Simple Interest Calculator
Fast, accurate and free online interest prostych calculator tool running directly in your browser.
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What is simple interest?
Simple interest is a method of calculating interest in which profit is calculated only on the initial capital. The earned interest is not added to the capital in order to calculate further interest (no so-called compound interest).
Simple interest formula:
- I – amount of accrued interest (Interest).
- P – initial capital (Principal).
- r – annual interest rate expressed as a decimal fraction (rate).
- t – investment/loan duration in years (time).
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The simple interest calculator is used to quickly calculate interest calculated linearly, i.e. without capitalization. In practice, this means that interest does not increase the capital during the period, and the total amount due increases in proportion to the time and interest rate. The tool allows you to enter the starting amount, the annual rate and the time in days, months or years, and then immediately gives you the interest rate and the final amount to be paid or withdrawn. Thanks to flexible time units, you can use them in personal finance, civil contracts, simple loans, commercial settlements and for educational comparisons with compound interest.
Formulas and theory
Definition of simple interest
Simple interest is generated by linear calculation of remuneration from the initial capital at a fixed interest rate. There is no capitalization, so the amount of interest in a given period depends only on the initial capital, rate and time, and not on the growing base. This model is sometimes used in short-term settlements and in education, where transparency and ease of interpretation of the result are important.
Basic formula for simple interest
I = P · r · t
- P– initial capital,
- r– annual interest rate in decimal terms, e.g. 7% is 0.07,
- t– time in years, e.g. 6 months is 0.5 year, 90 days is 90/365 year.
Final Amount
A = P + I = P · (1 + r · t)
Interest increases linearly with time, so doubling the time doubles the interest, keeping the rate constant.
Day counting conventions
Settlements use various conventions to convert time into fractions of a year. The most common ones areActual/365(the number of actual days in the numerator and 365 in the denominator),Actual/360and30/360. The influence of conventions is greater at high rates and short periods. The calculator allows you to choose a day convention so that the result meets contract requirements or an industry standard.
How to use the calculator
- Enterstarting capital Pin PLN.
- Enter theannual interest rate rin percent or per mille, converting to a decimal if necessary.
- Selecttime unitand enter the period: days, months or years.
- Optionally selectday convention(Actual/365, Actual/360, 30/360) when using days.
- ClickCalculateto seeinterest AND i final amount AND.
The result is calculated immediately and shows both the interest and the total due. Additionally, you can compare the impact of different day conventions and check how the result changes after changing the rate or time.
Examples
Example 1. Loan for 90 days
Data:P = PLN 10,000, r = 8% per annum, t = 90 days, Actual/365 convention.
Calculations:t = 90/365 = 0.2466, I = 10,000 · 0.08 · 0.2466 ≈ PLN 197.28, A ≈ PLN 10,197.28.
At the same rate, interest is proportional to the 90-day portion of the year.
Example 2. Monthly trade credit
Data:P = PLN 25,000, r = 12% per year, t = 1 month ≈ 1/12 of the year.
Interest:I = 25,000 · 0.12 · (1/12) = PLN 250,A = PLN 25,250.
The simple model well reflects quick trade settlements and contractual penalties calculated without capitalization.
Example 3. Short period according to 30/360
Data:P = PLN 50,000, r = 7%, t = 20 days, 30/360 convention.
Calculations:t = 20/360 = 0.0556, I = 50,000 · 0.07 · 0.0556 ≈ PLN 194.44.
Changing the day convention slightly shifts the result compared to Actual/365, which is important in corporate contracts.
Example 4. Half a year without capitalization
Data:P = PLN 12,000, r = 6%, t = 0.5 year.
Interest:I = 12,000 · 0.06 · 0.5 = PLN 360,A = PLN 12,360.
Doubling the time at a constant rate will double the amount of interest because the formula is linear in time.
Auxiliary tables
| Input | Unit | Converter to years | Notes |
|---|---|---|---|
| Days (Actual/365) | days | t = days/365 | Most common in personal finance |
| Days (Actual/360) | days | t = days/360 | Found in banking and corporate settlements |
| Days (30/360) | days | t = days/360 | Assumption 30 days a month, 360 days a year |
| Months | months | t = m/12 | Simple conversion to the calendar year |
| Years | years | t = years | Direct substitution to the formula |
Typical applications
Commercial settlements and contractual penalties
Interest for late payment, calculation of simple interest on unpaid invoices and quick comparisons of the cost of delay between contractors.
Education and Comparison
Demonstrate the difference between simple accrual and compound interest, test time and rate sensitivity, and prepare for more advanced calculations.
Frequently Asked Questions
What is the difference between simple interest and compound interest?
In simple interest, the basis remains constant because the interest is not added to the principal. In compound interest, the interest increases the base and subsequent periods are counted from the increasing amount.
How to choose the correct day convention?
Follow the contract or industry standard. Actual/365 works well for personal finances and simple settlements. In banking, there are also Actual/360 and 30/360.
What if the rate is on a monthly basis?
Convert it to an annual or adjust the time. For example, 1% per month for 3 months is r = 0.12 per year and t = 0.25 years, or r = 0.01 and t = 3 months converted into years.
Does the calculator round?
The results are presented with two decimal places for monetary amounts, which facilitates practical settlements. Internal accuracy remains higher.
Can I calculate statutory interest for delay?
Yes, enter the principal, the number of days late and the appropriate annual statutory interest rate, then select the appropriate day convention according to your billing practice.
How to use – step by step
- Enter the starting capitalPin PLN.
- Enter the annual rateras a percentage, e.g. 9 means 9%.
- Specify the timetin days, months or years, and select the day convention if you are working with days.
- ClickCalculateto see the interestIand the final amountA.
- Change parameters if necessary and compare alternative cost scenarios.
Summary
The simple interest calculator simplifies settlements because it uses a linear accrual model and clear time conversions. If you need a quick answer about late payment interest, short-term loans or educational comparisons, three inputs and one click are all it takes. Set the principal, rate and time, then select the day convention and pressCalculateto instantly know the interest rate and final amount.