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Interest equals principal times the annual rate divided by 100 times the time in years: I = P × r × t. The total amount is principal plus interest, and the monthly equivalent is the total divided by the number of months (years × 12). Because interest is never added to the principal, each period earns the same amount. Results are estimates for planning, not professional financial advice.
Simple interest is calculated only on the original principal, never on previously earned interest. The formula is: I = P × r × t, where I is the interest, P is the principal, r is the annual rate as a decimal (rate percent divided by 100), and t is the time in years. You will also see the compact form I = Prt, where P is principal, r is the annual rate as a decimal, and t is time in years. The total amount is A = P + I.
Because the rate is annual and the time is in years, shorter periods become fractions of a year: 6 months is 6/12 = 0.5 years, 90 days is 90/365 = 0.2466 years, and 3 weeks is 3/52 = 0.0577 years. The interest earned in each period is identical, so the balance grows in a straight line rather than a curve. That predictability is exactly why lenders use simple interest for short-term loans and why schools teach it as the first finance formula.
Enter the principal amount (the loan you take or the money you invest), the annual interest rate as a percentage, and the time in years (fractions are fine, 6 months is 0.5 years). Choose your currency from the list.
The calculator shows the total interest, the total amount (principal plus interest), and the monthly equivalent, which is the total divided by the number of months. A chart splits the total into your principal versus the interest it earned or cost. Results are estimates for planning, not professional financial advice.
Suppose you invest $10,000 at 5% per year for 3 years. These examples use round numbers so you can check the arithmetic by hand.
Interest = 10,000 × 5 / 100 × 3 = $1,500. Total = $10,000 + $1,500 = $11,500. Monthly equivalent = $11,500 / 36 = $319.44.
A shorter example: $20,000 in a 6-month fixed deposit at 6% per year. Time = 6/12 = 0.5 years. Interest = 20,000 × 6 / 100 × 0.5 = $600. At maturity you receive $20,600. The same math works from the borrower side: $5,000 at 10% for 3 years costs $1,500 in interest for a total repayment of $6,500.
Simple interest is always calculated on the original principal, so the interest amount is the same every period. Compound interest is calculated on the principal plus all previously accumulated interest, so the interest grows every period.
| Feature | Simple interest | Compound interest | |---|---|---| | Calculated on | Original principal only | Principal + accumulated interest | | Growth pattern | Linear, straight line | Exponential, curve | | Interest per period | Always the same | Increases every period | | Formula | I = P × r × t | A = P × (1 + r/n)⁽ⁿ × t) | | Better for borrowers | Yes, lower total cost | No, higher total cost | | Better for investors | No, lower returns | Yes, higher returns |
For example, $10,000 at 8% for 3 years gives $2,400 in simple interest, but about $2,597 with annual compounding. The gap widens dramatically over 10, 20 or 30 years, which is why compound interest powers long-term investing while simple interest suits short, predictable borrowing.
Simple interest shows up wherever the cost of money must be transparent and fixed: short-term personal loans, car and vehicle loans, fixed deposits and certificates of deposit held for a defined term, treasury bills, promissory notes, education loans, and informal loans between family or friends.
It is also the standard method taught in school mathematics from Class 7 upward, so students use it for homework and exam problems. Some savings accounts and savings certificates also apply simple interest for short fixed terms. Most mortgages and credit cards instead use compound interest or amortization schedules, where the outstanding balance changes with each payment, so always check your loan agreement before assuming a flat simple-interest quote.
Mixing up the rate period. A quote of 1% per month is 12% per year, not 1%. Always convert the rate to an annual figure before using I = P × r × t, and convert the time to years to match.
Using the rate as a whole number instead of a decimal. Five percent means r = 5/100 = 0.05, not 5. Forgetting the divide-by-100 step inflates the answer a hundredfold.
Forgetting that real loans add extras. Banks may add processing fees, use compound interest, or apply different day-count conventions (360 vs 365 days), so treat the calculator result as the pure interest figure and confirm the all-in cost with the lender.
Assuming the rate never changes. Simple interest assumes one constant rate for the whole term. If the rate changes mid-term, split the calculation into segments at each rate and add the results.
Multiply the principal by the annual rate (as a decimal) by the time in years: I = P × r × t. For example, $10,000 at 5% for 3 years gives 10,000 × 0.05 × 3 = $1,500 of interest, and the total is $11,500. For periods under a year, convert to a fraction of a year first, for example 6 months is 0.5 years.
Simple interest is charged only on the original principal, so the interest is identical every period and the balance grows in a straight line. Compound interest is charged on the principal plus accumulated interest, so it grows exponentially. Over long periods compound interest is much larger, for example $10,000 at 8% for 3 years gives $2,400 simple versus about $2,597 compounded annually.
Yes. Convert the period to a fraction of a year: divide months by 12 and days by 365. For example, $2,000 at 6% for 9 months gives 2,000 × 0.06 × 9/12 = $90 of interest. The monthly equivalent shown by this calculator is the total amount divided by the number of months, which is useful for budgeting.
Personal loans, car loans, short-term fixed deposits, treasury bills, promissory notes, education loans, and informal lending between family or friends. It is also the standard method in school mathematics. Most mortgages and credit cards use compound interest or amortization instead, so check your loan agreement.
For pure simple interest it is exact, but real bank loans often differ: many use compound interest, add processing fees, or use different day-count conventions such as 360 versus 365 days. Treat the result as the clean interest figure and always confirm the all-in cost with your lender before signing.
Simple interest assumes one constant rate for the whole term. If the rate changes, split the timeline into segments: calculate the interest for each segment at its own rate, then add the segment interests together. For example, 6% in year one and 8% in year two means computing each year's interest separately and summing them.
Most home loans use compound interest or an amortization schedule, not pure simple interest, because the outstanding balance falls with each monthly payment. Some short-term bridge loans, land loans and certain government housing schemes do use simple interest. Read the loan agreement: fixed monthly payments against a shrinking balance mean a reducing-balance method.
Divide by the number of months in a year: months / 12. Six months is 0.5 years, 9 months is 0.75 years. For days, use days / 365, and for weeks use weeks / 52. The rate must always be annual to match, so a monthly quoted rate should be multiplied by 12 first.
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