Retirement math comes down to one question: how big does the pot need to be? Here is how to calculate yours.
Retirement planning sounds impossibly complex, but the underlying math is surprisingly tidy. Two formulas — the 4% rule and the 25× rule — get most people 90% of the way there. This guide shows you how to use them, what they assume, and how to adjust for your country's retirement system.
Forget income. Retirement is funded by what you spend, not what you earn. Track your annual expenses honestly — housing, food, transport, healthcare, fun. A common starting estimate is 70–80% of your final pre-retirement income, because commute, work clothes and retirement-account contributions all disappear, but healthcare and travel often rise.
Target nest egg = annual spending × 25.
Want $40,000 a year? Target $1,000,000. Want $80,000? Target $2,000,000.
This comes from the 4% safe withdrawal rate — the famous Trinity Study showed that a balanced 60/40 portfolio could sustain 4% annual withdrawals (adjusted for inflation) for 30+ years with very high probability. 4% of your pot ≈ your annual spending; the inverse is 25×.
Most countries give you some baseline retirement income — Social Security in the US, State Pension in the UK, Age Pension in Australia, CPP/OAS in Canada, Riester/Statutory Pension in Germany. If those will pay you $20,000/year, you only need to fund the remaining $60,000 yourself → $1.5M.
$1,000,000 sounds like a lot. At 3% inflation, in 30 years it has the purchasing power of about $412,000 today. Always do retirement math in today's money, not future numbers — and revisit the plan every 5 years to recalibrate.
A 35-year-old in the UK wants to retire at 65 with the equivalent of £40,000/year in today's money.
Using the future-value formula, contributing £730/month for 30 years at 6% real return → roughly £710,000. Combined with State Pension (~£11,500/yr ≈ £290k notional), the gap is closed.
If you are 45 with $50k saved and want $1M by 65:
Tough but possible. The two biggest levers when you start late are saving rate (push to 25–30%) and delaying retirement by even 2–3 years.
1. Ignoring fees. A 1% annual fund fee can cost you 25% of your final pot over 30 years. 2. Holding too much cash. Inflation guarantees real losses. 3. Taking Social Security / pension early without modeling. Delaying often pays. 4. Treating your home as a retirement asset unless you genuinely plan to downsize. 5. Forgetting healthcare — pre-65 in the US, private cover in Australia, etc.
Is the 4% rule still valid? Multiple updates (Kitces, Pfau) suggest 3.3–3.5% is safer in low-yield environments. Use 4% as a planning estimate, 3.5% if you are conservative.
What return should I assume? 5–7% real is reasonable for a diversified stock-heavy portfolio. Adjust down if you hold more bonds.
What if I want early retirement (FIRE)? Aim for 30–33× annual spending and assume 3% withdrawal — money has to last 40–50 years.
Try our Savings Goal Calculator and our Compound Interest Calculator to model your own retirement number in any currency.