Investment Calculator
Starting capital plus monthly investing at an assumed return: the ending value, what you put in, and how much of the final figure growth contributed — with the smooth-return warning attached.
The two piles every portfolio is made of
At the end of any investing plan there are exactly two piles of money: what you put in, and what growth added. This calculator keeps the piles separate on purpose — in a young portfolio your contributions dwarf growth; in a mature one growth should be doing most of the talking, and the crossover is the story of compounding told personally.
Ending value = start grown monthly + contributions accumulated at the same return
Example: $10,000 start, $300 a month, 8% for 20 years: you put in $82,000 and finish near $196,000 — growth contributed about $114,000, more than you did. At year 10 the piles were nearly even; the second decade belongs to growth.
The smooth-line warning, attached as it should be
Real returns arrive as a jagged sequence, and the sequence matters when money is being added or withdrawn. A constant 8% is a planning fiction — useful for scale, disqualifying as a promise. Any projection that doesn't make you slightly uncomfortable about its smoothness is underselling risk.
How this is calculated
Monthly compounding at the nominal annual return ÷ 12; contributions at month end (site convention). Taxes, fund fees and inflation are excluded — each can be layered mentally (a 1% annual fee, for instance, behaves roughly like shaving a point off the return, which the form lets you test directly).
Frequently asked questions
What return should I enter for a stock portfolio?
A planning figure, not last year's: broad US equities have compounded around 10% nominal over very long runs, global portfolios somewhat less, bonds much less. Many planners project 6–8% for a mixed portfolio to leave margin for disappointment.
How do fees enter this picture?
As a return deduction: a fund charging 1% against an 8% gross return compounds you at 7%. Over 30 years that single point costs roughly a quarter of the ending value — enter net-of-fee returns.
Is the growth pile taxed?
In a taxable account, eventually (dividends yearly, gains at sale); in retirement accounts, per their rules. The calculator is pre-tax; the IRA pages address the tax layer for their account types.
Why monthly contributions instead of yearly?
Because that is how people invest — per paycheck. Monthly timing also slightly out-earns year-end annual deposits at the same total, a difference the model captures honestly.
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