Calculate simple interest and the final amount — or leave one field empty to find the principal, rate or time.
Simple interest is the oldest and most transparent way to charge for money over time: the interest is always a fixed percentage of the original principal, never of the interest already accumulated. The formula is J = C × i × t — principal times rate times time — and the final amount is simply M = C + J. A principal of R$ 1,000 at 2% per month for 6 months earns R$ 120 in interest, for a total of R$ 1,120, because each of the six months adds exactly R$ 20.
The most common mistake is mixing a monthly rate with a time in years, or the other way round. This calculator asks for the period of the rate and the unit of the time separately, and converts the time into the rate period before doing any math: 24% per year over 6 months becomes 24% × 0.5 = 12%, exactly the same R$ 120 of the example above. Whenever a conversion happens, the tool tells you which value it actually used, so nothing is hidden behind the result.
The reverse mode uses the same equation solved for a different variable. Leave the principal blank and it computes C = M ÷ (1 + i × t); leave the rate blank and it computes i = (M ÷ C − 1) ÷ t; leave the time blank and it computes t = (M ÷ C − 1) ÷ i. This is useful to check an instalment plan or to find out what rate someone is really charging you. Be aware, though, that simple interest is not what most Brazilian loans, credit cards, overdrafts and financing plans actually use — those charge compound interest, where interest itself earns interest and the debt grows much faster. All the arithmetic runs in JavaScript on your own device; no value you type is ever sent anywhere.
J = C × i × t and M = C + J, where C is the principal, i the rate as a decimal (2% = 0.02) and t the time expressed in the same period as the rate. Solved backwards: C = M ÷ (1 + i × t), i = (M ÷ C − 1) ÷ t, t = (M ÷ C − 1) ÷ i.
Simple interest is always calculated on the original principal, so it grows in a straight line; compound interest is calculated on the running balance, so it grows faster and faster. Over 6 months at 2%, R$ 1,000 becomes R$ 1,120 with simple interest and R$ 1,126.16 with compound interest.
For simple interest, divide by 12: 24% per year is 2% per month. This shortcut only works here — with compound interest you would need the twelfth root, not a division.
Yes. Fill in the principal, the time and the amount you ended up paying, leave the rate empty, and the tool returns the simple rate implied by those numbers.
Probably not. Banks, cards and financing in Brazil almost always use compound interest, and the contracted rate may also include fees and taxes such as IOF. Use this calculator to understand the mechanics or to check short, explicitly simple agreements.
No. Every calculation runs in JavaScript on your device. Nothing is uploaded, logged or stored.
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