Why use this calculator
Simple interest is what short-term loans, some car finance, many bonds and most informal lending between people actually use, and it behaves very differently from the compound interest that governs savings and mortgages. Because the interest is calculated only on the original amount, the cost is completely predictable and the arithmetic is something you can do in your head.
This calculator does that arithmetic and, more usefully, shows the same money under compound interest alongside it. Which is the comparison that tells you whether a quoted rate is generous or whether the structure is doing the work. That distinction matters most on flat-rate finance, where a modest-sounding percentage can cost close to double what the same number would under a reducing balance.
How to use it
- Enter the principal, which is the amount borrowed or invested at the outset and which never changes under simple interest. Enter the annual rate exactly as quoted.
- The time period accepts decimals, so enter 0.5 for six months or 1.25 for fifteen months.
- A common use is pricing a short-term loan between friends or working out what a bond coupon pays over a partial year.
- The comparison figure shows what the same money would have earned with monthly compounding, which is the fairer benchmark if you are choosing between products.
What each result means
- Total interest
- The full interest over the period, calculated only on the original principal.
- Total amount
- Principal plus interest: the full sum repaid by a borrower or received by a saver at the end.
- Interest per year
- The annual charge, which under simple interest is identical in every year because the rate always applies to the original principal.
- Interest per month
- The total spread evenly across the months, useful for budgeting a repayment.
- Same amount with monthly compounding
- What the identical principal, rate and term would produce if interest compounded monthly.
- Compounding advantage
- The gap between the two. It is small over short periods and grows dramatically as the term lengthens.
The formula
I = P × r × t
- I
- the interest
- P
- the principal
- r
- the annual rate expressed as a decimal
- t
- the time in years
The total repayable or receivable is P + I. Because the rate always applies to the original principal rather than a growing balance, the interest is the same in every period. Which is exactly what distinguishes it from compound interest, where A = P(1 + r/n)^(nt).
How this number is used in practice
The direction of benefit depends on which side you are on. For a borrower, simple interest is better: the debt does not accelerate. For a saver, compound interest is better for exactly the same reason. Any product advertising simple interest on savings is quietly offering you the worse structure.
Flat-rate car and personal finance is where this catches people. A quoted flat rate of six per cent on a loan repaid in instalments is not comparable to a six per cent reducing-balance rate, because you are paying interest on the full original amount even as you repay it.
The effective rate is often close to double the quoted one, which is why disclosure rules require an annual percentage rate alongside it.
For very short periods the two are nearly identical, which is why simple interest is standard for bridging finance, trade credit terms and short bond coupons. The difference only becomes material once compounding has had several periods to work.
Worked examples
$5,000 at 5% for 3 years
$750 of interest and a total of $5,750, against $808 if the same rate compounded monthly.
$10,000 at 8% for 1 year
$800 of interest and a total of $10,800. The compounding advantage over one year is only about $30.
$20,000 at 6% for 6 months
$600 of interest, because the half-year is entered as 0.5 and the rate applies pro rata.
When people use it
- Pricing a short-term loan between individuals
- Calculating a bond coupon or term deposit paid without compounding
- Checking a flat-rate finance quote against a reducing-balance equivalent
- Working out interest on an overdue invoice at a contractual rate
- Teaching the difference between simple and compound growth
Tips
- Simple interest favours the borrower and compound interest favours the saver. Check which structure a product is actually using.
- A flat rate on an instalment loan is not comparable to a reducing-balance rate; the effective cost is often close to double.
- Use decimals for part-years: 0.5 for six months, 0.25 for a quarter. The formula scales linearly with time.
- Over periods under a year, simple and compound interest differ by very little. The gap grows with the term.
- Always ask for the annual percentage rate when comparing finance offers, since it accounts for the repayment structure.
- Interest on an overdue invoice usually requires a contractual clause. Check your terms before charging it.
What it assumes, and where it stops being accurate
- Interest is calculated only on the original principal, never on accumulated interest.
- The rate is fixed for the entire period and no repayments reduce the principal along the way.
- Fees, charges and any capitalisation of unpaid interest are excluded.
- The comparison figure assumes monthly compounding, which is the most common savings convention.
- Tax on interest earned is not deducted.
Terms used on this page
- Principal
- The original amount borrowed or invested, which does not change under simple interest.
- Flat rate
- A rate applied to the original amount throughout the term, even as repayments reduce the balance.
- Reducing balance
- Interest charged only on the amount still outstanding, which falls with each repayment.
- Annual percentage rate
- A standardised figure showing the true annual cost of credit including the repayment structure.
Questions people ask
- What is the simple interest formula?
- I = P × r × t, where P is the principal, r is the annual rate as a decimal and t is time in years. So $5,000 at 5% for three years earns $5,000 × 0.05 × 3 = $750. The total repayable is the principal plus the interest.
- When is simple interest used instead of compound?
- Short-term loans, some car and personal finance quoted at a flat rate, many bond coupons, trade credit terms and informal lending between individuals. Savings accounts, mortgages and investments almost always compound, because compounding favours whoever is owed the money.
- Is simple interest better than compound interest?
- It depends which side you are on. As a borrower, simple interest is better because the debt does not accelerate. As a saver, compound interest is better for the same reason. A savings product advertising simple interest is offering you the structurally worse deal.
- How do I calculate interest for part of a year?
- Enter the time as a decimal fraction of a year: 0.5 for six months, 0.25 for three months, or days divided by 365 for an exact period. Because simple interest is linear in time, the result scales exactly with the fraction you enter.
- What is the difference between a flat rate and a reducing-balance rate?
- A flat rate charges interest on the full original amount for the whole term, even as you repay it. A reducing-balance rate charges only on what you still owe. A 6% flat rate on an instalment loan can cost close to what an 11% reducing-balance rate would, which is why disclosure rules require an annual percentage rate.
Use this with
Simple Interest Calculator answers one part of the question. These pick up where it stops: Compound Interest Calculator, Loan Repayment Calculator, Savings Goal Calculator and Investment Return Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- ASIC Moneysmart: loans and credit. Independent Australian guidance on borrowing costs
- Reserve Bank of Australia: indicator lending rates. Published lending and deposit rates
Simple Interest Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.