Formula
Simple interest = principal × annual rate × time. Use the annual rate as a decimal, so 6% becomes 0.06. Total amount = principal + simple interest + optional fixed fees.
Finance
Simple Interest Calculator
Calculate simple interest, total repayment and effective yearly interest from principal, rate and time.
Calculator
Working calculator
Live result180.00 simple interest1,180.00 total including 0.00 fixed fees · 6.00% yearly cost with fees
Formula used
Simple interest = principal × annual rate × time. Use the annual rate as a decimal, so 6% becomes 0.06. Total amount = principal + simple interest + optional fixed fees.
This is the method behind the answer, so the result can be checked rather than simply trusted.What-if check
Rate and time sensitivity
Simple interest moves in a straight line: double the rate or double the time and the interest doubles, as long as the principal stays fixed.
| Annual rate | Interest | Total with fees |
|---|---|---|
| 4.00% | 120.00 | 1,120.00 |
| 6.00% | 180.00 | 1,180.00 · current |
| 8.00% | 240.00 | 1,240.00 |
| Time | Interest | Total with fees |
|---|---|---|
| 2.00 years | 120.00 | 1,120.00 |
| 3.00 years | 180.00 | 1,180.00 · current |
| 4.00 years | 240.00 | 1,240.00 |
Visual proof
Fixed principal, separate interest
Interest is 180.00 because 1,000.00 × 0.0600 × 3.00 years. Total with fees is 1,180.00.
Printable calculation report
Result: 180.00 simple interest. Assumption: Interest is calculated only on the original principal, not on accumulated interest.
- Principal amount
- 1000
- Annual simple interest rate
- 6
- Time
- 3
- Extra fees or charges
- 0
- Page/date context
- 2026-05-16 UTC page version
- Page URL
- https://calculationtime.com/calculators/simple-interest-calculator
Worked example
Principal 1,000 × 0.06 annual rate × 3 years = 180 simple interest. Add the principal back: 1,000 + 180 = 1,180 before any optional fixed fees.
Professional note
Master’s Tip: confirm whether the real agreement uses simple interest, compound interest, daily interest, fees, penalties or an annual percentage rate disclosure. A simple-interest check is useful because it makes the base calculation visible, but many loans and investments use more than this one formula.
Regional and unit assumptions
Standard or basis: transparent simple-interest arithmetic using years as the time unit. Currency symbols, taxes, compounding, APR/APY disclosure rules, day-count conventions and lender-specific fees are not assumed.
Assumptions and limitations
- Interest is calculated only on the original principal, not on accumulated interest.
- The annual rate is entered as a percentage and converted to a decimal inside the formula.
- Time is entered in years; partial periods can be entered as decimals when that matches the agreement being checked.
- Fees are kept separate from interest so the printed report does not hide charges inside the rate.
- This is arithmetic for planning and checking. It is not financial, tax, lending or legal advice.
Methodology & Accuracy
How this calculator is checked
CalculationTime pages are built around visible arithmetic: the formula, assumptions, worked example and practical limitations are shown so the result can be checked rather than simply trusted.
Formula used
Simple interest = principal × annual rate × time. Use the annual rate as a decimal, so 6% becomes 0.06. Total amount = principal + simple interest + optional fixed fees.
Standard or basis
Standard or basis: transparent simple-interest arithmetic using years as the time unit. Currency symbols, taxes, compounding, APR/APY disclosure rules, day-count conventions and lender-specific fees are not assumed.
Where a calculator follows a named legal, trade or industry standard, that standard is cited visibly. Otherwise the page uses transparent general arithmetic and states its limits.Master's Tip
Master’s Tip: confirm whether the real agreement uses simple interest, compound interest, daily interest, fees, penalties or an annual percentage rate disclosure. A simple-interest check is useful because it makes the base calculation visible, but many loans and investments use more than this one formula.
Related calculators
Questions
How do you calculate simple interest?
Multiply the principal by the annual rate written as a decimal, then multiply by the time in years.
What is the difference between simple and compound interest?
Simple interest is calculated on the original principal only. Compound interest adds interest to the balance, so later interest can be earned or charged on earlier interest.
Can I use months in a simple interest calculation?
Yes, if you convert months to years first. For example, 6 months is 0.5 years and 18 months is 1.5 years.
Do fees count as simple interest?
No. Fees are separate charges. This calculator can add optional fixed fees to the total, but it keeps them separate from the interest formula.
Is this suitable for an official loan quote?
Use it as an arithmetic check only. Official borrowing costs can depend on compounding, APR rules, fees, penalties, tax and local disclosure law.
Calculation note
Simple interest is one of the clearest finance formulas because the interest base stays fixed. That makes it useful for classroom examples, quick loan checks, invoice late-fee notes and comparing simple agreements before more complex compounding or APR rules enter the discussion.
The principal stays fixed
The defining feature of simple interest is that interest is measured against the original principal throughout the period. If the principal is 1,000, the formula keeps using 1,000 as the base rather than growing the base each year.
Time must use the same unit as the rate
An annual rate needs time expressed in years. Six months can be entered as 0.5 years and three months as 0.25 years when the agreement uses ordinary proportional simple-interest timing.
Simple interest is not APR or compounding
Real finance documents may quote APR, APY, daily interest, monthly compounding, establishment fees, late charges or tax effects. This page keeps simple interest visible and limited so it can be used as a transparent checking step, not as a complete loan disclosure.
Sources and further readingInvestor.gov: Compound InterestCFPB: What is APR?