Is 7% a realistic return to use in compound interest calculations?

It's a reasonable historical average. The S&P 500 has returned roughly 10% per year on average before inflation, and about 7% after inflation. No one knows future returns. Using 7% in projections is a conservative-ish estimate, not a guarantee. Markets vary significantly year to year — 7% is an average, not a steady annual result.

Does compound interest apply to savings accounts?

Yes. When your savings account pays interest, that interest is added to your balance, and next month you earn interest on the larger balance. With savings accounts, the compounding is modest (0.01-5% APY) but the same mathematical principle applies.

How often does interest compound?

It depends on the account or investment. Savings accounts typically compound daily or monthly. Investment returns don't compound on a fixed schedule — they grow with the market. The more frequently interest compounds, the faster growth accelerates, though the difference between daily and monthly compounding is small.

What's the Rule of 72?

Divide 72 by your expected annual return rate to estimate how many years it takes to double your money. At 7% return: 72 ÷ 7 = about 10 years to double. At 4%: 72 ÷ 4 = 18 years to double. It's a quick mental math shortcut, not a precise calculation.

How Compound Interest Works (With Real Numbers) — Fiscal Word

Put $5,000 in an account earning 7% per year. Come back in 30 years.

With simple interest — where you only ever earn on your original $5,000 — you’d have about $15,500. Not bad.

With compound interest — where you earn on your original $5,000 plus all the interest you’ve already accumulated — you’d have about $38,000.

Same starting amount. Same rate. Same time period. Nearly $23,000 difference. That gap is compound interest.

Simple Interest vs. Compound Interest

Simple interest means you earn a fixed return on your original amount every year, and nothing more. If you put $5,000 in an account paying 7% simple interest, you earn $350 every single year — on the original $5,000, always, forever. After 30 years: $5,000 + (30 × $350) = $15,500.

Compound interest means your interest earns interest. In year one, you earn $350 on your $5,000. But in year two, you earn 7% on $5,350 — your original balance plus the $350 you already earned. That’s $374.50 instead of $350. Small difference. Then in year three, you earn 7% on $5,724.50. And on it goes — each year’s interest base is larger than the year before.

After 30 years of 7% compound growth, that $5,000 becomes approximately $38,000. That’s the power the math textbooks mean when they call compound interest “the eighth wonder of the world.” It doesn’t feel like much in year two. It’s staggering in year 30.

Why Time Matters More Than Amount

Here’s the part of compound interest that most people don’t fully absorb until they see the numbers: time matters more than how much you invest.

Consider two people, both investing $5,000 one time and never adding another dollar:

Maya invests $5,000 at age 25 and leaves it alone until age 65, earning 7% annually. After 40 years, her $5,000 grows to approximately $74,872.

Raj invests the same $5,000, but waits until age 35. He has 30 years until age 65 at the same 7% return. His $5,000 grows to approximately $38,061.

Same amount invested. Same rate. Ten years’ difference in start date. Raj ends up with about $37,000 less — roughly half of Maya’s total — because Maya’s money had an extra decade to compound.

Those first ten years of compounding produce more long-term wealth than the last ten years do. This is counterintuitive, but it’s simply how exponential math works. The sooner the clock starts, the more time the snowball has to roll.

Why This Number — 7%?

Throughout this article, I’m using 7% as an assumed rate of return. That’s meant to approximate the long-term inflation-adjusted average return of the US stock market (specifically, the S&P 500), which has historically returned around 10% per year before inflation and roughly 7% after accounting for inflation.

This is an average over long periods, not a guarantee. Any given year might be up 25% or down 30%. But for planning purposes — for understanding what consistent long-term investing can produce — 7% is a reasonable baseline. The actual returns on your investments will differ.

The Same Math Works Against You

Compound interest is wonderful when it’s working for you. It’s brutal when it’s working against you.

Credit card debt at 22% APY compounds just as aggressively as investing — except in reverse. If you carry a $3,000 balance on a credit card at 22% and pay only the minimum, the interest compounds monthly on a growing balance. Within a few years, you’ve paid back far more than $3,000 and may still owe close to the original balance.

The same principle that turns $5,000 into $38,000 over 30 years turns $3,000 in credit card debt into a debt spiral that takes years and thousands of dollars in interest to escape. This is why eliminating high-interest debt tends to be one of the best guaranteed “returns” available — you’re stopping compound interest from working against you at 22% rather than getting it working for you at 7%.

The Rule of 72

Here’s a simple mental shortcut: divide 72 by your expected annual return rate to estimate how many years it takes for your money to double.

At 7% return: 72 ÷ 7 ≈ 10 years to double
At 4% return: 72 ÷ 4 = 18 years to double
At 10% return: 72 ÷ 10 ≈ 7 years to double

So if you have $10,000 invested and expect roughly 7% returns, you’d expect it to become about $20,000 in 10 years, $40,000 in 20 years, and $80,000 in 30 years — just from the compounding, with no additional contributions. Add regular contributions and the numbers grow significantly faster.

The Rule of 72 is a back-of-the-envelope tool, not a precise calculator. But it’s useful for quickly grasping why starting earlier produces such dramatically different results than starting later.

”I’ll Start When I Have More Money”

This is one of the most common reasons people delay investing — and compound interest reveals exactly why it’s costly.

Waiting five years to start investing doesn’t delay your results by five years. It permanently removes the most productive compounding years from your timeline. Money invested at 25 has 10, 15, or 20 more years to compound than money invested at 35.

You don’t need a large sum to begin. Index funds at major brokerages have no minimum investment. Investing $50 a month starting at 25 will produce more wealth at 65 than investing $100 a month starting at 35 — because the early years of compounding are so valuable.

Start with whatever you have. The amount matters, but the starting date matters more.

To put compound interest to work in a tax-advantaged account, read about index funds as the vehicle and 401(k) accounts as one of the best places to hold them. If you’re ready to take the next step, the start investing path walks through the full process.