Compound vs Simple Interest: 10 Rate Scenarios Over 30 Years

2026-05-30 · ~2,000 words · 10 min read

1. Same $100K, Same Rate, 30 Years — One Gets $432K, the Other $250K

The difference comes down to one thing: whether your interest earns interest.

Meet Alice and Bob. Both invest $100,000 at 5% for 30 years.

Alice's account uses simple interest. Each year, she earns exactly $5,000 — the same amount, every year, calculated only on her original $100,000. After 30 years: $250,000.

Bob's account uses compound interest. In year one, he earns $5,000 — same as Alice. But in year two, he earns 5% on $105,000 = $5,250. By year 30, he's earning over $20,000 in interest that year alone. Final balance: $432,194.

That's a $182,194 gap — from the exact same starting point. This isn't magic. It's the exponential function doing what it does: looking unremarkable for years, then exploding.

2. The Math: Simple vs Compound Formulas

Simple Interest

Interest is calculated only on the original principal. Every year's interest payment is identical.

$100K at 5% simple for 30 years: 100,000 × (1 + 0.05 × 30) = $250,000. Total interest = $150,000 ($5,000/year × 30).

Compound Interest

Each period's interest joins the principal for the next period's calculation. Interest earns interest.

$100K at 5% compound for 30 years: 100,000 × (1.05)³⁰ = $432,194. That's 73% more than simple interest.

3. Interactive: Watch the Lines Diverge

Adjust the sliders below to see how the straight line (simple) and the accelerating curve (compound) pull apart over time.

4. 10 Rate Scenarios: Full Comparison

Same $100K principal, 30 years — across different rates:

The pattern: at 1%, compound barely beats simple (1.04×). At 6%, it's 2×. At 10%, it's 4.4×. At 12%, it's 6.5×. The rate isn't an additive lever — it's a multiplicative lever on an exponential curve.

Data source: standard compound and simple interest formulas. Historical market returns: Damodaran, NYU Stern.

5. Time as an Amplifier: Year 10 / 20 / 30

$100K at 5%. Watch the gap grow:

In the first decade, the gap is just $12.9K — easy to dismiss. By decade three, it's $182K. Time doesn't just help compounding — time IS the mechanism through which compounding works.

6. The Rule of 72 Only Works With Compounding

The Rule of 72 is a quick mental shortcut — but it's meaningless under simple interest.

At 10%, compounding doubles your money nearly 3 years faster than simple interest. That extra cycle of doubling is where fortunes diverge.

7. When Simple Interest Actually Makes More Sense

Compound isn't always "better" — context matters:

Short-term loans (≤1 year)

You lend a friend $10K for 6 months at 5%. Simple interest: $10,000 × 0.05 × 0.5 = $250. Clean, transparent, no arguments. Monthly compounding would yield $252.62 — $2.62 more, but with extra calculation hassle. For short, simple transactions, clarity beats precision.

Fixed-term CDs

A 5-year CD at 4% simple: $100K → $120K at maturity. The bank won't auto-compound for you — because the spread between what they pay you and what they earn lending your money out is their business model.

Bonds with periodic coupon payments

Most bonds pay interest as cash to your account. Unless you manually reinvest those payments, your bond position grows at simple interest. Interest on paper ≠ compound interest in practice. You need to actively reinvest.

8. How to Tell What You're Actually Getting

A quick field guide to real-world products:

The one-question test: ask "Does interest automatically join the principal and earn more interest?" Yes = compound. No = simple.

9. FAQ

Q1: How big is the compound vs simple gap over 30 years?

At $100K and 5%, compound ($432K) beats simple ($250K) by $182K — a 73% advantage. At higher rates, the gap explodes: 10% gives you $1.74M vs $400K.

Q2: Are my bank deposits earning compound interest?

Savings and money market accounts: yes (daily/monthly). Fixed-term CDs: no (simple, paid at maturity). Look for "APY" (Annual Percentage Yield) — if you see it, it's compound. If you see "interest rate" without "APY," check the fine print.

Q3: Does the Rule of 72 apply to simple interest?

No. The Rule of 72 is derived from the exponential nature of compounding. For simple interest, doubling years = 100 ÷ rate (e.g., 5% takes 20 years). Always longer than compound doubling.

Q4: Which products use compound vs simple interest?

Compound: savings accounts, reinvested ETF/mutual fund dividends, DRIP stocks, universal life insurance, and credit card debt (it works against you). Simple: bank CDs, government bonds, corporate bonds, personal loans. When in doubt: does the interest automatically join the principal? Yes = compound.

10. Summary

1. The gap is exponential, not linear. Same $100K at 5%: compound beats simple by 73% over 30 years. Higher rates and longer horizons widen the gap dramatically.
2. Short-term: prefer simplicity. Long-term: demand compounding. Under 1 year, the difference is negligible and transparency matters more. Beyond 5 years, skipping compounding is leaving serious money on the table.
3. Compounding doesn't happen automatically. You need to actively choose "reinvest dividends" on your brokerage, roll over maturing CDs, and verify that your products actually compound. Paper returns ≠ real returns if you're not reinvesting.