Interest Calculator: Simple vs Compound Interest Explained

What Is Interest?

Interest is the cost of borrowing money — or the reward for lending it. When you deposit money in a savings account, the bank pays you interest for the right to use your funds. When you take out a loan, you pay the lender interest for the same privilege.

Two types dominate personal finance: simple interest and compound interest. They use different formulas, grow at different rates, and appear in different financial products. Understanding both changes how you think about every loan, savings account, and investment you'll ever touch.

Simple Interest: The Straightforward Formula

Simple interest is calculated only on the original principal. The formula:

I = P × r × t

Where:

• I = interest earned or owed
• P = principal (original amount)
• r = annual interest rate (as a decimal)
• t = time in years

Example: $10,000 at 5% simple interest for 3 years earns $10,000 × 0.05 × 3 = $1,500 in interest, for a total of $11,500. Every year, you earn exactly $500. No more, no less.

Simple interest is straightforward and predictable. It's used in most short-term personal loans, some car loans, and U.S. Treasury bills. The interest never compounds — year 3 earns the same as year 1.

Compound Interest: The Exponential Formula

Compound interest is calculated on the principal and all previously accumulated interest. That means you earn interest on your interest.

A = P(1 + r/n)^(nt)

Where:

• A = final amount
• P = principal
• r = annual interest rate (as a decimal)
• n = compounding periods per year
• t = time in years

Same example: $10,000 at 5% compounded annually for 3 years gives A = $10,000 × (1.05)^3 = $11,576. That's $76 more than simple interest — a small gap at first that balloons dramatically over decades.

Simple vs Compound: A 30-Year Comparison

Here's what the difference looks like on a $10,000 investment at 6% over 30 years:

The 10-year gap is about $1,900. By year 30, the gap is nearly $30,000. And this is on a static deposit — not a retirement account with monthly contributions. That acceleration is the entire point of "making your money work for you."

How Compounding Frequency Affects Growth

Compound interest can compound annually, quarterly, monthly, or daily. More frequent compounding means interest starts earning interest sooner — but the difference is smaller than you'd expect.

The gap between annual and daily compounding is just $181 over 10 years. Don't let banks use "daily compounding" as a marketing trick while hiding a lower APY. The Annual Percentage Yield (APY) already accounts for compounding frequency — compare APYs, not nominal rates. Our savings calculator lets you model any compounding scenario.

5 Places Simple vs Compound Interest Matters Most

1. High-Yield Savings Accounts

The FDIC reports the national average savings rate at just 0.41% APY as of early 2026. But high-yield savings accounts at online banks are paying 4.0–4.75% APY — all compounded daily or monthly. Moving $25,000 from a traditional bank to a high-yield account adds roughly $850–$1,000 per year in interest, compounding continuously over time.

2. Certificates of Deposit (CDs)

CDs use compound interest and typically compound daily or monthly. A 12-month CD at 4.5% APY on $20,000 earns about $900. CDs are FDIC-insured up to $250,000, making them one of the safest compounding vehicles available. For strategies to maximize CD returns, our compound interest guide covers CD laddering.

3. Mortgages

Mortgages use amortizing interest — functionally similar to compound interest. Each monthly payment covers that month's interest on the outstanding balance first, then chips away at principal. On a $400,000 mortgage at 6.5% for 30 years, total interest paid exceeds $510,000. The Federal Reserve's 2025 consumer credit data shows the average 30-year fixed mortgage rate stayed above 6% for most of 2025.

4. Credit Cards

This is compound interest working against you at maximum velocity. The Federal Reserve reports average credit card APRs above 21% in 2025 — with many cards charging 24–29%. Interest compounds daily. A $5,000 balance at 24% APR making minimum payments takes over 20 years to pay off and costs more than $8,000 in total interest. See our loan calculator to model payoff scenarios.

