Retirement: How much do I need? A framework for the hardest question in personal finance

"How much do I need to retire?" is the most common question in personal finance. It also has no universal answer — which is not a reason to avoid it, but a reason to ask it more carefully.

Most people approaching retirement planning want a number. The financial industry is happy to provide one: $1 million, $2 million, "25 times your annual expenses," "replace 70–80% of your pre-retirement income." These figures are useful as rough orientation but dangerous as targets, because they carry assumptions that may not apply to your situation — about how long you will live, what you will spend, and what return your portfolio will generate. The right approach is not to find the number but to understand the three variables that determine it.

Variable 1: What will you spend?

Retirement spending is the most underestimated variable in retirement planning. Most projections use pre-retirement income as a proxy, but research on actual retiree spending consistently shows a more complex pattern.¹

Spending in early retirement (ages 60–75) is often higher than expected, not lower. Many people retire into an active phase: travel, hobbies, home improvements, time with family, and the activities they spent their working life postponing. The 70–80% income replacement rule assumes a quiet retirement; an active early retirement may require 90–100% of working income or more.

Spending typically declines in mid-retirement (ages 75–85) as physical activity naturally reduces. It then rises again in later retirement (ages 85+) as healthcare costs, care costs, and support requirements increase. This "smile" pattern — high, then lower, then higher again — is the more accurate model of retirement spending.

The starting point for your retirement spending estimate is not what financial projections assume you will spend but what you actually want your life to look like. Start with the life, then price it.

Variable 2: How long will you need it?

Life expectancy at birth is a misleading figure for retirement planning. What matters is life expectancy at retirement — conditional survival probability for someone who has already reached 60 or 65.

In the UK, a 65-year-old man has a 50% probability of reaching 86 and a 25% probability of reaching 92.² A 65-year-old woman has a 50% probability of reaching 89 and a 25% probability of reaching 94. These are median and upper-quartile figures — meaning roughly half and a quarter of people will live beyond them, respectively.

Planning to age 85 — common default in many retirement calculators — leaves a quarter of people running out of money before they die. The conservative planning assumption is to age 90–95. For someone with family history of longevity or their own good health profile, planning to 100 is not paranoid — it is actuarially defensible.

100 Great Years plans to age 100 by default. This is not optimism for its own sake — it is the recognition that building a great life across a full century requires financial resources that last a full century. A plan that runs out of money at 88 is not a plan for 100 great years. Overestimating how long you live costs you some unnecessary frugality; underestimating it can cost you financial independence in the years you need it most.

The consequence is stark: a 65-year-old who plans for a 20-year retirement needs a materially smaller fund than one planning for a 35-year retirement. The difference between planning to 85 and planning to 100 is not marginal. At a 3.5% withdrawal rate, a 35-year retirement requires a fund approximately 70% larger than a 20-year retirement to deliver the same annual income.

Variable 3: What return will your portfolio generate?

The withdrawal rate — the percentage of your portfolio you draw down each year — determines how long your money lasts, and it is sensitive to investment returns in ways that are non-linear.

The widely cited 4% rule originated from William Bengen's 1994 research, which found that a 60/40 portfolio (60% equities, 40% bonds) had historically supported a 4% annual withdrawal for at least 30 years in virtually all historical periods in the US market.³ Subsequent research by the Trinity Study reached similar conclusions.

Important caveats that are frequently omitted:

• The 4% rule was calibrated for 30-year retirements. For 35 or 40-year retirements, the safe rate may be closer to 3.3–3.5%.
• It was based on US market data. International portfolios have shown different success rates.
• It applies to a portfolio of real investments, not cash. A retiree holding 80% cash will exhaust their savings on any realistic withdrawal rate.
• "Safe" in the Trinity Study means succeeded in over 95% of historical scenarios — not 100%. Some sequences of returns remain problematic.

A reasonable working framework for a UK or international investor planning a 30–35 year retirement is to use a 3.5% withdrawal rate as a baseline, adjusting upward if willing to accept some spending flexibility in bad market years.

Putting it together: a worked example

A 55-year-old planning to retire at 65, targeting annual spending of $60,000 in retirement, planning to age 92 (a 27-year retirement), using a 3.5% withdrawal rate:

Required portfolio = $60,000 ÷ 0.035 = $1,714,000

That figure needs to exist at age 65, in today's money. Inflation between now and retirement means the nominal target is higher. A financial planning tool or adviser can model this precisely.

How to improve your position

• Estimate your retirement spending from your desired life, not from rules of thumb — build a budget for what you actually want to do in retirement, including travel, hobbies, healthcare, and gifts to family.
• Plan to age 100 — 100 Great Years plans to 100 by default. Planning to 85 leaves a meaningful proportion of people financially exposed in their final decades. The cost of over-saving is a modest lifestyle adjustment; the cost of under-saving is running out of money when you are most vulnerable.
• Use 3.5% as your withdrawal rate baseline if retiring before 70 — this buffer above the 4% headline figure reflects longer retirement horizons and international market variability.
• Build in adaptability — the withdrawal rate is not a fixed contract — if markets fall significantly in the early years of retirement, reduce discretionary spending temporarily: pause non-essential travel, defer large purchases, or consider part-time income until portfolio values recover. Retirees who treat the withdrawal rate as guidance rather than a hard rule, adjusting up or down as conditions demand, dramatically improve their plan's resilience. Conversely, if markets substantially outperform expectations, the plan can accommodate more generous spending.
• Model multiple scenarios — what if markets perform poorly in your first decade of retirement (sequence-of-returns risk)? What if you live to 100? What if care costs are significant? Stress-testing your plan is more valuable than optimising the base case.
• Account for state pension / Social Security — these guaranteed income streams reduce the portfolio drawdown requirement meaningfully. In the UK, the full new State Pension (£11,500/year in 2025/26) reduces the required portfolio at a 3.5% withdrawal rate by approximately £329,000.
• Review the plan every three to five years — retirement planning is not a one-time calculation. Life expectancy estimates, market returns, and spending patterns all evolve.

The 100 Great Years perspective

The question "how much do I need?" is ultimately a question about what kind of life you want to lead, for how long, and with what margin of safety. 100 Great Years frames this through the lens of wealthspan: not maximising the number, but ensuring your financial resources support the life you want for as long as you live. The three variables — spending, longevity, and returns — are not fixed facts to look up; they are choices and assumptions you make. Making them consciously, with honest inputs, is the most important work in retirement planning.

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This article is for educational purposes only and does not constitute financial advice. Past performance is not a reliable indicator of future results. Always consider your personal circumstances and consult a qualified financial adviser before making investment decisions.