Compound Interest Explained
Compound interest is interest earned on interest: your returns are reinvested and then earn returns of their own. At a 7% annual return, a one-time $10,000 investment grows to about $76,000 in 30 years — and investing $500/month for 30 years at the same return grows to roughly $610,000, of which only $180,000 was your own money. The Rule of 72 estimates doubling time: divide 72 by your annual return (72 ÷ 7 ≈ 10.3 years).
What Is Compound Interest?
Compound interest is the interest you earn on your original money plus the interest you earned before. Your returns get reinvested, and then those returns earn returns of their own — so growth feeds on itself.
Here's the difference with simple interest. If you invest $10,000 at 7% simple interest, you earn $700 every year, forever. After 30 years you have $31,000. With compound interest, your $10,000 earns $700 in year one, then 7% of $10,700 in year two, and so on. After 30 years it's roughly $76,000.
The formula is A = P(1 + r/n)^(nt), where P is your starting amount, r is the annual rate, n is how often interest compounds per year, and t is years.
Why Compounding 'Accelerates' Over Time
Compound growth looks slow at first, then suddenly fast — because the growth is exponential, not linear.
Take $10,000 at 7%. After 10 years it's about $20,000. After 20 years, $39,000. After 30 years, $76,000. The second decade adds more than the first, and the third adds more than the second. Each decade's growth is larger than the last because there's more money compounding.
This is why people say the first $100,000 is the hardest. It takes years of saving to reach it, but once the portfolio is large, market returns alone routinely add far more than your contributions — and that snowball is what FIRE is built on.
The Rule of 72
The Rule of 72 is a quick mental shortcut to estimate how long it takes money to double at a given return rate.
Years to double = 72 ÷ annual return rate. At 7%, 72 ÷ 7 ≈ 10.3 years. At 10%, it's about 7.2 years. At 4%, about 18 years.
The same rule works in reverse for inflation: at 3% inflation, prices double in about 24 years, which is why your retirement number needs to grow over time too.
Our Compound Interest Calculator does this math precisely — enter your starting amount, monthly contribution, and expected return to see the exact growth curve year by year.
How Compound Interest Powers FIRE
FIRE is a game of compound growth plus a high savings rate. The portfolio doesn't do the work alone — contributions get it started, but compounding does the heavy lifting over the long run.
Consider saving $500/month at 7% for 30 years. Total contributions: $180,000. Ending balance: roughly $610,000. More than two-thirds of the final number came from compounding, not from money you saved.
The flip side: every 0.5% of extra fees or lower return costs a lot over 30 years. A 6.5% vs 7% return on that same plan is roughly a $35,000 difference. Keeping costs low and staying invested are the two biggest levers you control.
Why Starting Early Beats Saving More
Time is the multiplier in compound interest, which makes starting early remarkably powerful.
Example: Alice invests $300/month from age 25 to 35 (10 years, $36,000 total), then stops. Bob invests $300/month from 35 to 65 (30 years, $108,000 total). At 7%, Alice's account at 65 is about $530,000 — more than Bob's $365,000, even though Bob saved three times as much money.
That's the case for funding retirement accounts aggressively in your 20s and 30s, and it's why retirement-account-types and the FIRE Calculator are worth exploring before you set your savings plan.
Frequently Asked Questions
What is compound interest in simple terms?
Compound interest is interest on interest. You earn a return on your original money, that return is reinvested, and then you earn returns on the reinvested amount too. Over time this creates exponential — not linear — growth, which is why a modest amount saved early can grow into a large retirement nest egg.
How does compound interest grow so fast?
Because the base grows every year. Each period's return is calculated on the previous total, so growth accelerates. At 7%, money doubles roughly every 10 years (Rule of 72: 72 ÷ 7 ≈ 10.3). Over 30 years that's three doublings — your money multiplies roughly 8 times.
What is the Rule of 72?
A shortcut to estimate how long an investment takes to double: divide 72 by the annual return rate. At 7% it's about 10.3 years; at 10% about 7.2 years. It also works for inflation — at 3% inflation, prices double in about 24 years.
Does compound interest work on loans and debt?
Yes, and against you. Compound interest on credit card or loan balances grows the same exponential way — which is why high-interest debt grows so fast. Paying it off is mathematically identical to earning the interest rate tax-free, so clearing debt above roughly 7-8% usually beats investing.