Most calculators just ask you to pick "simple" or "compound" and move on, but the actual difference between the two compounds (no pun intended) over time in a way that's worth understanding before you commit money to anything long-term.
Simple interest: linear growth
Simple interest is calculated only on the original principal, every period, for the life of the loan or deposit. The formula is:
Interest = Principal x Rate x Time
If you put in $1,000 at 5% simple interest for 3 years, you earn $50 every single year, for a total of $150. The interest never earns interest of its own -- it grows in a straight line.
Compound interest: growth on growth
Compound interest is calculated on the principal plus whatever interest has already accumulated. Each period's interest gets added back into the base, so the next period's interest is calculated on a slightly larger number.
Amount = Principal x (1 + Rate/n)^(n x Time)
where n is how many times per year the interest compounds (annually, monthly, daily, etc.). That same $1,000 at 5% compounded annually for 3 years grows to about $1,157.63 -- roughly $8 more than the simple-interest version over just 3 years. Stretch the same comparison to 20 or 30 years and the gap becomes dramatic, because each year's gains are now earning their own gains.
Why the compounding frequency matters too
Two accounts can advertise the same nominal annual rate and still pay out differently depending on how often interest compounds. Monthly compounding earns slightly more than annual compounding at the same stated rate, and daily compounding earns slightly more than monthly. The difference per period is small, but it's real, and it's why the "annual percentage yield" (APY) disclosed by banks is often a more honest number than the plain interest rate -- APY already accounts for compounding frequency.
Where each one shows up in practice
Simple interest is common in short-term loans and certain bonds. Compound interest is how most savings accounts, fixed deposits, and long-term investments actually work, which is also why starting early matters more than the size of any single contribution -- time is the multiplier that compounding needs to do its work.
Do the math instead of estimating it
Because compounding involves an exponent, it's genuinely hard to eyeball accurately, especially over longer time horizons or with monthly contributions layered in. Small differences in rate or compounding frequency add up to real money over years.
The Interest Calculator handles both simple and compound interest, so you can compare exact numbers instead of approximating them in your head.