Compound Interest Explained Simply

March 2026 · 5 min read

Compound interest is the most powerful force in personal finance. It's why someone starting at 25 can end up with twice as much as someone starting at 35.

Simple vs compound interest

Simple interest: $1,000 at 5% = $50/year forever. After 30 years: $2,500. Compound interest: same $1,000 at 5% = $4,322 after 30 years. The difference grows dramatically with time.

The 10-year head start

• Start at 25: $200/mo at 7% → $525,000 at 65 (contributed $96K)
• Start at 35: $200/mo at 7% → $243,000 at 65 (contributed $72K)

$24K more contributed, $282K more in the account. That's compound interest. Try different scenarios with the compound interest calculator.

Where it works for you

High-yield savings (4-5% in 2026), index funds (~7-10% historically), retirement accounts (tax-advantaged compounding). Start with whatever you can — even $50/month. Find that first $50 by canceling a subscription you don't use.

Where it works against you

Credit card debt compounds too — against you. A $5,000 balance at 20% APR costs $8,000+ in interest if you pay minimums. Use the debt payoff calculator to see the true cost.