What it is
The Simple Interest Calculator works out interest that is charged only on the original amount, never on the interest already accumulated.
The formula
Interest = Principal × Rate × Time, and the final amount is the principal plus that interest.
Lend R$ 1,000 at 2% a month for 10 months and the interest is 1,000 × 0.02 × 10 = R$ 200, giving R$ 1,200. Each month adds exactly R$ 20, never more, because the 2% always applies to the original R$ 1,000.
Simple against compound
This is the whole point of the distinction, and it is worth seeing in numbers.
The same R$ 1,000 at 2% a month for 10 months under compound interest gives about R$ 1,219, because each month's interest joins the base for the next. Over 10 months the gap is small. Over 5 years it is enormous: simple interest gives R$ 2,200, compound gives roughly R$ 10,765.
That difference is why almost everything in real life uses compound interest. Simple interest survives mainly in short-term agreements, some fines and late fees, informal loans between people, and school exercises.
Where to be careful
Match the rate to the period. A monthly rate with a number of months, or an annual rate with years. Mixing them is the most common error, and it produces a result that is wrong by a factor of twelve.
For anything involving instalments, financing or savings over time, use the compound interest calculator instead, because that is what those actually charge.
Simulation for information only. It does not replace the contract or a financial adviser.