Interest Rate Calculator: How to Calculate Simple & Compound Interest Online

Interest Rate Calculator (2026)

When to Use This Tool

John recently received $100,000 in spare cash and plans to put it in a 3-year fixed-term deposit. The bank offers an annual interest rate of 2.5%, and he wants to know how much interest he will earn after 3 years. By opening our interest rate calculator, selecting the 'Simple Interest' mode, and entering a principal of 100,000, an annual interest rate of 2.5%, and a term of 3 years, he clicks calculate—the right side immediately shows $7,500 in interest and a total balance of $107,500. Whether you are considering a standard mortgage or comparing the actual returns of different investment products, this tool can also come in handy.

Principles and Formulas

There are two basic methods for calculating interest:

Simple Interest: Interest = Principal × Annual Interest Rate × Term (Years)
Example: 100,000 × 2.5% × 3 = $7,500. Only the principal earns interest; the interest does not earn additional interest.
Compound Interest: Total Balance = Principal × (1 + Annual Interest Rate)^Term (Years)
Interest = Total Balance - Principal. With compound interest, the interest earned in each period is added to the principal to earn more interest, commonly known as "interest on interest."

If you select "Monthly Compound Interest," the tool divides the annual interest rate by 12 and multiplies the term by 12, using the same compound interest formula. Mathematical intuition: Because compound interest generates interest on interest, the final return is higher than simple interest given the same rate and term.

How to Use

Enter your initial amount in the "Principal" input field, for example, 100000.
Enter the rate in the "Annual Interest Rate (%)" field. Do not include the percent sign (e.g., enter 2.5).
In the "Term" section, select "Years" or "Months," then enter the number next to it, such as 3.
In the "Interest Type" dropdown menu, select [Simple Interest] or [Compound Interest]. If you need it compounded monthly, select [Compound - Monthly].
Click the [Calculate] button. The results area on the right will display the "Total Interest," "Total Balance," and "Equivalent Annualized Rate."

Complete Examples

Example 1: Simple Interest Savings
Principal: $100,000, Annual Interest Rate: 2.5%, Term: 3 Years (Simple Interest)
Interest = 100,000 × 2.5% × 3 = $7,500
Total Balance = $107,500
Explanation: Bank fixed-term deposits often calculate interest using simple interest, paying the principal and interest in a lump sum at maturity. Over 3 years, the interest is exactly 7.5% of the principal.

Example 2: Compound Interest Investment (Compounded Annually)
Principal: $100,000, Annual Interest Rate: 2.5%, Term: 3 Years (Compound Interest)
Total Balance = 100,000 × (1 + 2.5%)^3 = 100,000 × 1.07689 ≈ $107,689
Interest ≈ $7,689
Compared to simple interest, this yields an extra $189 because the interest from the first year generates its own interest in the second year.

Comparison: If Compounded Monthly
Monthly Interest Rate = 2.5% / 12 ≈ 0.20833%, Number of Periods = 3 × 12 = 36 months
Total Balance = 100,000 × (1 + 0.20833%)^36 ≈ 100,000 × 1.07792 ≈ $107,792
Interest ≈ $7,792, which is $103 more than annual compounding. The higher the compounding frequency, the greater the final return.

How to Interpret the Results

The calculation results include two main figures:

Total Interest: The actual amount of money you earn (or need to pay).
Total Balance: The final total amount you will receive (or need to pay off), including principal and interest.

In addition to these two, we also display an Equivalent Annualized Yield (or effective interest rate). For example, with monthly compounding, the nominal annual rate is 2.5%, but the effective annual rate = (1 + 2.5%/12)^12 - 1 ≈ 2.53%, which is slightly higher than the nominal rate. If you find that the calculated effective interest rate on your loan is much higher than the advertised rate, you should watch out for hidden fees.

Common Mistakes / Pitfalls to Avoid

Confusing simple and compound interest: The advertised rate for financial products is often an annualized simple interest rate, but the actual return is higher when calculated with compound interest. You can use our tool to instantly convert and compare them.
Forgetting how percentages are formatted: Simply enter "2.5" in the interest rate field; do not enter "0.025" as the tool handles the percentage automatically. If entered incorrectly, the result will be off by a factor of 100.
Selecting the wrong term unit: If the deposit term is 3 months, select "Months" in the "Term" section and enter 3. Do not select "Years" and enter 0.25; while mathematically correct, it is easy to confuse.
Ignoring the compounding period: For the same amount of money, compounding quarterly, semi-annually, or annually yields different results. Choose the mode that matches your actual product to get accurate figures.

Frequently Asked Questions (FAQ)

Q: Which is a better deal, simple or compound interest?
A: For the same rate and term, compound interest yields a higher return. However, in actual financial products, the nominal rate of a compound interest product might be lower than that of a simple interest product, so you need to calculate the total balance to compare them accurately.
Q: I pay my mortgage every month. Can I use this tool to calculate it?
A: This tool is primarily designed for calculating lump-sum interest payments and is not suitable for amortized mortgages (equal principal and interest). Mortgages require a dedicated amortization calculator.
Q: Can I enter decimals in the interest rate field? For example, 3.125%?
A: Yes, you can enter 3.125 directly. The tool supports up to three decimal places.
Q: Why is the interest I calculated slightly different from what the bank offers?
A: Some banks calculate interest based on the exact number of days (365 or 360 days), while we simplify it to full years or months. For precise daily calculations, a more specialized tool is needed, but our results are perfectly adequate for everyday reference.
Q: How do I calculate it if the deposit term is less than a year?
A: Select "Months" and enter the actual number of months; the annual interest rate will be prorated automatically. For example, for an 8-month deposit with a 2.5% annual rate, Interest = Principal × 2.5% × (8/12).

Important Notes

This calculator is suitable for the most common lump-sum interest scenarios for deposits and loans. It is not applicable in the following situations:

Installment loans (mortgages, auto loans, etc.) require a dedicated amortization calculator.
When complex terms such as discounting, early withdrawal, or penalty interest are involved, the results should only be used as an estimate.
In actual financial products, some calculate interest based on a 360-day year (banks), while others use 365 days (investments). Our tool defaults to an average of 365 days/12 months, which may result in minor discrepancies.
The calculation results are for reference only and do not constitute investment or loan advice. The exact amount is subject to the formal contract provided by your bank or financial institution.

Now you can try entering your own numbers in the interest rate calculator above. For example, compare the difference between simple and compound interest on a $100,000 deposit over 5 years.