Interest Calculator
Calculate simple interest on savings or loans.
Compare How Interest Accumulates
Use this calculator to see how an interest rate can affect money over a specific period. You can calculate either simple interest or compound interest and adjust the compounding frequency to see how different calculation methods change the final amount.
This can help when comparing savings and borrowing scenarios, estimating the growth of money over time, or understanding why the same interest rate can produce different results depending on how interest is calculated and compounded.
Choose the interest method that matches the situation
Interest can be calculated in different ways, so the selected method matters as much as the rate. Simple interest applies the rate to the original principal under the assumptions of the simple-interest formula. Compound interest accounts for previously accumulated interest becoming part of the balance used for later periods. When compound interest is selected, the compounding frequency determines how often that calculation is applied.
See how time and compounding change the result
To understand the calculator's output, keep the principal and annual rate unchanged while testing different time periods or compounding frequencies. This isolates the effect of each variable. A longer period can increase accumulated interest, while more frequent compounding can change the final amount under a compound-interest model. These comparisons are especially useful for learning why two scenarios with the same stated annual rate can produce different final balances.
Use the result as an estimate of the selected model
The displayed result follows the inputs and interest model selected on this page. Real financial products may use specific day-count conventions, account rules, payment schedules, promotional rates, fees, or other contractual terms. When comparing a savings product, investment illustration, or borrowing arrangement, use the calculator to understand the mathematical relationship between principal, rate, time, and compounding, then compare the result with the institution's stated terms.
A simple way to test your understanding
Try keeping the principal and rate fixed while changing only the time period. Then restore the time period and change only the compounding frequency for a compound-interest scenario. Looking at one variable at a time makes it easier to identify why the final amount changes and helps you interpret the calculator without mixing several assumptions together.
About Interest Calculator
Interest is the amount earned on money that is saved or invested, or the cost charged when money is borrowed. The amount of interest depends on several factors, including the original principal, annual interest rate, length of time, and the method used to calculate or compound the interest. Understanding these factors can help you estimate how money may grow over time or how borrowing costs can accumulate.
This Interest Calculator supports both simple and compound interest. Simple interest is calculated from the original principal throughout the entire period, while compound interest allows previously accumulated interest to become part of the balance used for future calculations. Compound interest can therefore produce increasingly larger gains over longer periods.
Simple Interest
Simple interest is calculated only from the original principal. Because previously earned interest is not added to the principal for future calculations, the amount of interest generated during each equal period remains constant when the interest rate does not change.
A = P + I
In these formulas, I represents total interest, P is the original principal, r is the annual interest rate expressed as a decimal, t is the time in years, and A is the final amount after interest is added.
Compound Interest
Compound interest differs from simple interest because accumulated interest is added to the balance and can then earn additional interest. This creates a compounding effect that becomes more noticeable as the time period increases.
I = A − P
In the compound interest formula, A is the final amount, P is the initial principal, r is the annual interest rate as a decimal, n is the number of compounding periods per year, and t is the number of years.
How Compounding Frequency Affects Growth
Compound interest can be added to the balance at different frequencies. Common options include annual, semiannual, quarterly, and monthly compounding, while some financial products use daily compounding. When the nominal annual rate and time period are otherwise unchanged, more frequent compounding generally results in a higher final amount.
The effect of changing the compounding frequency may be modest over a short period, but the difference can become more significant over longer periods. This is one reason it is useful to examine both the interest rate and the compounding method when comparing financial products.
Factors That Affect Interest
The principal, interest rate, and time period are the main factors used in these calculations. Increasing the principal increases the amount of interest generated at the same rate and time. A higher interest rate also increases the amount earned or charged, while a longer period provides more time for interest to accumulate.
For compound interest, compounding frequency is another important factor. More frequent compounding can increase the final balance because interest is added to the account more often and can begin generating additional interest sooner.
Simple Interest vs. Compound Interest
The key difference between simple and compound interest is how previously accumulated interest is treated. Simple interest continues to use the original principal, while compound interest recalculates future interest using a balance that includes accumulated interest.
For the same principal, rate, and time period, compound interest will generally produce a higher final amount than simple interest when the rate is positive and interest is compounded more than once over the period. The difference becomes increasingly important as the investment or loan remains outstanding for longer.
Understanding the Results
The calculator displays the original principal, annual interest rate, calculation period, selected interest method, total interest, interest relative to the original principal, and final amount. For compound interest, the selected compounding frequency is also included in the calculation.
The growth table provides an additional view of how the balance changes over time. For compound interest, the balance can increase at an accelerating rate because accumulated interest contributes to future growth. For simple interest, the increase remains linear when the rate and principal are constant.
Using Interest Calculations in Financial Planning
Interest calculations can be useful when comparing savings accounts, certificates of deposit, investments, loans, or other financial arrangements. However, the result from a mathematical interest formula may not exactly match the amount shown by a financial institution. Real-world products can use different day-count conventions, payment schedules, variable rates, fees, taxes, deposits, withdrawals, or other contractual rules.
For borrowing, it is also important to distinguish the interest rate from the annual percentage rate (APR), because APR can incorporate certain fees and other borrowing costs depending on the product and applicable regulations.
Important Considerations
This calculator assumes a constant interest rate and does not account for additional deposits, withdrawals, loan payments, taxes, or changes in the interest rate. The results are mathematical estimates based on the information entered and should be used as a general planning and comparison tool.
Actual returns or borrowing costs may differ according to the terms of the specific financial product and the institution providing it.
Financial Disclaimer
Results are estimates only and should not be considered financial, investment, tax, or legal advice. Please consult a qualified professional before making financial decisions.