5. Student Loans

Federal unsubsidized student loans accrue interest from the day funds are disbursed — even while you're still in school. That interest capitalizes (gets added to principal) after your grace period ends. On a $30,000 unsubsidized loan at 6.5% over a 4-year degree, roughly $7,800 in interest capitalizes before you make a single payment — turning your $30,000 loan into a $37,800 starting balance.

The Rule of 72

The Rule of 72 is a mental math shortcut: divide 72 by your annual interest rate to estimate how many years it takes for money to double. It works for investments growing toward you and for debt growing against you.

At 8% investment returns, $10,000 becomes $20,000 in about 9 years, $40,000 in 18 years, and $80,000 in 27 years. The same math applies to debt: a $5,000 credit card balance at 24% APR doubles to $10,000 in about 3 years if you make no payments. That last row in the table should make every credit card holder uncomfortable.

APR vs APY: The Number That Actually Matters

Banks and lenders quote two different numbers on purpose.

• APR (Annual Percentage Rate): the nominal rate before compounding. Lenders prefer quoting APR because it looks smaller.
• APY (Annual Percentage Yield): the effective rate after compounding. Savings accounts quote APY because it looks larger.

A savings account with 5.00% APR compounding daily has an APY of 5.13%. The difference seems small, but matters on large balances. The formula: APY = (1 + APR/n)^n − 1. Always compare APYs when evaluating savings accounts, and APRs when comparing loans.

Real-World Example: $10,000 Over 20 Years

Put $10,000 into a high-yield savings account at 4.5% APY (compounded daily) and leave it untouched for 20 years:

By year 20, compound interest produced $5,107 more than simple interest on the same $10,000 at the same rate. The gap is entirely from interest earning interest — not from any additional contributions.

When Simple Interest Is Actually Better (For Borrowers)

If you're taking out a loan, simple interest is usually preferable to compound interest. Most auto loans use simple interest: you pay interest only on the remaining principal each month. Make an extra payment early in the loan and you reduce the principal directly, cutting all future interest charges.

Some payday lenders and installment loans use compound interest or add-on interest (which front-loads all interest costs). Always ask a lender whether interest accrues on the original balance or the remaining balance — the difference can be thousands of dollars.

Frequently Asked Questions

What is the difference between simple and compound interest?

Simple interest is calculated only on the original principal using I = P × r × t. Compound interest is calculated on the principal plus accumulated interest, using A = P(1 + r/n)^(nt). Over time, compound interest grows exponentially while simple interest grows linearly — the gap becomes enormous over decades.

What is the Rule of 72?

The Rule of 72 estimates how many years it takes to double your money. Divide 72 by your annual interest rate. At 6%, your money doubles in about 12 years (72 ÷ 6 = 12). At 9%, it takes about 8 years. The rule is accurate for rates between 2% and 15% and applies equally to debt growing against you.

How much does compounding frequency matter?

Less than most people think. On a $10,000 deposit at 5% for 10 years, daily compounding produces $16,470 vs $16,289 for annual compounding — a $181 difference. The interest rate and time invested matter far more. Don't chase daily-compounding accounts at lower rates over monthly-compounding accounts at higher rates.

Do mortgages use simple or compound interest?

Mortgages use amortizing interest, which works like compound interest. Each monthly payment covers that month's interest on the outstanding balance first, then reduces principal. Early payments are mostly interest. A $400,000 mortgage at 6.5% for 30 years costs over $510,000 in total interest over the life of the loan.

What interest rate do savings accounts currently pay?

The FDIC reports the national average savings rate at 0.41% APY as of early 2026, but high-yield savings accounts at online banks are paying 4.0–4.75% APY. That's a 10x+ difference on the same FDIC-insured deposit. Moving $25,000 from a traditional bank to a high-yield account adds roughly $850–$1,000 per year.

How does compound interest work on credit cards?

Credit card interest compounds daily on your outstanding balance. The Federal Reserve reports average credit card APRs above 21% in 2025. A $5,000 balance at 24% APR making minimum payments takes over 20 years to pay off and costs more than $8,000 in interest — more than the original balance.