6.3 Simple Interest

3 min readjune 18, 2024

Simple interest is a straightforward way to calculate how much you'll earn or owe on money over time. It's based on the initial amount, , and . This method is commonly used for short-term loans and basic savings accounts.

Understanding simple interest helps you make informed financial decisions. You can figure out loan payments, savings growth, and even how much to invest now to reach future goals. It's a fundamental concept in personal finance and basic investing.

Simple Interest Fundamentals

Calculation of simple interest

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  • calculates the interest earned on a amount over a given time period at a constant interest rate: [I](https://www.fiveableKeyTerm:I)=Prt[I](https://www.fiveableKeyTerm:I) = Prt
    • II represents the total interest earned
    • [P](https://www.fiveableKeyTerm:P)[P](https://www.fiveableKeyTerm:P) represents the or initial amount invested or borrowed
    • [r](https://www.fiveableKeyTerm:r)[r](https://www.fiveableKeyTerm:r) represents the annual interest rate expressed as a decimal (6% = 0.06)
    • [t](https://www.fiveableKeyTerm:t)[t](https://www.fiveableKeyTerm:t) represents the time in the principal is invested or borrowed
  • formula determines the total amount after earning interest, found by adding the principal and interest: [FV](https://www.fiveableKeyTerm:FV)=P+I[FV](https://www.fiveableKeyTerm:FV) = P + I
    • FVFV represents the future value or total amount after interest is added
  • Converting time periods to years for use in simple interest calculations:
    • to years: divide the number of months by 12 (6 months = 0.5 years)
    • Days to years: divide the number of days by 365, or 360 for some financial institutions (90 days = 0.25 years)
  • Example: $5,000 principal invested at 4% annual interest for 2 years
    • I=50000.042=400I = 5000 * 0.04 * 2 = 400
    • FV=5000+400=5400FV = 5000 + 400 = 5400

Loan balances and partial payments

  • formula calculates the amount owed on a loan after making a , subtracting the payment from the future value: [RB](https://www.fiveableKeyTerm:RB)=FV[PP](https://www.fiveableKeyTerm:PP)[RB](https://www.fiveableKeyTerm:RB) = FV - [PP](https://www.fiveableKeyTerm:PP)
    • RBRB represents the remaining balance owed on the loan
    • PPPP represents the partial payment amount applied to the loan
  • is equivalent to the remaining balance, representing the total amount needed to fully repay the loan
  • Example: 10,000loanat510,000 loan at 5% annual interest for 3 years, with a 2,000 partial payment made after 2 years
    • I2=100000.052=1000I_2 = 10000 * 0.05 * 2 = 1000
    • FV2=10000+1000=11000FV_2 = 10000 + 1000 = 11000
    • RB2=110002000=9000RB_2 = 11000 - 2000 = 9000
    • I3=90000.051=450I_3 = 9000 * 0.05 * 1 = 450
    • FV3=9000+450=9450FV_3 = 9000 + 450 = 9450
    • Payoff amount after 3 years = $9,450

Monthly payments and present values

  • formula determines the equal periodic payments needed to repay a loan over a given term, dividing the future value by the total number of payments: MP=FV[n](https://www.fiveableKeyTerm:n)MP = \frac{FV}{[n](https://www.fiveableKeyTerm:n)}
    • MPMP represents the monthly payment amount
    • nn represents the total number of monthly payments over the loan term
  • formula calculates the initial investment needed to reach a future goal amount, discounting the future value based on the interest rate and time: [PV](https://www.fiveableKeyTerm:PV)=FV1+rt[PV](https://www.fiveableKeyTerm:PV) = \frac{FV}{1 + rt}
    • PVPV represents the present value or initial investment required
  • Example: $15,000 loan at 6% annual interest for 4 years
    • I=150000.064=3600I = 15000 * 0.06 * 4 = 3600
    • FV=15000+3600=18600FV = 15000 + 3600 = 18600
    • n=412=48n = 4 * 12 = 48 monthly payments
    • MP=1860048=387.50MP = \frac{18600}{48} = 387.50 per month
  • Example: $100,000 future retirement goal in 20 years, assuming 5% annual interest
    • PV=1000001+0.0520=37688.95PV = \frac{100000}{1 + 0.05 * 20} = 37688.95
    • Initial investment of 37,688.95neededtoreach37,688.95 needed to reach 100,000 in 20 years at 5% interest

