Compound Interest Basics
Learn how compounding works, why time matters, and which assumptions drive long-term growth estimates.
5 min read
What compounding means
Compound interest means that earned returns are added back to the balance, so future returns can grow on both the original principal and prior gains. This is why time horizon can matter as much as the starting amount.
A simple compound model is not a promise. It is a way to compare scenarios under a consistent return assumption.
The main formula
For a lump sum, a common formula is FV = PV x (1 + r / n) ^ (n x t). FV is future value, PV is starting principal, r is annual return, n is compounding periods per year, and t is years.
Recurring contributions require a cash-flow model because each contribution has a different amount of time to compound.
What to watch
Small changes in return assumptions can create large differences over decades. Use conservative, balanced, and optimistic cases instead of relying on one exact number.
Fees, taxes, inflation, and market volatility are not always visible in simple calculators, but they can materially affect real outcomes.
Key takeaway
Use compound interest estimates to compare assumptions, not to predict guaranteed portfolio values.
Use the calculators
After reading this guide, you can return to the CompoundX calculators to compare your own assumptions. Results are estimates only and do not constitute investment advice.
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