Simple interest only ever earns interest on the original amount — never on previously earned interest. Predictable, easy to compute, but slower-growing than compound interest.
Simple interest
I = P × r × t
P = principal, r = rate per period, t = number of periods
Try it
$1,000 at 5% per year for 3 years.
I = 1000 × 0.05 × 3 = $150. Total = $1,150.