. Lower assumptions give a more conservative projection.