Compound interest
See how a starting amount, regular deposits, and time could grow your money.
Saving & investingSaving & investing
The little-spend calculator calculator
No latte shaming. Just the trade-off.
Your numbers
See what a recurring expense adds up to—and what the same money could grow into.
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THE MATH, WITHOUT THE MYSTERY
What’s actually happening here?
Opportunity cost is what you give up when you choose one option over another. Here, the comparison is between a recurring purchase and investing the same monthly-equivalent amount.
The alternative scenario deposits the monthly equivalent at each month’s end. The monthly growth rate is (1 + annual effective return)^(1/12) − 1. The return is uncertain, and the enjoyment or usefulness of the purchase is real but not priced here.
THE FORMULA
Monthly equivalent = Cost × Occurrences per year / 12
A small example
A 10 weekly expense adds up to 520 a year, or a monthly equivalent of 43.33. At a 0% return, setting it aside for 10 years builds 5,200.
Further reading
SEC Investor.gov · Compound interest calculatorGOOD QUESTIONS
A few things worth knowing.
Does this say my coffee is a bad decision?
Absolutely not. Money exists to support a life you value. This tool makes a financial trade-off visible; it cannot measure happiness.
Why use a monthly equivalent?
It makes different spending frequencies comparable. Actual daily or weekly investing would produce a slightly different path.