Use this simple interest calculator to find how much interest you'll earn or owe on a loan, investment, or savings account. Enter your principal amount, interest rate, and time period. The calculator returns your total interest and final balance instantly.
Simple interest is interest calculated only on the original principal. It does not compound. That makes it straightforward to estimate borrowing costs or earned interest on fixed-rate instruments.
How the Simple Interest Calculator Works
The calculator takes three inputs: principal amount, annual interest rate, and loan or deposit term. It applies the simple interest formula and returns your total interest along with the combined principal and interest.
Results update as you change any input. This lets you compare scenarios quickly. For example, you can see how a different rate or term changes the amount of interest you'll pay or earn.
This calculator estimates interest for planning purposes. It does not account for fees, payment schedules, or rate changes. For binding figures, consult your lender or financial advisor.
Simple Interest Formula and Interest Rate
Use the Simple Interest Formula to Calculate Interest
The formula for simple interest is:
SI = P × R × T
- P (Principal): the original amount of a loan or deposit.
- R (Rate): the annual interest rate, expressed as a decimal. For example, 5% becomes 0.05.
- T (Time): the duration in years.
To find the total amount you'll repay or receive, add the interest to the principal:
A = P + SI
If your time period is in months, divide by 12. If it's in days, divide by 365. The formula stays the same. You're just converting T into a fraction of a year.
Example: $10,000 at a 6% annual interest rate for 3 years.
SI = 10,000 × 0.06 × 3 = $1,800
Total amount = $10,000 + $1,800 = $11,800.
The 6% interest of $10,000 over one year is $600. Over three years, it's $1,800. Simple interest scales linearly with time.
Simple Interest Rate vs. Compound Interest Rate
A simple interest rate charges interest only on the original principal amount. Interest that accumulates never gets added back to the balance. Your interest payment stays the same each period.
A compound interest rate charges interest on the principal plus any previously earned interest. That means interest on interest. Over time, compounding produces a larger total than the same rate applied as simple interest.
Here's a quick comparison for $1,000 at a rate of 5% over 3 years:
| Method | Year 1 Interest | Year 2 Interest | Year 3 Interest | Total Interest |
|---|---|---|---|---|
| Simple | $50 | $50 | $50 | $150 |
| Compound (annually) | $50 | $52.50 | $55.13 | $157.63 |
The difference grows larger with higher rates, longer terms, and more frequent compounding.
Calculate Simple Interest Using the Calculator
How to Use This Calculator
- Enter the principal amount. This is the starting balance of your loan or deposit.
- Enter the interest rate. Use the annual rate as a percentage (for example, 7 for 7%).
- Enter the time period. Select years, months, or days depending on your situation.
- Read your results. The calculator returns total interest and the combined final amount.
You can also calculate simple interest monthly. Enter your term in months, and the calculator converts it for you. The same applies to daily interest calculations.
Want to find the rate instead? Rearrange the formula: R = SI / (P × T). Plug in known values to solve for the missing variable.
Using the Simple Interest Calculator for an Interest Loan
Many personal loans, auto loans, and short-term financing products use simple interest. The interest is the cost of borrowing, and since simple interest is calculated on the original balance only, payments are predictable.
To estimate your borrowing cost as interest on a loan:
- Enter the loan amount as your principal.
- Enter the annual interest rate from your loan agreement.
- Enter the loan term.
The result shows how much interest you'll pay interest over the full term, assuming no early payments. It also shows the total amount you'll repay (principal and interest combined).
This is useful for comparing loan offers. A lower rate or shorter term reduces the amount of interest. Use the calculator to test different combinations before committing.
Keep in mind: real loan payments may include fees, insurance, or other charges. This calculator estimates interest only.
Simple Interest in Practice
Simple Interest Loan and Loan Calculator Examples
Simple interest applies to several common financial instruments:
- Auto loans. Many car loans use simple interest. Interest accrues daily on the remaining principal, so early payments reduce your total cost.
- Personal loans. A fixed-rate personal loan often calculates interest on the original balance.
- Short-term business loans. Lenders may use simple interest for terms under a few years.
- Government prompt payment interest. Some government obligations calculate interest using a simple interest method.
