What is compound interest? (In plain English)
Compound interest is one of those phrases that sounds scary in a textbook but is actually pretty simple. Once you see it once, you cannot unsee it.
The one-sentence version: compound interest is interest on your interest.
Pat invested $24,000 more than Sam — and ended up with more than twice as much money. The extra $280,000 is the magic of compound interest. It is the years, not the dollars.
- P — how much you start with
- r — annual rate (7% = 0.07)
- n — compoundings per year (12 = monthly)
- t — years
Open the compound interest demo and slide the years from 5 to 40. Watch the curve. It bends upward. Hard.
Time matters more than amount
$25/month from age 16 to 22 (just six years) can grow to more than $50/month started at age 32.
Do not wait until you "have enough" to start
Start now with $20 and increase it later. The point is to get the snowball rolling.
Pay off high-interest debt first
A 22% credit card eats compound interest in reverse — it works against you. Always pay debt down before you invest at lower expected returns.
- ✗ A guarantee of 7%. Real returns vary. Some years are negative.
- ✗ Financial advice. Talk to a trusted adult before investing real money.
- ✗ A recommendation of any specific stock, fund, or product.
Start the snowball
- Open the compound interest demo and slide the years.
- If you have high-interest debt, plan to pay that down first.
- When ready, set a small recurring transfer — $20–50/month is plenty to start.