Simple vs Compound Interest: What Is the Difference?

Two loans or savings accounts can advertise the same interest rate yet pay out very differently, and the reason is whether the interest is simple or compound. Understanding simple vs compound interest is one of the most valuable pieces of money knowledge there is, because it explains why savings snowball and why debt can spiral. In short, simple interest is charged only on the original amount, while compound interest is charged on the amount plus all the interest already added. This guide explains both with clear formulas and a side-by-side example, so you can see exactly how the gap grows. To model compound growth on your own numbers, use our compound interest calculator. Figures use generic currency units.

What is simple interest?

Simple interest is calculated only on the original amount you invested or borrowed, called the principal. The interest each period stays the same, because it never counts previously earned interest.

So if you invest 1,000 at 10% for one year, you earn 100. Do it for five years and you earn 100 each year, for 500 in total. Straightforward and linear.

What is compound interest?

Compound interest is calculated on the principal plus all the interest already added. Each period, the interest itself starts earning interest, which is why it accelerates.

The first year looks the same as simple interest, but from year two onward you earn interest on a growing balance. Our full guide to how compound interest works covers this in depth.

A side-by-side example

Invest 1,000 at 10% a year for 5 years.

Method	Calculation	Interest earned	Final amount
Simple	1,000 x 0.10 x 5	500	1,500
Compound	1,000 x 1.10 to the power of 5	~610	~1,610

In plain English: both start with the same 10% rate, but compound interest earns about 110 more over five years, because it keeps paying interest on interest. Stretch it to 20 or 30 years and the difference becomes enormous.

See compounding in action

Our compound interest calculator lets you test different rates, amounts, and time periods to see how fast money grows. It is the clearest way to appreciate why compound interest is so powerful over the long term.

Why the gap keeps growing

With simple interest, your balance grows in a straight line, the same amount every year. With compound interest, it grows as a curve that gets steeper over time, because each year’s interest is calculated on a bigger balance than the last.

This is why time is the most important ingredient in compounding. The longer money compounds, the more dramatically it outpaces simple interest, which is the whole basis of long-term investing and the Rule of 72.

Where each one shows up

• Simple interest: some short-term loans, certain bonds, and car finance often use it.
• Compound interest: savings accounts, most investments, credit cards, and mortgages typically compound.

The lesson is simple: you want compound interest working for you in savings and investments, and you want to avoid letting it work against you on high-interest debt like credit cards.

Frequently asked questions

What is the difference between simple and compound interest?

Simple interest is calculated only on the original principal, so it earns the same amount each period. Compound interest is calculated on the principal plus all previously earned interest, so it grows faster over time. On 1,000 at 10% for 5 years, simple interest earns 500, while compound interest earns about 610.

Which is better, simple or compound interest?

It depends on which side you are on. For savings and investments, compound interest is better because your money grows faster. For borrowing, simple interest is cheaper because you pay interest only on the original amount. In short, you want compound interest earning for you and simple interest, or low rates, when you owe.

How do I calculate compound interest?

Multiply the principal by (1 plus the interest rate) raised to the number of periods. For 1,000 at 10% for 5 years, that is 1,000 times 1.10 to the power of 5, which is about 1,610. Subtract the principal to find the interest earned, roughly 610. A compound interest calculator does this instantly for any inputs.

Why does compound interest grow faster?

Because it pays interest on interest. After the first period, your balance includes the interest already earned, so the next period’s interest is calculated on a larger amount. This creates a compounding curve that steepens over time, unlike simple interest, which stays flat by only ever using the original principal.

Does credit card debt use compound interest?

Yes, most credit cards compound interest, often daily or monthly, on any unpaid balance. This means unpaid interest is added to what you owe and then charged interest itself, which is how card debt can grow so quickly. Paying the balance in full each month avoids these compounding interest charges entirely.

The bottom line

Simple interest is charged only on your original principal, while compound interest is charged on the principal plus accumulated interest, so it grows much faster. On 1,000 at 10% for 5 years, simple earns 500 and compound earns about 610, and that gap widens dramatically over decades. Aim to have compound interest working for your savings and to avoid it on expensive debt. Explore the effect with our compound interest calculator.

Further reading

For authoritative background on this topic, see Interest on Wikipedia.