Simple Interest Calculator
Introduction
Simple interest is a straightforward method to calculate the interest charged or earned on a principal amount over a specific period. It is commonly used for short-term loans, savings accounts, or investments where interest is not compounded. This calculator helps you determine the simple interest, total maturity amount, and equivalent monthly or daily earnings based on your inputs.
Simple Interest Formula
The simple interest I can be calculated using the formula:
where:
- P is the principal amount (initial sum of money)
- r is the annual interest rate (expressed as a decimal)
- t is the time the money is invested or borrowed for, expressed in years
Since the calculator allows time input in years, months, or days, the time value is converted to years before applying the formula:
- Years:
t = time-value - Months:
t = time-value / 12 - Days:
t = time-value / 365
The maturity amount M is the sum of the principal and the interest earned or owed:
If there are any upfront fees or discounts, these are added or subtracted from the maturity amount accordingly.
Interpreting the Results
The calculator outputs:
- Simple Interest: The total interest earned or owed over the specified period.
- Maturity Amount: The total amount at the end of the period including principal and interest, adjusted for any upfront fees or discounts.
- Equivalent Monthly and Daily Earnings: These values show how the interest breaks down over shorter time frames, useful for understanding periodic returns or costs.
These results help you evaluate the cost or benefit of a loan or investment under simple interest terms.
Worked Example
Suppose you invest $5,000 at an annual interest rate of 6% for 9 months, with no upfront fees or discounts.
- Principal, P = $5,000
- Annual interest rate, r = 6% = 0.06
- Time, t = 9 months = 9/12 = 0.75 years
- Upfront fees/discount = $0
Calculate simple interest:
Maturity amount:
Equivalent monthly interest:
$225 / 9 months = $25 per month
Equivalent daily interest (assuming 365 days/year):
$225 / (9 × 30) ≈ $0.83 per day
Comparison Table: Simple Interest vs Compound Interest
| Feature | Simple Interest | Compound Interest |
|---|---|---|
| Interest Calculation | On principal only | On principal + accumulated interest |
| Interest Growth | Linear | Exponential |
| Typical Use Cases | Short-term loans, simple investments | Long-term investments, savings accounts |
| Complexity | Easy to calculate | Requires compounding frequency |
| Calculator Available Here | Yes (this calculator) | No (use compound interest calculator) |
Limitations and Assumptions
- This calculator assumes interest is calculated using the simple interest method without compounding.
- Time is converted to years based on the selected unit; fractional years are supported.
- Upfront fees or discounts are treated as a one-time adjustment to the maturity amount.
- Does not account for taxes, inflation, or other financial factors.
- Not suitable for loans or investments where interest compounds periodically.
- Assumes a 365-day year for daily calculations; leap years are not considered.
Frequently Asked Questions
How does the time unit affect the calculation?
The time unit (years, months, days) determines how the time value is converted to years for the formula. For example, 6 months is converted to 0.5 years. Accurate time input ensures correct interest calculation.
What impact do upfront fees or discounts have?
Upfront fees or discounts adjust the maturity amount by adding or subtracting a fixed dollar amount. They do not affect the interest calculation itself but change the final amount you receive or owe.
Can I use this calculator for compound interest?
No, this calculator only computes simple interest. For compound interest calculations, use a dedicated compound interest calculator that accounts for interest compounding periods.
Why is simple interest used instead of compound interest?
Simple interest is easier to calculate and is often used for short-term loans or investments where compounding is not applied. Compound interest better reflects growth over longer periods.
Are partial periods supported?
Yes, you can enter fractional time values (e.g., 1.5 years or 45 days) and the calculator will convert these appropriately to compute interest.
Is the calculator suitable for all currencies?
The calculator uses dollar signs for display but the calculations apply to any currency as long as the principal and fees are entered in the same currency.
How simple interest is computed
Simple interest grows linearly with time and does not compound. The accumulated balance is , where is the principal, is the annual interest rate expressed as a decimal, and is the time in years. The interest earned is simply . Because the formula is linear, each additional day of holding contributes the same dollar amount as every other day.
This tool converts months into fractional years by dividing by 12 and days by 365 so you can mix billing cycles and due dates. The optional fee field subtracts upfront costs from the net payout, highlighting how origination charges reduce the effective return for investors or raise the cost for borrowers.
Example schedules
| Scenario | Principal | Rate | Time | Interest | Maturity total |
|---|---|---|---|---|---|
| Short-term equipment loan | $4,500 | 6.5% | 18 months | $438.75 | $4,938.75 |
| 90-day treasury bill | $10,000 | 4.2% | 90 days | $103.29 | $10,103.29 |
| One-year bridge loan with fee | $250,000 | 9.0% | 12 months | $22,500.00 | $272,500.00 |
Compare with other money tools
After running the simple interest projection, explore how compounding changes the outcome using the Compound Interest Calculator, evaluate structured payments with the Loan Payment Calculator, and set long-term targets via the Savings Goal Calculator.
Interest Dash Mini-Game
Catch linear growth in motion—ride simple interest drops, dodge surprise fees, and finish the run with the healthiest balance you can.
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Tip: Simple interest grows linearly—time and rate stretch the same slope, so steady catches beat risky spikes.