$1,800.00
$11,800.00
$50.00
$1,800.00
$11,800.00
$50.00
Simple interest is the most straightforward method of calculating interest on a principal amount. Unlike compound interest, where interest is earned on previously accumulated interest, simple interest is calculated only on the original principal — making it grow linearly over time rather than exponentially.
Simple interest is used in a variety of practical financial contexts: short-term personal loans, car loans, some types of bonds (particularly US Treasury bills and notes, which use an act/365 convention), consumer installment loans, and some savings certificates. It is also the basis for understanding more complex interest calculations and forms the foundation of financial mathematics education.
The calculation is intuitive: if you borrow $10,000 at 6% simple interest for 3 years, you pay $1,800 in interest ($10,000 × 6% × 3) for a total repayment of $11,800. There are no hidden complexities — the rate applies uniformly to the principal for each period of time.
In comparison to compound interest, simple interest always produces less growth on investments (or less cost on debt) for the same nominal rate over the same period. This makes it preferable from a borrower's perspective but less desirable for savers. Our Simple Interest Calculator computes interest amount, total repayment, and monthly interest for any principal, rate, and time.
The simple interest formula is:
$$I = P \times r \times t$$
where $$P$$ is the principal, $$r$$ is the annual interest rate (as a decimal), and $$t$$ is the time in years.
The total amount (principal + interest) is:
$$A = P + I = P(1 + r \times t)$$
The monthly interest (assuming the rate is annual):
$$I_{\text{monthly}} = P \times \frac{r}{12}$$
For example, a $10,000 loan at 6% simple interest for 3 years:
$$I = 10{,}000 \times 0.06 \times 3 = \$1{,}800$$
$$A = 10{,}000 + 1{,}800 = \$11{,}800$$
$$I_{\text{monthly}} = 10{,}000 \times 0.06/12 = \$50/\text{month}$$
Note that in a true simple interest scenario, the monthly interest remains constant throughout the loan ($50/month), unlike an amortizing loan where the interest portion decreases as the outstanding balance decreases.
The Simple Interest is the total cost of borrowing (or earnings from lending) over the entire period. The Monthly Interest is useful for understanding the periodic interest accrual. For borrowers, simple interest is generally more favorable than compound interest at the same nominal rate — the difference grows with time and interest rate. For investors, simple interest products (some savings bonds, bills) are less powerful than compound interest products at comparable rates. When comparing financial products, always confirm whether the rate is simple or compound interest.
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A $5,000 personal loan at 8% simple interest for 18 months (1.5 years) accrues $600 in interest for a total repayment of $5,600.
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A $25,000 bond paying 5% simple annual interest over 2 years earns $2,500 in interest, for a total redemption value of $27,500.
Simple interest is calculated only on the original principal (I = P × r × t). Compound interest is calculated on the principal plus all accumulated interest [A = P(1 + r/n)^(nt)]. Simple interest grows linearly; compound interest grows exponentially. For $10,000 at 6% over 10 years: simple interest = $6,000 total interest, compound (monthly) = $8,194 total interest — 37% more from compounding alone.
Common simple interest loan types: (1) Auto loans — most US car loans use simple interest with daily accrual, meaning extra payments reduce the principal directly and save total interest, (2) Short-term personal loans, (3) Payday loans (often expressed as simple interest but at extreme annual rates), (4) Some installment loans. Note: most mortgages use amortization schedules (which achieve a similar effect to compound interest), not pure simple interest.
The Rule of 72 is designed for compound interest. For simple interest, the doubling formula is simply $$t = \frac{100}{r}$$ (where r is the percentage rate). At 6% simple interest, money doubles in 100/6 = 16.67 years. At 6% compound interest (annual), it doubles in 72/6 = 12 years. The 4.67-year difference shows why compound interest builds wealth faster.
US Treasury bills (T-bills) are issued at a discount and redeemed at face value. They use an actual/360 or actual/365 day count convention (depending on the specific instrument). The discount rate and investment yield are both simple interest concepts. For example, a 91-day T-bill with a 5% discount rate: discount = face × 0.05 × 91/360. The investment yield (equivalent simple annual rate) is slightly different from the discount rate and is used for comparison with other investments.
Simple and compound interest are equal at exactly one period (t=1 year for annual compounding). Before t=1, compound interest accumulates more slowly than simple interest (the curve starts below the line). After t=1, compound interest overtakes and increasingly exceeds simple interest. At very short time periods, the difference is negligible; over decades, it is enormous. This is why the time dimension is so critical in long-term financial planning.
Yes — all else being equal. With simple interest, the interest cost scales linearly with time and does not accelerate due to interest-on-interest. On a $10,000 loan at 6% for 10 years: simple interest = $6,000 total; compound interest (monthly) = $8,194 total — 37% more in interest payments. However, borrowers should be cautious: many loans described with a simple interest rate actually use compound interest calculation methods, or charge daily compounding. Always confirm the exact calculation method, not just the quoted rate.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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