5 min read · Updated August 2, 2026
Compound Interest Explained With Real Numbers
Compounding means your growth starts earning growth of its own. Over one year the effect is barely visible. Over thirty it dominates everything else in the calculation.
Time beats rate
Depositing $300 a month at 7% for 30 years produces a balance in the region of $365,000, of which only about $108,000 is money you deposited. The rest is growth on growth.
Start the same plan ten years later and the balance roughly halves, even though you only skipped a third of the deposits. The earliest contributions are the ones with the most time to compound.
Compounding frequency is a minor detail
Monthly versus annual compounding changes the outcome by a fraction of a percent at typical rates. Contribution size, contribution consistency and the length of the horizon each matter far more.
Treat the rate as an assumption
Any projected rate is an assumption, not a forecast. Model a pessimistic case alongside your central case, and remember that inflation reduces the purchasing power of the final figure.