How Does Compound Interest Work?

Compound interest is one of the most powerful concepts in personal finance. Unlike simple interest — which is calculated only on your original principal — compound interest is calculated on your principal and on the interest you've already earned.

The basic formula

The standard compound interest formula is:

A = P(1 + r/n)^(nt)

Where:

- A = final amount

- P = principal (initial investment)

- r = annual interest rate (as a decimal)

- n = compounding frequency per year (12 for monthly, 365 for daily)

- t = time in years

Why it matters

If you invest $10,000 at 7% annual return for 30 years:

- With simple interest: $10,000 + ($10,000 × 7% × 30) = $31,000

- With compound interest (annual): $10,000 × (1.07)^30 ≈ $76,123

That's a 2.5x difference from the same rate, just from compounding.

The role of time

Time is the most important variable. Starting at 25 vs 35 can mean hundreds of thousands of dollars at retirement. Here's a comparison of $500/month invested at 7% annual return:

- Starting at age 25, retiring at 65 (40 years): ~$1.32M

- Starting at age 35, retiring at 65 (30 years): ~$680K

The 10-year head start nearly doubles the final amount.

Compounding frequency

More frequent compounding yields slightly more, but the difference is usually small:

- Annually: $10,000 × (1.07)^10 = $19,672

- Monthly: $10,000 × (1 + 0.07/12)^120 = $20,097

- Daily: $10,000 × (1 + 0.07/365)^3650 = $20,137

For most people, monthly compounding (which is what most savings accounts use) is sufficient.

Use our calculator

See exactly how your savings will grow with our Compound Interest Calculator. You can adjust the principal, monthly contributions, interest rate, and time horizon to match your specific situation.

The earlier you start, the more compound interest works in your favor.