At 4.5% simple interest, $20,000 earns $900 over one year. This is a straightforward calculation: $20,000 × 0.045 × 1 = $900. If your savings account actually uses compound interest (as most do), you'd earn slightly more, around $918 with daily compounding, making simple interest a conservative estimate.

Most traditional savings accounts use compound interest, but some basic savings products and money market instruments calculate returns using simple interest. Understanding simple interest is still valuable because it serves as the baseline for comparing financial products. Any compound interest product will always yield more than its simple interest equivalent over the same period.

Calculating simple interest gives you a quick, conservative estimate of your minimum earnings and helps you understand the foundation of all interest calculations. The difference between simple and compound interest for short periods like one year is small, so simple interest provides a good approximation. It's also useful for comparing products and understanding exactly how much of your return comes from compounding versus the base rate.