Two numbers, two units
“I want $3,000 a month in retirement” is a statement about purchasing power today. Your investment projection, meanwhile, produces a nominal balance — the number the statement will show in thirty years, in money that buys less. Comparing the two directly overstates how ready you are, sometimes by a factor of two.
Fix it by converting one into the other:
Nominal amount = Today's amount x (1 + i)^Y Today's amount = Nominal amount / (1 + i)^Y i = annual inflation rate Y = years until retirementA worked conversion
Age 30, retiring at 62, wanting $3,000 a month in today's money
- Annual spending: 3,000 x 12 = $36,000 in today's money.
- Target using the 25x rule: 36,000 x 25 = $900,000 — in today's money.
- 32 years at 2.5% inflation multiplies prices by 1.02532 = 2.20.
- So the goal at age 62 is 900,000 x 2.20 = $1,983,000 nominal.
Now the savings side. Starting with $20,000, saving $500 a month, at a 7% nominal return:
- Projected balance at 62: about $900,900 nominal.
- That same balance in today's money: 900,900 / 2.20 = about $408,800.
- Shortfall against the nominal goal: about $1,082,000.
Compare $900,900 with the $900,000 “target” and it looks like a bullseye. It is not — it is roughly 45% of the goal. That single comparison error is why the retirement calculator now shows both units side by side.
Note also how sensitive this is: at 2% inflation the goal is $1,696,000; at 3% it is $2,318,000. A one-point assumption changes the target by more than $600,000. Treat any single projection as one scenario among many.
The real-return shortcut
There is a simpler way to handle the same problem: work entirely in today's money by using a real return instead of a nominal one.
Real return = (1 + nominal) / (1 + inflation) - 1 7% nominal with 2.5% inflation = 1.07 / 1.025 - 1 = 4.39% real (the rough version, 7 - 2.5 = 4.5%, is close enough for planning)Project your savings at 4.39% and compare the result directly with the $900,000 target — no conversion needed. Both methods give the same answer; what you must never do is mix them.
One caveat: contributions. If you hold your monthly saving flat in nominal terms, its real value shrinks every year. Projecting in real terms with a flat real contribution therefore assumes you increase your saving with inflation — which is realistic if your income keeps pace, and optimistic if it does not.
Inflation does not stop when you retire
The 4% / 25x guideline already contains an inflation assumption that people often miss: the original US studies modelled withdrawing 4% in year one and then increasing that withdrawal with inflation every year thereafter. If you plan to hold withdrawals flat in nominal terms, you are planning to get steadily poorer.
A 30-year retirement at 2.5% inflation means your final year's spending needs to be about 2.1 times your first year's, just to stand still. Things that help:
- Inflation-linked income — state pensions and index-linked annuities in countries that offer them; check whether yours is linked to prices, wages, both, or nothing.
- Keeping growth assets in the portfolio rather than moving entirely to cash, which guarantees a real loss.
- Inflation-linked bonds, where available, for the portion of spending you cannot flex.
- Flexible spending. The single most effective response to a bad decade is trimming withdrawals for a while — something no fixed rule models.
What this model does not tell you
- Your inflation is not the headline rate. Official indices such as CPI measure a national average basket. Retirees typically spend more on healthcare and housing and less on transport and education, so personal inflation can differ persistently from the published figure.
- The 4% rule is US, historical and 30-year. It came from studies of past US market returns over 30-year windows. Different countries, valuations, fee levels and retirement lengths give different sustainable rates — some higher, several lower.
- Nothing here models sequence risk. A poor first decade of returns damages a withdrawal plan far more than the same poor decade later, even with identical averages.
- Taxes, state pensions, property and healthcare costs are excluded, and each of them can move the answer more than the inflation assumption does.
Treat the output as a direction and a rough magnitude — “roughly half way, and the gap is large” — rather than a number to plan a life around. Then re-run it every few years.
Sources and further reading
Primary sources are preferred: regulators, central banks, statistical agencies, tax authorities, index providers and original research. Links open on the publisher's own site; FinSage has no commercial relationship with any of them.
- U.S. Bureau of Labor Statistics — Consumer Price Index, and equivalents such as the UK Office for National Statistics and the ECB's HICP, for measured inflation in your own economy.
- Cooley, P. L., Hubbard, C. M. & Walz, D. T. (1998), “Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable” (the “Trinity study”), AAII Journal. Together with Bengen, W. P. (1994), “Determining Withdrawal Rates Using Historical Data”, Journal of Financial Planning — the original sources of the 4% figure.
- U.S. Social Security Administration — cost-of-living adjustments, an example of how an inflation-linked state pension is indexed.