Simple vs. Compound Interest Calculator

Two loans or savings accounts can quote the exact same interest rate and still produce very different results, depending on whether that interest is simple or compound. Use the calculator below to see how your money compounds over time, and use the worked comparison further down to see exactly how much more (or less) you'd have under simple interest instead.

Simple vs. Compound Interest: What's the Real Difference?

Simple interest is calculated only on the original principal, for the entire term. It never changes, no matter how long the money sits or how many interest payments have already accrued. The formula is:

A = P × (1 + r × t)

where P is the principal, r is the annual interest rate (as a decimal), and t is the number of years.

Compound interest, which the calculator above models, is calculated on the principal plus any interest already earned, at each compounding period. Every time interest is added to the balance, the next period's interest is calculated on that larger amount — so the balance grows faster the longer it compounds, and the gap between simple and compound results widens every year. The formula is:

A = P × (1 + r/n)^(n×t)

where n is the number of compounding periods per year.

Key Differences Between Simple and Compound Interest

• Growth Curve: Simple interest grows in a straight line; compound interest grows on a curve that steepens over time.
• What Interest Is Calculated On: Simple interest is always calculated on the original principal only; compound interest is calculated on principal plus all previously accrued interest.
• Impact of Time: The longer the time horizon, the larger the gap between the two — over short periods the difference is small, but over decades it becomes enormous.
• Impact of Compounding Frequency: For compound interest, more frequent compounding (daily vs. monthly vs. annually) produces a slightly higher effective return at the same stated annual rate.
• Where You'll See Each: Compound interest is standard for savings accounts, credit cards, and most investment accounts; simple interest is common for some auto loans, short-term personal loans, and certain bonds.
• Which Favors the Borrower: Simple interest favors borrowers (you pay interest only on the original amount owed); compound interest favors savers and investors (your returns generate their own returns).

How to Use This Calculator to Compare Both

1. Enter Your Principal, Rate, and Time Horizon

Input the starting principal, the annual interest rate, and the number of years, and choose a compounding frequency (for example, monthly or annually). Leave the monthly contribution at 0 for a pure lump-sum comparison.

2. Read the Compound Interest Result

The calculator's final balance and yearly chart show how the principal grows under compound interest at your chosen compounding frequency.

3. Calculate the Simple Interest Result By Hand

Using the same principal, rate, and years, apply the simple interest formula above: multiply the principal by one plus the rate times the number of years. This is what the same money would grow to under simple interest — with no compounding at all.

4. Compare the Gap

Subtract the simple interest result from the compound interest result. That difference is the extra return compounding generates over the same time period — and you'll notice it grows larger the longer the time horizon or the higher the rate.

5. Try Different Compounding Frequencies

Switch the compounding frequency between annual, monthly, and daily with the same principal, rate, and time. You'll see the compound result increase slightly as compounding becomes more frequent, even though the stated annual rate hasn't changed — this is the difference between a nominal rate and an effective annual rate.

Which Matters More for You?

If you're borrowing money, simple interest is generally better for you, since you only ever pay interest on the original principal, not on interest that's already accrued. When comparing loan offers, always check whether interest is simple or compound (and how often it compounds) — a loan advertising a slightly higher simple-interest rate can cost less overall than a lower compound rate compounded frequently.

If you're saving or investing, compound interest works in your favor, and more frequent compounding is better for you. Starting early matters enormously here: money given more years to compound grows disproportionately more than the same amount contributed later, even at the same rate.

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Tips for Working With Interest Calculations

1. Always Check How Interest Compounds: A loan or account's stated annual rate doesn't tell the whole story — ask whether it's simple or compound, and if compound, how frequently.
2. Start Compounding as Early as Possible: Because compound growth accelerates over time, the biggest lever for long-term savings is usually time in the market, not a marginally higher rate.
3. Watch for APY vs. APR: APY (annual percentage yield) already accounts for compounding frequency and is the more accurate figure for comparing savings products; APR does not.
4. Reinvest, Don't Withdraw: Compound growth only continues if earned interest stays in the account — withdrawing interest as it's earned effectively converts a compound account into a simple-interest one.

Common Mistakes to Avoid

• Assuming All Interest Compounds: Some short-term loans genuinely use simple interest — treating them as compound overstates what you'll actually owe.
• Comparing Rates Without Comparing Compounding Frequency: A 5% rate compounded daily produces a higher effective return than a 5% rate compounded annually.
• Underestimating Long Time Horizons: Because compound growth is exponential, its effects over 20-30 years are far larger than intuition based on a few years suggests.
• Ignoring Fees That Erode Compounding: Account fees or high expense ratios reduce the effective growth rate and can meaningfully shrink long-term compound returns.

Conclusion

Simple and compound interest can produce dramatically different results from the exact same rate, principal, and time period — the only difference is whether interest earns interest. Use the calculator above to see your compound growth, work the simple interest formula by hand for the same numbers, and let the size of that gap guide which loan or savings product actually serves you better.