Simple interest
Simple interest is interest paid only on the original principal, so the amount added each period stays the same for as long as the rate and the principal do.
The formula is , with the original sum, the rate as a decimal, and time in the same unit as the rate. The total is . Because the rate never sees interest already paid, the balance climbs in a straight line.
Compound interest is the other rule: each period pays on the current balance, so the amount added grows. The two agree over a single compounding period, and they agree for as long as every period's interest is paid out and never joins the principal. Past that point they separate.
The simple interest calculator runs . The pair is laid out on simple against compound interest.