Brian Preston's wealth multiplier calculator

The Money Guy wealth multiplier turns a dollar invested at any age into a figure at 65 — $647 at birth, $88 at 20, $7 at 40. Reproduce it from the show’s own return schedule, then deflate it, which the source states it does not do.

Brian Preston Reviewed Aug 7, 2026 Rule located in Money Guy, 2026

Interactive model

Put your numbers through the rule

The source establishes the rule. The values below belong to you, and the output is JMM's deterministic calculation.

Make the assumptions yours

Every field recalculates immediately. Changed values can be copied into a shareable URL.

Your age today

years

Invested once, today

Invested every month from now on

Age you want the figure at

years

Your inflation assumption

Nominal value at 65$23,063

Multiplier on each dollar: 23.1×9.0% a year for 35 years
Value in today’s money: $8,196deflated at 3.0% a year
Purchasing power lost to inflation: $14,867the line the source leaves out

From the amount invested today

$23,063

From monthly contributions

Real value of both

$8,196

The return schedule is the show’s own: 10% for a dollar invested at 20 or younger, falling 0.1 percentage points for each year after 20, to 5.5% at 65. Compounding is monthly, which is what reproduces the published $647, $88 and $7 figures. Every monthly contribution is priced at the rate for the age it goes in, not the rate for your age today, because that is what the schedule says — a deposit made at 45 earns 7.5% whoever makes it. Inflation is JMM’s addition, because the source states it excludes inflation and taxes.

The multiplier by the age the dollar goes in

The show’s multiplierAfter your inflation assumptionAge at investment

Inspect the calculationThe same result in a readable record view

Invested at 0

Assumed return: 10.0%
Multiplier by 65: 647.5×
Multiplier after inflation: 94.8×

Invested at 20

Assumed return: 10.0%
Multiplier by 65: 88.4×
Multiplier after inflation: 23.4×

Invested at 30

Assumed return: 9.0%
Multiplier by 65: 23.1×
Multiplier after inflation: 8.2×

Invested at 40

Assumed return: 8.0%
Multiplier by 65: 7.3×
Multiplier after inflation: 3.5×

Invested at 50

Assumed return: 7.0%
Multiplier by 65: 2.8×
Multiplier after inflation: 1.8×

Source and translation

What the source says, and what the calculator adds

Source-supported ruleA dollar invested at a given age earns one lifetime rate from that age until 65: 10% at 20, declining by 0.1 percentage points for each year after 20, reaching 5.5% at 65. The guide prints the resulting figures for a single dollar, including $647 at birth, $88 at 20 and $7 at 40, and states that the model excludes inflation and taxes.

JMM calculationJMM rebuilds the schedule instead of copying the printed table, which is how the monthly compounding convention was established: only monthly compounding reproduces the guide’s own $647, $88 and $7. It then applies the deflator the source says it leaves out, so the nominal figure and the figure in today’s money appear side by side rather than one without the other.

Formula and methodRate = 10% for a dollar invested at 20 or younger, otherwise 10% − 0.1 × (age − 20), floored at 5.5%. Multiplier = (1 + rate ÷ 12)^{12 × (target age − age)}. Monthly contributions are not an annuity: the schedule gives each deposit its own rate, so they are summed one at a time, each compounded at the rate for the age it goes in. Real value = nominal ÷ (1 + inflation)^years.

Money Guy Jul 29, 2026The Wealth Multiplier: what $1 invested today is worth at 65Section “What is the Wealth Multiplier Formula?”, which states: “While returns do change when investing at different ages (for example, 10% at 20, 9% at 30, and 8% at 40), if one dollar is invested at age 20, it is assumed that it will earn 10% from age 20 to 65.” The schedule is completed under “How accurate are the assumptions behind the Wealth Multiplier?”: “Lifetime returns decrease by 0.1% for each year after 20, reaching a terminal return of 5.5% at age 65.” The same page states the model “does not account for variables such as inflation and taxes.” Bylined Daniel May, CFP; last updated July 29, 2026.

Decision notes

What changes the answer

The published numbers reproduce exactly, and that is the point

Running the guide’s own schedule returns 647.47, 88.35 and 7.34 against the printed 647, 88 and 7. Most creator arithmetic cannot be checked because the assumptions are not published. This one can, and it survives the check.