Additional Loan Terms

  • : The individual or entity receiving the loan and responsible for repaying it with interest
  • : The financial institution or individual providing the loan funds
  • : The final date by which the loan must be fully repaid
  • : The total cost of borrowing, including interest and any additional fees
  • : The process of gradually paying off a loan through regular payments that cover both principal and interest

Key Terms to Review (40)

1099 form: A 1099 form is a tax document used to report various types of income other than wages, salaries, and tips. It is commonly used for freelance earnings, interest, dividends, and other miscellaneous income sources.
Absolute value of an integer: The absolute value of an integer is the distance between that number and zero on a number line, always expressed as a non-negative integer. It essentially removes any negative sign from the integer.
Amortization: Amortization is the process of gradually paying off a debt over time through regular payments that cover both principal and interest. This approach allows borrowers to systematically reduce their loan balance while managing their financial commitments, making it a key concept in various financial transactions, including loans and mortgages.
Annual percentage rate: Annual percentage rate (APR) is the yearly interest rate charged on borrowed money or earned through an investment, expressed as a percentage. It provides a clearer understanding of the cost of borrowing or the yield on an investment over one year, helping consumers compare financial products more easily. The APR includes not just the interest cost but also any additional fees that may be charged, making it a comprehensive measure of the true cost of credit.
Borrower: A borrower is an individual or entity that takes out a loan from a lender with the agreement to repay the borrowed amount, typically with interest, over a specified period. Borrowers can use the funds for various purposes such as purchasing a home, financing education, or investing in a business. The relationship between the borrower and lender is governed by a contract that outlines the terms of the loan, including repayment schedules and interest rates.
Certificate of deposit: A certificate of deposit (CD) is a financial product offered by banks and credit unions that allows individuals to deposit a fixed amount of money for a specified period of time, earning interest at a higher rate compared to traditional savings accounts. CDs are considered low-risk investments and are ideal for savers looking to grow their money over time without the volatility associated with stocks or other investment vehicles.
Compound interest: Compound interest is the interest calculated on the initial principal and also on the accumulated interest from previous periods, allowing investments to grow at a faster rate over time. This concept connects to various financial topics, including how investments can appreciate more significantly compared to simple interest, the role of exponents in calculating growth, and the importance of percentages in determining returns. Understanding this concept is crucial for making informed decisions about savings, investing, and managing loans or mortgages.
Division: Division is a mathematical operation that represents the process of splitting a quantity into equal parts or determining how many times one number is contained within another. This operation plays a critical role in various mathematical concepts, providing a foundation for understanding relationships between numbers and forming the basis for operations involving fractions, ratios, and proportions. Division interacts with other operations like multiplication and is essential in contexts like calculating averages and understanding rational numbers.
Division in a base: Division in a base is the process of dividing numbers expressed in a specific numeral system, such as binary, octal, or hexadecimal. It follows similar principles to division in the decimal system but requires adjustments based on the base.
Finance charge: A finance charge is the cost of borrowing money, typically expressed as an interest rate, that is applied to the outstanding balance on a loan or credit account. This charge represents the additional amount a borrower must pay to compensate the lender for the risk taken and the opportunity cost of not having that money available for other uses. Understanding finance charges is essential when considering loans or credit options, especially in relation to how they accumulate over time based on the principal amount borrowed.
Fixed interest rate: A fixed interest rate is an interest rate on a loan that remains constant throughout the term of the loan. It does not fluctuate with market conditions, providing predictable payments for borrowers.
Future Value: Future value refers to the amount of money an investment will grow to over a period of time at a given interest rate. This concept is essential for understanding how savings and investments can increase in value due to interest, and it plays a significant role in making financial decisions. Future value helps individuals plan for financial goals by estimating how much money they will have in the future based on current investments, whether it's through simple interest, compound interest, or savings methods.
FV: FV, or Future Value, refers to the amount of money an investment will grow to over a specific period of time at a given interest rate. It helps in understanding how much an initial sum will be worth in the future, considering factors like interest rates and time. FV is crucial in financial calculations, as it allows individuals and businesses to plan for future expenses, savings goals, and investment returns.