Example: You borrow $15,000 for a car at 4.5% annual simple interest for 4 years.
SI = 15,000 × 0.045 × 4 = $2,700
Total repayment = $17,700. Your interest payments total $2,700 over the life of the loan.
Example: A $5,000 personal loan at 8% for 18 months.
SI = 5,000 × 0.08 × 1.5 = $600
You'd owe $5,600 at the end of the term.
Calculate Simple Interest on Investments and Savings
Some savings instruments and bonds pay simple interest. Treasury bills, certain certificates of deposit, and some fixed deposits calculate interest on the principal amount only.
Example: You deposit $20,000 into a fixed deposit at 3.5% annual simple interest for 2 years.
SI = 20,000 × 0.035 × 2 = $1,400
Your earned interest is $1,400. The future value of your deposit is $21,400.
Simple interest is also useful for estimating short-term returns. If you park $10,000 in a savings product at 4% for 90 days:
SI = 10,000 × 0.04 × (90/365) = $98.63
For longer investment horizons, compound interest calculators give more accurate projections because most investments reinvest earnings.
How Simple Interest Works Compared to Compound Interest
Simple or Compound Interest: When to Use Simple Interest
Use simple interest when interest is calculated only on the original principal and does not compound. These situations include:
- Short-term loans. The difference between simple and compound interest is small over short periods.
- Car loans and some personal loans. Lenders that apply interest to the original balance use simple interest.
- Quick estimates. Simple interest calculations are easier to determine than compound interest calculations. They give a fast approximation.
- Fixed-rate bonds and notes. Some debt instruments pay a flat interest amount each period.
Simple interest is "simple" because the math is linear. Double the time, double the interest. That predictability makes budgeting straightforward.
When to Use Compound Interest and a Compound Interest Calculator
Use a compound interest calculator when interest compounds annually, monthly, or daily. Most real-world savings and investment products compound:
- Savings accounts. Banks typically compound interest daily or monthly.
- Credit cards. Unpaid balances accrue compound interest.
- Mortgages. Monthly interest calculations on the remaining balance behave like compounding.
- Investment portfolios. Reinvested dividends and gains compound over time.
Which will pay out more over time? Compound interest always produces a larger total than simple interest at the same rate and term. That's beneficial when you're saving, but costly when you're borrowing.
If you're comparing a simple interest loan to a compound interest loan, the simple interest version will cost less in total interest (all else being equal). As a borrower, simple interest works in your favor. As an investor, compound interest works in yours.
Simple and Compound Interest Calculator Help
Using the Calculator to Compare Simple and Compound Interest
To compare simple and compound interest side by side:
- Calculate your simple interest result using this calculator.
- Use a compound interest calculator (available on this site) with the same principal, rate, and term.
- Compare the total interest from each method.
The gap between simple and compound interest grows with:
- Higher interest rates
- Longer time periods
- More frequent compounding intervals
For a $10,000 deposit at 5% over 10 years, simple interest yields $5,000. Compound interest (compounding annually) yields $6,288.95. That's a $1,288.95 difference from interest on interest alone.
Simple Interest Calculator Help for Common Questions
What is simple interest? Simple interest is an interest calculation method based on the original principal amount only. It does not add earned interest back to the balance.
How do I calculate simple interest on a loan? Multiply the principal by the annual interest rate and the loan term in years: SI = P × R × T. This calculator does it for you.
How do I calculate simple interest monthly? Enter your term in months. The calculator converts it to a fraction of a year automatically. Or divide your annual rate by 12 to find monthly interest manually.
How do you calculate simple interest daily? Use days as your time unit. The formula becomes SI = P × R × (days / 365).
What is principal in simple interest? The principal is the original amount borrowed or invested, before any interest is added.
Do interest rates on principal amounts change over time in simple interest? No. Simple interest uses a fixed rate applied to the original principal for the entire term.
How do you find the future value of simple interest? Add the interest to the principal: A = P + (P × R × T). The result is your future value.
Is this calculator free? Yes. This simple interest calculator is free to use, with no signup required.
What is the difference between simple and compound interest? Simple interest is calculated on the principal only. Compound interest is calculated on the principal plus accumulated interest. Compound interest grows faster over time.