Simple Interest Calculator
Calculate simple (non-compounding) interest on a principal over time.
The Simple Interest Calculator computes interest charged only on the original principal, with no compounding. It is the standard method for short-term loans, some bonds and promissory notes, many auto finance agreements, and most classroom problems, and it remains the clearest starting point for understanding how interest works at all. Because the interest amount is the same in every period, the arithmetic is transparent: multiply principal by rate by time. This page shows the formula, defines each variable, and explains exactly when simple interest applies and when using it would understate the real cost of borrowing.
How to use this calculator
- Enter the principal (starting amount).
- Enter the annual rate and the time in years.
- The result shows interest separately from the total.
The formula
I = P × r × t Total = P + I- I
- Interest earned or owed
- P
- Principal
- r
- Annual rate as a decimal
- t
- Time in years
How it works
Simple interest is charged only on the original principal — interest does not earn interest. Growth is linear: the same dollar amount of interest accrues every year.
This makes it easy to compute by hand, but over long periods it grows far more slowly than compound interest.
Worked example
$5,000 at 4% simple interest for 3 years.
- 1I = 5,000 × 0.04 × 3
- 2I = 600
Interest = $600; total = $5,600. (Compounded annually it would be ≈ $5,624.32.)
When simple interest is the right model
Simple interest fits arrangements where interest is settled each period rather than added to the balance: short-term commercial paper, many personal IOUs, some fixed-term deposits that pay interest out, and loans of under a year.
It is the wrong model for credit cards, savings accounts that retain interest, and long-term investments, all of which compound. Using simple interest there will understate the total substantially over multi-year horizons.
Time units and part-years
Rate and time must use the same unit. An annual rate needs time in years, so a nine-month loan is entered as 0.75 years. Mixing a monthly rate with a term in years is the most common error and inflates the result twelvefold.
For day-count conventions, financial contracts differ: some use 360-day years, others 365. For rough estimates the difference is negligible; for contractual amounts, follow the convention written into the agreement.
Comparing with compound interest
Over one period, simple and compound interest give the same result. The gap opens from the second period onward and widens as the horizon extends.
If you are choosing between offers quoted on different bases, convert both to a total amount repaid before comparing, rather than comparing headline rates.
Where simple interest is actually used
Simple interest charges on the original principal only, never on accumulated interest. It is the convention for many short-term loans, car finance in some markets, bonds paying periodic coupons, and statutory interest on late payments.
Court awards, tax refunds, and some government-set rates are calculated on a simple basis because it is transparent and easy to audit.
Anything held for more than a year that reinvests its earnings is compound, not simple — using the wrong model then understates growth substantially.
Day-count conventions matter
For part-year periods, the answer depends on how days are counted. Common conventions are actual/365, actual/360, and 30/360, and they give slightly different results on the same loan.
Actual/360, common in commercial lending, produces a marginally higher charge because it divides by fewer days while counting the real ones.
If you are checking a lender's figure and it differs by a small margin, the day-count convention is the usual explanation.
Simple versus compound over time
Over a single year at the same rate, simple and annually compounded interest are identical. The gap opens only when interest is left to earn interest.
Ten thousand at 5% for 20 years earns 10,000 simple but 16,533 compound. Over short periods the choice barely matters; over long ones it dominates.
As a borrower, prefer simple interest. As a saver, prefer compound.
Common mistakes
Mixing the units of rate and time. An annual rate needs a time in years; using months with an annual rate overstates the charge twelvefold.
Applying simple interest to a savings account that in fact compounds, which understates the balance.
Forgetting that repayments reduce the principal. On an amortising loan, simple interest on the original balance overstates the true cost.
Worked examples
Borrowing 4,000 at 7% for 18 months: 4,000 x 0.07 x 1.5 = 420 interest, so 4,420 repayable.
A 90-day invoice at 8% statutory interest on 12,500 using actual/365: 12,500 x 0.08 x 90/365 = 246.58.
A bond with a 5,000 face value and a 4% coupon pays 200 a year in simple interest regardless of how long you hold it.
Glossary and verification
Principal: the sum on which interest is charged. Rate: the periodic percentage. Time: the period in the same unit as the rate. Accrued interest: interest earned but not yet paid.
Verify by dividing the interest by the principal and the time — you should recover the rate exactly.
For part-years, verify the fraction separately: 90 days on a 365-day basis is 0.2466 of a year.
Where simple interest is still used
Simple interest is standard on many short-term instruments: treasury bills, certificates with interest paid out rather than reinvested, car finance in some markets, and statutory late-payment interest on unpaid invoices.
It also governs court-awarded interest in many jurisdictions and the interest element of some personal loans between individuals, where compounding would be considered unfair.
Because the interest never earns interest, the total cost rises in a straight line with time — predictable, easy to audit, and easy to verify by hand.
Simple versus compound over time
Over a single year at the same rate, simple and annual compound interest are identical. The gap opens with time and grows with the rate.
