The retirement number: how the 4% rule works

Retirement math sounds intimidating because most of it is presented as a giant lump-sum target — 'you need $1.6 million' — with no explanation of where the number came from. Almost all of it comes from one piece of research, the Trinity study, and one derived rule of thumb: the 4% rule. Once you know the mechanics, the target and the savings rate that gets you there become simple arithmetic.

What the 4% rule actually says

The Trinity study, published in 1998 and updated many times, looked at historical returns of a diversified stock/bond portfolio and asked: what's the highest annual withdrawal rate that would have survived a 30-year retirement in every historical period, even the bad ones? The answer was roughly 4% of the starting portfolio, adjusted upward for inflation each year.

In plain English: if you retire with $1,000,000 in a diversified portfolio, you can withdraw about $40,000 the first year and give yourself an inflation raise each year after that, and historically the money outlasts a 30-year retirement in nearly every case.

The formula in one line

If you want $60,000/year in retirement (in today's dollars), you need $60,000 × 25 = $1,500,000. If you want $40,000/year, you need $1,000,000. The multiplier is 25 because 1 ÷ 4% = 25. That's it. That's the whole rule.

Back into today's savings rate

A rough shortcut: assume real (inflation-adjusted) returns of about 5% on a diversified portfolio. Then the fraction of income you save every year roughly determines how many working years you need before you can retire on that income level. Save 15% → about 40 years. Save 25% → about 30 years. Save 40% → about 20 years. Save 50% → about 15 years. The math is almost linear in savings rate, because the person saving 50% is also learning to live on 50% — so their target number is smaller.

A worked Canadian example

A dual-income household spending $75,000/year today. Estimate retirement spending at 75% of current ($56,250) since mortgage and commuting drop off. Subtract expected CPP + OAS at age 65 (~$25,000/year for two average earners). The portfolio has to produce $31,250/year → target $781,000 (in today's dollars). If you're 35 with $50,000 already saved and 30 years to go, you need to accumulate another $731,000 real. At 5% real return, that's about $850/mo of new contributions. Doable — and much less scary than 'you need $1.5 million.'

A worked US example

Same household in the US spending $80,000/year, retirement spending at $60,000. Subtract Social Security at 67 (~$35,000/year for a middle-income couple). Portfolio produces $25,000 → target $625,000. Again, adjusting for existing savings and years to go, contribution needed is often $600–$1,000/mo — well within reach for households currently spending $80,000, but only if it's deliberately routed and automated.

What the 4% rule does not account for

• Government benefits (CPP, OAS in Canada; Social Security in the US) — these reduce how much your own portfolio has to produce, sometimes by a lot
• A paid-off house — dramatically lowers annual spending needs in retirement
• Early retirement (30–40 year horizons) — the 4% rate was calibrated to 30 years; longer horizons need closer to 3.3–3.5%
• One-time costs (kids' education, a wedding, a home renovation) — plan those separately, don't try to hide them in your withdrawal rate

Sequence-of-returns risk

The 4% rule's biggest weakness isn't average returns — it's the order they arrive in. A retiree hit with a 30% market drop in year one has a much worse outcome than the same average return sequenced later. That's why most planners recommend 1–3 years of expected withdrawals in cash or short-term bonds at retirement, so a bad first year doesn't force selling equities at the bottom.

Frequently asked questions

Is 4% still safe with today's valuations? Debated. Some researchers argue 3.5% is safer given current stock valuations and bond yields; others show 4% still holds. The honest answer: build to a 25× target and be willing to flex spending down 10–20% in early bad years — that resilience matters more than the exact rate.

What if I'll have a pension? Value it as an annuity (pension income × ~20 = its 'portfolio equivalent') and subtract that from your target. Government defined-benefit plans work the same way.

Does home equity count? Only if you'd realistically downsize. A $800,000 house you plan to stay in doesn't produce retirement income. A $500,000 downsize with $300,000 released to the portfolio does.

The takeaway

You don't need a financial planner to know your retirement number in five minutes. Estimate what you'd want to live on in retirement (in today's dollars), multiply by 25, subtract any home equity you'd downsize out of and government benefits you'll receive, and that's the portfolio target. Then decide what savings rate closes the gap in the timeframe you want. It's a calculation, not a mystery.