I: In the context of simple interest, 'I' represents the interest earned or paid on a principal amount over a specified period of time. This concept is crucial for understanding how money can grow or incur costs when borrowed or invested. The calculation of 'I' typically involves the principal amount, the interest rate, and the time period, making it an essential element in finance and investment decisions.
Interest rate: An interest rate is the percentage at which interest is charged or paid for the use of money, typically expressed as an annual percentage. This rate affects how much money you can earn from savings or how much extra you have to pay on borrowed funds. Understanding interest rates is crucial because they influence loan affordability, savings growth, and overall financial planning.
Lender: A lender is an individual or institution that provides funds to borrowers with the expectation of receiving the principal amount back along with interest. In the context of simple interest, lenders play a crucial role in financing loans, as they set the terms for how much interest is charged and how repayment is structured. Understanding the role of lenders is essential for grasping how borrowing and lending transactions operate in financial scenarios.
Loan amortization: Loan amortization is the process of paying off a loan through scheduled, periodic payments that cover both principal and interest. Over time, the portion of each payment that goes towards interest decreases while the portion covering principal increases.
Maturity date: The maturity date is the specified date on which a financial instrument, such as a loan or bond, becomes due for repayment. This date is critical as it determines when the principal amount along with any interest must be paid back to the lender or investor. Understanding the maturity date helps in managing cash flows and planning investments effectively.
Monthly payment: A monthly payment is a fixed amount of money that is paid each month towards a loan or financial obligation. It is crucial in understanding how loans, interest rates, and repayment schedules work, as it determines the affordability and financial planning necessary for borrowers. Monthly payments can vary depending on the loan amount, interest rate, and term length, affecting overall costs and budgeting.
Months: Months are units of time that divide a year into segments, typically consisting of 28 to 31 days each. They play a crucial role in financial calculations, particularly in determining interest accrued over time, as they help quantify the duration for which money is borrowed or invested.
Multiplication: Multiplication is a mathematical operation that combines groups of equal sizes to find the total quantity. It serves as a fundamental building block in mathematics, allowing us to simplify and solve problems involving repeated addition, scaling quantities, and working with rational numbers. This operation is also essential for understanding various mathematical concepts, including the order of operations, base systems, and financial calculations such as interest.
Multiplication and division in bases: Multiplication and division in bases involve performing these arithmetic operations within non-decimal numeral systems, such as binary (base-2) or hexadecimal (base-16). These operations follow similar principles to base-10 but require adjustments for different digits and place values.
N: 'n' is a variable commonly used to represent the number of time periods in the calculation of simple interest. This term is integral to understanding how interest accumulates over time, as it directly influences the total amount of interest earned or paid. The value of 'n' can change depending on whether interest is calculated annually, semi-annually, quarterly, or monthly, making it essential for accurate financial calculations and projections.
P: In the context of simple interest, 'P' represents the principal amount, which is the initial sum of money that is borrowed or invested before interest is applied. Understanding 'P' is crucial because it serves as the basis for calculating the total interest accrued over time, which is determined by applying a specific interest rate to this initial amount for a designated period. The larger the principal, the more interest will accumulate, making it a key factor in determining overall returns or costs.
Partial payment: Partial payment refers to the payment of a portion of the total amount owed on a debt or loan, rather than paying the full amount at once. This method allows borrowers to reduce their outstanding balance incrementally while still being subject to interest calculations on the remaining balance. The terms of partial payments can vary, and it's essential to understand how they impact the overall cost of borrowing, including interest rates and the timeline for repayment.
Payoff amount: The payoff amount is the total sum of money required to pay off a loan or financial obligation in full at a specific point in time. This amount typically includes the principal, which is the original amount borrowed, as well as any accrued interest and fees that may be due at the time of payoff. Understanding the payoff amount is crucial for borrowers to know exactly how much they need to settle their debt completely.
Percentage: A percentage is a way of expressing a number as a fraction of 100, denoting a proportion or ratio in relation to a whole. It is widely used in various calculations to determine parts of a total, making it crucial in fields such as finance, statistics, and everyday decision-making. Understanding percentages allows for clearer interpretations of data, comparisons, and assessments in different contexts.