The deflator does more damage than any input on the page

Move the inflation control and watch the headline collapse. At 3% a year the $647 a dollar invested at birth reaches by 65 is worth about $95 in today’s money — still a striking number, and a completely different one.

Who else has run the numbers on this rule

Everyone below has published a position on this specific rule with a figure attached. Each row names the document, the date and the passage, so you can check the number rather than take ours.

BJavier EstradaProfessor of finance at IESE Business School, Barcelona, publishing in The Journal of Wealth ManagementAgainst the rule
6.5% annual compound real US equity return, 1900-2014, at 20.0% volatility
Estrada reports the long-run US equity return in the form an investor can actually spend — compound, and after inflation — using the Dimson, Marsh and Staunton series. Over 1900 to 2014 that figure is 6.5% a year at 20% volatility. The multiplier’s 10% is a nominal figure and cannot be compared with it directly, which is exactly the gap: substitute 6.5% real for 10% nominal at age 20 and a dollar reaches about $18 by 65 rather than $88.
stocks and bonds had mean annual compound (real) returns of 6.5% and 0.9%, with annual volatility of 20.0% and 4.6%
The Journal of Wealth ManagementMar 31, 2016Buffett’s Asset Allocation Advice: Take It … with a TwistSpring 2016 issue, page 61, opening sentence of the page, describing the Dimson-Marsh-Staunton data used in the study’s 1900-2014 sample.

The arithmetic is honest to the dollar. The number it produces is not the number people hear.

JMM rebuilt this model from the show’s published schedule and it reproduces every printed figure, which is more than can be said for almost any rule this site has audited. The problem is not the maths, it is the unit. $647 is nominal, pre-tax, and assumes a flat 10% every year for sixty-five consecutive years — a return nobody receives in that shape, on a dollar nobody has at birth. Deflate it at 3% and the same dollar is worth about $95 in today’s money, and $95 is still an argument for investing early. That is JMM’s objection in full: the honest version of this number is persuasive on its own, so printing the nominal one alone trades credibility for a headline it did not need. The show says on the same page that it excludes inflation and taxes. Almost nobody quoting it repeats that sentence.

Written by the JMM editor. This is a view, not a recommendation for your circumstances. Who writes this

Not the calculator’s limits. The rule’s.

A flat rate for forty-five years is not what a portfolio does
The model earns exactly its assigned percentage every year. A real investor earns a sequence, and two sequences with the same average produce different balances once contributions or withdrawals are involved. The multiplier is the smooth path through a distribution, not a point on it.
It prices a dollar you already have spare
The multiplier answers what a dollar becomes, never whether you had a dollar to spare. Applied to somebody carrying a card balance at 20%, it argues for investing money that the same show’s own step order says should clear the debt first.
The schedule is asserted, not derived
Nothing on the source page explains why lifetime returns should decline by exactly a tenth of a point per year of age, and no market mechanism makes an older investor’s dollar earn less than a younger one’s in the same fund. It is a modelling convention presented in the grammar of a finding.

Limits

What this model does not know

The rate is fixed for the whole horizon by the age of the dollar. A real portfolio earns a sequence, and the order of that sequence changes the outcome even when the average does not.
Taxes are excluded, as the source states. The same nominal figure is worth materially different amounts inside a Roth account, a traditional account, and a taxable brokerage.
Fees are not modelled. A single percentage point of annual cost removes roughly a third of the terminal figure over 45 years.
The published schedule is defined to age 65. Choosing a different target age extends the same lifetime rate beyond where the source states it, which is JMM’s extrapolation and not the show’s.

Questions people ask

Before you use the result

Is the $647 figure real?

It is arithmetically correct under the assumptions the source publishes, and this page reproduces it to the dollar. It is nominal and pre-tax, so it is not $647 of purchasing power in today’s money — at 3% inflation it is closer to $95.

Why does this page compound monthly?

Because that is what matches the guide’s own printed figures. Annual compounding at the same 10% returns 72.9 for a dollar invested at 20, where the guide prints 88; monthly returns 88.35. The convention was derived from the published numbers rather than assumed.

Does the multiplier apply to money already invested?

No. It prices a dollar going in today, at today’s age. A balance already invested has its own history and its own remaining horizon, so use the monthly contribution field for new money and treat the lump sum as this month’s deposit.

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