At 8% over ten years, 10,000 earns 8,000 in simple interest but 11,589 compounded annually. Over thirty years the compound figure is more than three times the simple one.
When comparing two offers, confirm which basis each uses before comparing the headline rate — the basis often matters more than a percentage point of difference.
Partial periods and day-count conventions
For periods shorter than a year, interest is prorated by days. Which day count applies changes the answer: actual/365, actual/360, and 30/360 are all in common use.
Actual/360, common in commercial lending, produces slightly more interest than actual/365 for the same nominal rate, because the year is treated as shorter.
Check whether both the start and end dates are counted. A one-day difference is trivial on a small balance and material on a large one.
Verification and common errors
The formula is principal times rate times time. Verify by dividing the interest by the principal and the time; the result should return the rate.
The most common error is a rate and time mismatch — a monthly rate with a term in years, or an annual rate with a term in months. Convert both to the same unit first.
The second most common is entering the rate as 7 rather than 0.07 in a manual calculation, which inflates the result a hundredfold.
Where simple interest still applies
Short-term commercial paper, some personal loans, many bonds' coupon calculations, and statutory interest on late payments all use simple interest rather than compounding.
Car finance and point-of-sale credit are often quoted as flat rates, which is simple interest on the original balance even though you repay progressively.
A flat rate therefore understates the true cost badly: a five percent flat rate over four years is close to a nine percent effective annual rate.
Whenever a rate is described as flat, convert it to an effective rate before comparing it with a conventional loan.
Rearranging the formula
The relationship between interest, principal, rate, and time has four variables, so knowing any three gives the fourth by simple rearrangement.
To find the rate, divide the interest by the product of principal and time; to find the time, divide by principal and rate instead.
Keep rate and time in matching units — an annual rate needs time in years, and a monthly rate needs months.
Converting a daily rate to annual by multiplying by 365 is valid for simple interest, but not for compounding, where the factor is exponential.
Comparing simple with compound
Over a single period the two are identical; the gap opens from the second period onward and widens with the rate and the term.
For short horizons of a few months, treating a compound product as simple introduces very little error and makes mental checks easy.
For borrowers, simple interest is preferable at the same nominal rate; for savers, compounding is.
When a product statement does not say which method applies, the presence of an APR or AER figure usually indicates compounding.
When this calculator is useful
- Short-term personal loans between individuals
- Some auto and consumer financing quotes
- Bonds that pay interest without reinvesting
- Teaching the difference between simple and compound growth
Frequently asked questions
When is simple interest used in real life?
Some short-term loans, certain auto financing structures, and coupon payments on bonds. Most bank accounts and credit products compound instead.
How do I enter 18 months?
As 1.5 years. Six months is 0.5, one month is about 0.0833.
What is the simple interest formula?
Interest equals principal multiplied by the annual rate multiplied by time in years. Total repaid is principal plus that interest.
When is simple interest used in practice?
Short-term loans, many auto finance contracts, promissory notes, and some bonds that pay coupons out rather than reinvesting them.
How do I enter a term in months?
Divide by twelve. Six months is 0.5 years, eighteen months is 1.5 years, provided the rate is annual.
Is simple interest cheaper for borrowers?
For the same nominal rate and term, yes — nothing is charged on accumulated interest. That is why long-term credit is almost always written on a compounding basis.
Can the rate be entered as a decimal?
Enter it as a percentage, for example 7.5 rather than 0.075. The calculator converts internally.
Does the tool store my figures?
No. Everything is computed locally in your browser and nothing is sent to a server.
When is simple interest used instead of compound?
Short-term loans, bond coupons, statutory late-payment interest, and many auto loans use a simple basis.
Is simple interest always cheaper for a borrower?
At the same rate and term, yes, because interest never accrues on interest.
How do I handle a period of months?
Convert to years by dividing by 12, or use a monthly rate consistently. Never mix an annual rate with a month count.
What is a day-count convention?
The rule for turning days into a fraction of a year, such as actual/365 or 30/360. It slightly changes part-year interest.
Does it matter for a one-year period?
No. Simple and annually compounded interest give the same answer over exactly one year.
Is simple interest ever better for a borrower?
Yes. For the same rate and term beyond a year, simple interest always costs less than compound interest.
How do I calculate interest for part of a year?
Prorate by days using the applicable day-count convention, commonly actual/365 or actual/360.
Why does actual/360 cost more than actual/365?
It treats the year as 360 days, so each actual day carries a slightly larger share of the annual rate.
Does simple interest apply to credit cards?
No. Card balances compound, usually daily, which is why carried balances grow faster than a simple calculation suggests.
What is a flat rate?
Simple interest charged on the original balance for the whole term, which costs roughly twice its headline figure in effective terms.
How do I find the rate from the interest?
Divide the interest by the principal multiplied by the time, keeping both in matching units.
When do simple and compound interest agree?
Over one compounding period they are identical; they diverge from the second period onward.