Simple Interest Calculator

When simple interest applies and when it doesn't

Simple interest is interest worked out on the original principal only, never on interest already added. The formula is I = P × r × t: principal, times the annual rate as a decimal, times the years. Borrow 5,000 at 6% simple interest for 3 years and the interest is 5,000 × 0.06 × 3 = 900, so 5,900 is repaid in total, which is the figure this calculator returns, with the 900 shown separately. That is the easy part. The complication is that most modern financial products do not use simple interest; they compound, and knowing which one applies changes the total cost, sometimes by a lot.

Where you'll actually encounter simple interest

Some short-term personal loans quote repayment as principal plus a flat charge, which is simple interest under another name. Bridging loans and some development finance work the same way, with interest accruing on the amount drawn rather than rolling up monthly. Discount instruments such as treasury bills are priced so that the interest is the gap between what is paid and the face value received at maturity, again without compounding. Some bonds accrue interest annually but pay it in one sum at the end. And informal loans between people are almost always simple interest, because nobody sits down to compound them. In everyday finance it commonly turns up on loans of under a year, some pawn and short-term credit products, and money lent within families.

Why compound usually costs more

Same 5,000, same 6%, same 3 years, but compounded monthly: the total repaid is 5,983, against 5,900 simple, so compounding adds 83 over three years. Extend the term to 10 years and the gap opens: 8,000 simple against 9,097 compounded monthly, a difference of 1,097. Even compounding once a year, which is gentler, reaches 8,954. For long-term debt the compounding structure moves the total more than the small differences in headline rate that tend to get the attention.

Why compound usually grows more

Savings work the same way in the other direction. Put 5,000 at 6% simple interest for 10 years and it earns 3,000. Compounded monthly it earns 4,097, so the saver is 1,097 better off on the same pot over the same decade. Under two years the difference is small; over long horizons it becomes the dominant factor, which is why long-term savings and investment products generally compound while some short-term products still use simple interest.

Reading loan documents to spot which you have

The words to notice are flat rate and APR. A flat rate is simple interest on the original amount for the whole term, so a 3% flat rate over 3 years is 3% × 3 = 9% of the loan in total interest. Because the balance is being repaid every month, that same loan expressed as an annual percentage rate works out at about 5.7%, nearly double the flat figure, which is why flat-rate quotes make a loan look cheaper than it is. In the United States, Regulation Z defines the APR as a measure of the cost of credit, expressed as a yearly rate, that relates the amount and timing of value received to the amount and timing of payments made. The EU's consumer credit rules use the annual percentage rate of charge, described as the total cost of the credit, for the same job. Two loans compared on APR are being compared on the same footing; a flat rate against an APR is not a comparison at all.

The Rule of 78 trap

Some older instalment loans allocate interest using the Rule of 78, also called the sum-of-the-digits method. It is a simple-interest variant that front-loads the interest, so an early payoff saves far less than a straight-line split would suggest. On a 24-month loan settled after month 12, the digits 24 down to 13 add to 222 out of a total of 300, so 74% of the interest has already been charged, not 50%. In the United States, 15 U.S.C. § 1615 requires interest refunds on precomputed consumer credit running longer than 61 months to use a method at least as favourable as the actuarial method, which rules the method out for longer loans there, but it still appears on shorter terms and in other markets. An early-settlement figure that is much worse than expected is often this rule at work.

When simple interest favours the borrower

Simple interest is sometimes the better deal for the borrower. On a loan paid off in full within a year or two, compounding has had little time to build, and the rates quoted on some simple-interest products sit below the equivalent APR on compounding products. Bridging loans are the clearest case: with interest accruing on the amount drawn, a loan that is repaid earlier than planned costs proportionately less, although the arrangement fees on bridging finance usually dwarf the interest structure either way.

The tax difference

Tax treatment of interest varies by jurisdiction, and the difference that matters here is timing. Some tax systems treat interest as income when it accrues, others only when it is paid. A simple-interest instrument that pays everything at maturity can create a mismatch under the first kind of system, with a tax charge arriving in years when no cash has. Savings accounts that credit interest regularly tend to keep cash and tax in step, which is one reason they dominate consumer products. Local rules decide which applies.

When this calculator applies

This calculator fits a flat-rate personal loan quote, an informal loan between individuals, a bridging or development finance product, or a quick check on whether an advertised rate is being applied as stated. It does not fit savings accounts, mortgages, credit cards or long-term investments, all of which compound or amortise; for those, the compound interest calculator is the right model and this one understates or overstates the answer.

What this tool doesn't model

The figure here is the pure mathematical interest. Real products add arrangement fees, late-payment charges and early-repayment penalties on top, and none of those appear in the result. The rate is held constant for the whole term, there is no compounding, and no allowance is made for inflation or tax. Comparing loans on their APR, rather than on headline rates or raw interest figures, is what puts fees and timing back into the picture.