PP: In the context of simple interest, PP refers to the Principal Payment or the initial amount of money that is invested or borrowed. This term is crucial as it serves as the base amount on which interest calculations are made. Understanding the role of PP helps in calculating total returns on investments and the overall cost of loans, which are essential for personal finance decisions.
Present Value: Present value refers to the current worth of a sum of money that is to be received or paid in the future, discounted back to today's value at a specific interest rate. This concept is essential for understanding how money can grow over time, whether through simple interest, compound interest, savings methods, loans, or when considering the costs and benefits of renting versus owning a home. By calculating present value, individuals can make informed financial decisions based on the time value of money.
Principal: The principal is the original sum of money borrowed in a loan or invested, before interest or earnings. It is the base amount on which interest is calculated.
Principal: The principal refers to the initial amount of money that is either deposited or borrowed, which serves as the basis for calculating interest in various financial contexts. Understanding the principal is crucial, as it directly impacts how much interest will be earned or paid over time, influencing savings strategies and loan repayment plans.
PV: PV, or Present Value, refers to the current worth of a future sum of money or cash flow, given a specified rate of return. This concept is essential in finance, as it helps in understanding how much future money is worth today, allowing individuals and businesses to make informed investment decisions. Present Value considers the time value of money, which states that a certain amount of money today is more valuable than the same amount in the future due to its potential earning capacity.
R: In mathematics, 'r' typically represents the common ratio in geometric sequences, the rate of interest in simple interest calculations, and the correlation coefficient in statistics. Each of these uses of 'r' plays a critical role in understanding how values change over time or in relation to one another. Whether you're looking at how a sequence grows, how money accumulates, or how data points relate, 'r' helps quantify these relationships and rates.
RB: RB refers to the 'Rate of Borrowing' which is a crucial concept in finance that helps determine how much it costs to borrow money over time. This rate influences the calculations for simple interest, as it directly affects the amount of interest accrued on a loan or investment. Understanding RB allows individuals to make informed decisions regarding loans, savings, and investments by clearly seeing how borrowing rates can impact overall financial obligations.
Remaining balance: The remaining balance refers to the amount of money that is left to be paid off on a loan or credit account after accounting for any payments made. It is crucial in understanding how much debt is still owed, especially in relation to the interest that may accrue on that amount over time. This term is closely tied to financial calculations, helping individuals and businesses manage their finances effectively by keeping track of what they still owe.
Savings account: A savings account is a deposit account held at a financial institution that allows individuals to store money while earning interest on the balance. It provides a safe way to save funds for short-term goals or emergencies, and the interest earned can vary based on the institution and type of account. This type of account typically has lower liquidity compared to checking accounts, as it may have restrictions on the number of withdrawals.
Simple interest formula: The simple interest formula is a mathematical equation used to calculate the interest earned or paid on a principal amount over a specific period of time, without compounding. It connects the principal amount, the interest rate, and the time period to determine the total interest. Understanding this formula is crucial for making informed financial decisions regarding loans, investments, and savings.
T: In mathematical contexts, 't' is often used to represent time, particularly in formulas and equations related to growth, decay, or changes over time. It serves as a variable that allows for the modeling of different scenarios where time is a crucial factor, such as calculating interest accrued over a period or measuring distance traveled at varying speeds.
Time period: In finance, a time period refers to the duration over which an investment or loan is considered for interest calculations. It plays a crucial role in determining how much interest is accrued on a principal amount, as interest can be calculated annually, semi-annually, quarterly, monthly, or even daily depending on the terms of the agreement. Understanding the time period helps in evaluating the growth of investments and the cost of borrowing over time.
Years: In finance, years refer to the duration of time over which interest is calculated on a principal amount in simple interest calculations. This term is crucial because it determines how long the money is invested or borrowed, impacting the total amount of interest accrued. The longer the time period in years, the more interest will accumulate, demonstrating the time value of money.
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