Each Class 3 year costs about £900 and adds ~£359 a year of State Pension for life — a three-year breakeven. Who it helps, who it can't, and the checks to run before paying HMRC anything.
The full new State Pension requires 35 effective qualifying years of National Insurance. Each year short costs you 1/35th of the pension — about £359 a year at today's £12,548 full rate — every year for the rest of your life. And each gap year bought back restores exactly that, for a one-off Class 3 payment of roughly £900. Even after basic-rate tax on the pension, the payback is about three years; live twenty years past State Pension age and one £900 payment returns £5,700+, inflation-protected. Nothing else legal comes close.
People who will finish short of 35 effective years: career breaks, years abroad, long stretches of self-employment with small profits, early retirees who stopped work in their 50s. The last group is the classic case — retire at 55 with 30 years and no future earnings, and every year to 35 is worth buying.
Buying does nothing if future work gets you to 35 anyway: a 40-year-old with 22 years and 25 working years ahead needs no top-up. And pre-2016 records interact with contracting-out in ways that can make an extra year genuinely worthless. So the drill is: check your State Pension forecast and NI record on gov.uk; if the forecast already shows the full amount, stop; if anything is unclear, ring the Future Pension Centre before paying. You can normally fill gaps from the previous six tax years, and the self-employed can often pay Class 2 at roughly a fifth of the Class 3 price for the same benefit.
Hundred Summers' voluntary NI calculator runs the honest after-tax breakeven, and the full plan models your qualifying years and gap-filling inside the whole retirement picture.