When you borrow money, you pay interest to the lender. Interest can be estimated as simple interest, which is calculated by multiplying the amount of money loaned by the interest rate and the length of the loan. The mathematical equation to calculate simple interest is I = Cit. { Displaystyle I = Cit.}
Sin embargo, los bancos normalmente cobran un interés compuesto en los préstamos. Al interés compuesto, se le añade el interés al capital cada año del préstamo. El año siguiente, el interés se paga sobre la cantidad total del capital y el interés. Es más común ver el interés que se calcula cada año, pero el interés también puede calcularse mensualmente, o incluso de forma semanal o diaria. También puedes ganar interés (ya sea simple o compuesto) en inversiones que hagas.
Pasos
Método 1 de 2: Fórmula del interés simple
Step 1. Determine the total amount you have borrowed
Interest is paid on the total amount of money you have borrowed, also known as the principal. In the case of an investment, the principal is the total amount of money that you have invested. This amount is represented in the simple interest formula by a "C".
For example, suppose you bought a car for $ 12,000. You made a down payment of $ 3,000 and financed the rest. The principal of your car loan would be $ 9,000
Step 2. Convert the interest rate to a decimal value
Interest rates are normally expressed as a percentage. Divide the percentage rate by 100 to convert it to a decimal. Use that decimal in the formula.
For example, if your car loan had an annual interest rate of 7%, you would express this in the simple interest formula as 0.07
Tip:
some calculators will automatically convert a percentage to a decimal value. Just make sure to hit the percent key after the number.
Step 3. Use the correct time period for the duration of the loan
Loans are generally made for a certain number of years, represented by "t" in the simple interest equation. However, for some loans, such as car loans, the length of the period is expressed as a number of months. For this simple interest formula, you would need to convert the months to years.
 For example, if you took out a 60month car loan, you would divide 60 by 12 (the number of months in a year) to determine that the loan is for 5 years.

The formula I = Cin { displaystyle I = Cin}
Step 4. Find the total interest you owe using the formula I = Crt { displaystyle I = Crt}
Una vez que tengas valores numéricos para las porciones de la fórmula, multiplícalos para determinar la cantidad de interés que debes durante el plazo del préstamo o inversión. Continuando el ejemplo, has financiado un carro con un préstamo de $9000 con un interés de 7 % por 5 años:
 P=9000{displaystyle P=9000}
 r=0, 07{displaystyle r=0, 07}
 t=5{displaystyle t=5}
 El interés que debes es $3150. 3150=9000x0, 07x5{displaystyle 3150=9000x0, 07x5}
Step 5. Calculate the total amount you owe over the term of the loan
When you repay a loan with simple interest, you pay the amount of the principal that you originally borrowed plus the total interest on that amount. To find the total amount, add the interest back to the principal using the formula I + C { displaystyle I + C}
 Continuando con el ejemplo anterior, la cantidad total que debes sería $12 150. 3150+9000=12150{displaystyle 3150+9000=12150}
 Puedes combinar ambas ecuaciones si buscas la cantidad total de dinero que se acumulará en el plazo del préstamo o inversión usando la fórmula A=C(1+rt){displaystyle A=C(1+rt)}
Método 2 de 2: Interés compuesto
Step 1. Start with the initial amount that you have borrowed or invested
Like simple interest, compound interest is charged on principal. But unlike this, compound interest is added to principal. In the compound interest formula, principal is symbolized by a "C", just as in the simple interest formula.
For example, suppose you bought a house for $ 150,000. You have made a down payment of $ 50,000 and have taken out a home loan for the rest. The principal of the home loan would be $ 100,000
Step 2. Express the annual interest rate as a decimal
In the compound interest formula, as in the simple interest formula, the interest rate is symbolized by the letter "r". Divide the percent by 100 to get the decimal value.
For example, if the annual interest rate on your home loan is 8%, you would use 0.08 in the compound interest formula
Step 3. Determine the term of the loan or investment in years
In the compound interest formula, the letter "t" is used to symbolize the number of years the loan or investment will be in force. As with the simple interest formula, the value for "t" must be in years, so if the term is in months or weeks, you would need to convert to years.
For example, if you have a 10year mortgage on the house you bought, you would use 10 in the compound interest formula
Step 4. Find the number of times the interest will compound in 1 year
When interest is compounded, it is added to principal at the end of each compounding period. In the next period, the interest is calculated on the total amount of the principal and the interest of the first period. This cycle continues for the entire term of the loan, or until the loan is paid off in full.
 For example, if your home loan capitalizes interest on a monthly basis, it would compound 12 times in a year. In your compound interest formula, this value is represented by an "n".
 In the case of an investment, the interest would be compounded until the end of the deposit term, or until you have withdrawn your investment.
Step 5. Find the total amount accrued using the compound interest formula
Once you have numbers for all the securities, you can determine the total amount of money that will accumulate over the term of the loan or investment, including interest. This quantity is symbolized by the letter "A" in the formula. Use the formula A = C (1 + r / n)^{nt}.

P = 100,000 { displaystyle P = 100,000}
 r=0, 08{displaystyle r=0, 08}
 n=12{displaystyle n=12}
 t=10{displaystyle t=10}
 la cantidad total pagada durante el plazo del préstamo hipotecario sería $221 964. 221 964 = 100 000 (1 + 0, 08/12)^{12(10)}. el interés total pagado sería $121 964, 0.
 el interés compuesto puede ser significativamente mayor que el interés simple, en especial a largo plazo. si el mismo préstamo hipotecario tiene un interés simple, solo pagarías $80 000 de interés durante el plazo del préstamo.
consejo:
para encontrar la cantidad total del interés pagado, simplemente resta la cantidad del capital de la cantidad total de dinero acumulado. el resultado es el interés total pagado durante el plazo del préstamo o inversión.
vídeo de wikihow: cómo calcular el interés simple
ver
consejos
 reordena la fórmula si necesitas calcular el capital (c=i/rt{displaystyle c=i/rt}
 no confundas la tasa de interés con la tasa de porcentaje anual (apr, por sus siglas en inglés). la apr incluye tarifas además del interés. al comparar los préstamos, la apr puede darte una mejor idea de cuánto te costará pedir prestado esa cantidad de dinero.
), la tasa (r=i/ct{displaystyle r=i/ct}
) o el tiempo (t=i/cr{displaystyle t=i/cr}
).