The permanent ~5.8% a year uplift for waiting, the income you give up, the roughly age-83 breakeven, and the tax angle that often matters more — the statutory mechanics, worked through honestly.
Your State Pension starts when you claim it, not automatically at State Pension age — and under the new State Pension, every 9 weeks you wait adds 1% to every later payment: roughly 5.8% per full year, permanent, on top of whatever annual increases happen. (The old system's lump-sum option is gone for anyone reaching State Pension age after April 2016.)
Deferring a year gives up a year of pension — about £12,500 at the full rate — to gain about £730 a year for life. That money comes back at roughly £730 a year, so the breakeven sits about 17 years out: around age 83–84. Life expectancy at 66 is about 85 for men and 87 for women — a near-fair bet on average, clearly better if your health and family history point long, clearly worse if they don't. The uplift is not compounded: two years deferred means +11.6%, not +11.9%.
State Pension income is taxable. Take it while you're still earning and it stacks on top of your salary — a higher-rate earner keeps only 60p of each pension pound. Defer until you stop working and the same income lands in years where your personal allowance and basic-rate band may cover it. For someone working two years past State Pension age at 40%, deferral converts income taxed at 40% into a larger income likely taxed at 20% or less — a better deal than the headline uplift alone suggests.
Hundred Summers' State Pension deferral calculator finds your breakeven on the statutory rules, and the full plan models deferral inside your whole retirement — where the tax interaction, the triple lock assumption and your other income are all in the same picture.