How interest works — How to Adult

AThe super simple explanation

When you borrow money, is the extra amount you pay the lender for the privilege of using their money.

When you save money, is extra money the bank pays you for letting them use it.

is usually expressed as a percentage rate over a year.

The more time that passes, or the higher the rate, the more adds up.

Interest determines how expensive debt really is and how much saved or invested money can grow.
Understanding compounding helps explain why paying off high-interest debt quickly is so valuable.
It also explains why starting to save early, even small amounts, can add up to much more over decades.

is usually quoted as an annual percentage rate (like 5% per year).

Savings growing with compound interest

Priya puts $1,000 in a savings account earning 5% annual interest, compounded once a year.

After year one, she has $1,050. After year two, 5% is calculated on $1,050, not just the original $1,000, giving her $1,102.50.

Illustrative example with made-up people and numbers — not current rates or rules.

Principal: The original amount borrowed (or the part still owed), not counting interest.
Simple interest: Interest calculated only on the original principal, not on previously added interest.
Compound interest: Interest calculated on the principal plus any interest already added, so it grows faster over time.
Rule of 72: A rough shortcut: divide 72 by the interest rate to estimate how many years it takes money to double.

Assuming interest only applies to loans — savings and investments earn it too, and that's a good thing.
Not realizing that unpaid interest can get added to your balance and then earn its own interest.
Comparing two interest rates without checking how often each one compounds.
Underestimating how much even a small daily-compounding rate can add up on a large debt.

Totally optional. Nobody's grading you.

Q1. What is compound interest calculated on?

Q2. Priya has $1,000 at 5% annual interest, compounded yearly. What does she have after year one?

Q3. Why can unpaid credit card debt grow quickly?

1.Look up how often your savings account or credit card compounds interest.
2.Try estimating how long it'd take your savings to double using the Rule of 72.

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