Compound interest calculator

Why compounding accelerates

Each period you earn interest on a bigger base — your deposits plus all past interest. That is why the second decade of saving typically earns more than the first, and the third more than both combined. The donut above shows the split at your chosen number of years: notice how the "interest earned" slice grows disproportionately larger the longer you leave the money invested.

The Rule of 72

A quick mental-math shortcut: divide 72 by your annual return to estimate how many years it takes your money to double. At 6% that's 12 years; at 9% it's 8 years; at 12% it's just 6 years. It's not exact, but it's close enough to compare scenarios in your head.

Why starting early beats saving more

Starting early matters more than saving more: see the full explanation with examples. Someone who invests for 10 years in their 20s and then stops can end up with more money at retirement than someone who invests twice as much for 30 years starting in their 40s — purely because of extra decades of compounding.

Thinking about whether to overpay a mortgage instead of investing? Compare the two here.

Frequently asked questions

How does compound interest work?

You earn interest on your deposit plus previously earned interest — growth on growth.

What is the Rule of 72?

72 ÷ annual return ≈ years to double. At 8%, about 9 years.

How often does this compound?

Monthly — matching most savings accounts and investment growth models.

Is a higher return always better?

Higher expected returns usually come with higher risk and volatility — this calculator assumes a steady rate, but real markets fluctuate year to year.

Can I include monthly contributions, not just a lump sum?

Yes — enter your starting amount and a monthly contribution and this calculator with monthly contributions adds both to the compounding balance every month, alongside a lump-sum-only comparison in the results.