A compound-interest chart can rise impressively over several decades. Before accepting the picture, follow what happens over just two years. The mechanism is easier to see when the numbers remain small.
Investor.gov defines compound interest as interest on the principal and accumulated interest. The following numbers are an invented arithmetic example, not an available account rate or an investment forecast.
Keep the first step visible
Assume 1,000 units earn a fixed 5% after one year, with no withdrawals, fees or tax. The interest is 50 units, giving a balance of 1,050.
If that interest remains in the balance and the same annual rate applies again, the second year’s interest is 52.50. The balance becomes 1,102.50. The extra 2.50 arises because the second calculation includes the first year’s interest.
Compare the assumptions, not just the result
The example assumes annual compounding and a fixed rate. Changing the timing of interest, adding contributions or withdrawing money changes the calculation. Fees, taxes and inflation introduce further questions.
For investments with variable returns, a smooth constant-rate projection does not describe the actual sequence of gains and losses. A calculator can model an assumption without making that assumption likely.
Change one input deliberately
Use a calculator to compare different rates or contribution amounts while holding the other inputs steady. Label every result as a scenario. Do not select the most attractive line and treat it as the expected future.
The Investor.gov calculator is one tool for exploring inputs; it does not choose an appropriate product for you.
Bring the lesson back to the document
When a financial book presents a long projection, ask which assumptions create its shape and which costs it excludes. Our fee-reading exercise addresses one of those missing pieces. Understanding the mechanism is more useful than memorising an impressive future balance.