What Is Compound Interest?

Quick Answers

Compounds daily?

Most savings accounts

Rule of 72

Years to double = 72/rate

More frequent =

More growth/cost

Formula

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

Compound interest is interest calculated on the initial principal of a deposit or loan, plus all of the accumulated interest from previous periods. In simpler terms, it is interest on interest -- causing wealth or debt to grow faster over time than simple interest alone.

When it comes to savings or investments, compound interest works powerfully in your favor. As your money earns interest, that new larger balance earns even more interest in the next period. Over long time horizons, this compounding effect can generate significant wealth -- even from modest starting amounts.

However, with loans and credit cards, compound interest works against you. If you leave a balance unpaid, the interest charges are added to your principal, meaning you will pay interest on your interest in subsequent billing cycles. The more frequently interest compounds, the faster the balance grows.

How Compound Interest Is Calculated

The standard compound interest formula is:

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

A the final amount (principal + interest)
P the principal (starting balance)
r the annual interest rate as a decimal (e.g. 5% = 0.05)
n the number of times interest compounds per year
t the number of years

For example, $10,000 invested at 6% annual interest compounded monthly for 10 years grows to approximately $18,194 nearly double the original amount, with $8,194 coming entirely from compounding.

Compounding Frequency Matters

The more often interest compounds, the greater the effect. Common compounding schedules include:

Frequency	Compounds Per Year	Common Uses
Daily	365	Credit cards, high-yield savings
Monthly	12	Mortgages, personal loans, most savings accounts
Quarterly	4	Some bonds and CDs
Annually	1	Some savings bonds

Compound Interest on Loans vs. Savings

The same mechanism that builds wealth in a savings account erodes it in debt. Here is how compound interest plays out differently depending on context:

When it helps you

Savings accounts and CDs
Investment accounts and 401(k)s
Reinvested dividends
Any account where earnings are reinvested

When it works against you

Credit card balances carried month to month
Personal loans with unpaid interest
Student loans in deferment
Any debt where interest is capitalized

How to Minimize Compound Interest on Debt

Pay more than the minimum minimum payments on credit cards barely cover the monthly interest charge, letting the principal compound largely unchecked
Pay early in the billing cycle with daily compounding, every day you carry a balance costs money
Refinance to a lower rate reducing the interest rate directly reduces how fast balances compound
Avoid interest capitalization on student loans, capitalized interest (unpaid interest added to principal) compounds for the life of the loan

Common Compound Interest Mistakes

Comparing accounts by nominal rate alone. A 6% rate compounded daily earns more than a 6% rate compounded annually, even though the headline number is identical. Always compare the effective annual yield (APY), not just the stated rate, when shopping savings products or evaluating loan offers.

Assuming a "small" minimum payment keeps a balance from growing. On a credit card with daily compounding, a minimum payment that's close to the monthly interest charge means you're barely breaking even -- the balance can still creep upward even while you're making payments every month on time.

Key Facts

The Mechanics: Compound interest is calculated on both your original principal and any interest that has already been earned or charged.
The Benefit: In savings accounts and investments, compound interest works in your favor, helping your money grow exponentially over time.
The Cost: In loans and credit cards, compound interest works against you, increasing the total amount you owe beyond the original principal.
Frequency Matters: The frequency of compounding significantly impacts the final amount, with more frequent compounding resulting in greater growth or cost.
Time Is the Key Variable: The longer money compounds, the more dramatic the effect -- small differences in rate or time horizon produce large differences in outcome.

Frequently Asked Questions

How does compound interest differ from simple interest?

Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus any previously accumulated interest -- causing balances to grow faster over time.

How often does interest compound on loans?

Most mortgages and personal loans compound monthly. Credit cards typically compound daily, which makes carrying a balance especially expensive compared to other loan types.

Why is compound interest dangerous on credit card debt?

When you carry a balance, unpaid interest is added to your principal. The next cycle, you pay interest on that larger balance -- a snowballing effect that can make debt very difficult to escape without aggressive payoff strategies.

How can I use compound interest in my favor?

In savings and investments, compound interest grows your balance over time. The longer the time horizon, the greater the effect -- which is why starting to save early is so powerful even with small amounts.

What is the compound interest formula?

The standard formula is A = P(1 + r/n)^(nt), where A is the final amount, P is the principal, r is the annual interest rate (as a decimal), n is the number of compounding periods per year, and t is the number of years.

Does compounding frequency really matter?

Yes, significantly. A $10,000 balance at 20% APR compounded daily costs more than the same rate compounded monthly. The more frequently interest compounds, the faster the balance grows -- for both savings and debt.