What is Compound Interest?
Compound interest is interest calculated not only on the original amount of money (the principal) but also on the interest that has already been added[1][2]. In plain terms, it’s "interest on interest." Over time, that compounding effect can make balances grow much faster than they would under simple interest, especially when the money is left invested for years.
This concept shows up everywhere: savings accounts, certificates of deposit (CDs), retirement accounts, credit cards, mortgages, and many other financial products. Compounding can work for you (when you’re earning interest) or against you (when you’re paying interest on debt).
How compound interest works
With compounding, each period’s interest becomes part of the new base that future interest is calculated on. If interest is compounded monthly, interest is added 12 times per year; if daily, it’s added more frequently (often 365 times). All else equal, more frequent compounding leads to a higher ending balance[3].
A common way to represent periodic compounding is:
- A = P(1 + r/n)^(nt)
Where P is principal, r is the annual interest rate, n is the number of compounding periods per year, and t is time in years.
Simple interest vs. compound interest
Simple interest is calculated only on the original principal. If you deposit $1,000 at 5% simple interest, you earn $50 each year, and that yearly interest amount stays the same[4].
Compound interest is calculated on the principal plus accumulated interest. Using the same $1,000 at 5% but compounded annually, the first year adds $50 (to $1,050). The second year’s 5% is then applied to $1,050, so the interest earned becomes $52.50, and so on.
Over short periods the difference can look small, but over decades the gap can become substantial.
What affects how fast money compounds?
Several variables determine how powerful compounding will be:
- Time: More time generally has the largest impact; compounding benefits from staying invested.
- Rate: Higher rates compound faster, but often come with higher risk depending on the product.
- Compounding frequency: Daily or monthly compounding can produce a slightly higher result than annual compounding.
- Additional contributions: Regular deposits (e.g., monthly) can dramatically increase the end value.
If you want to see the impact of these inputs, sign into calm sea to see the effects of compound interest on our calculators.
Compounding in real life (including the “dark side”)
For savers and investors, compounding is the engine behind long-term wealth building—especially when returns are reinvested rather than withdrawn.
For borrowers, compounding can be costly. Credit card balances, for example, can grow quickly when interest is charged on prior interest and the balance isn’t paid down.