Simple Interest Calculator

Simple Interest Explained

The simple interest formula I = P × R × T is one of the most fundamental concepts in finance.^[1] It calculates interest only on the original principal, making it easy to predict total costs or earnings. While less common in modern banking (most accounts use compound interest), simple interest still applies to many loans and short-term instruments. Compare results with the Compound Interest Calculator to see the difference over time.

Simple vs Compound Interest: $10,000 at 5%

Simple Interest vs. Compound Interest

Simple interest charges a fixed percentage on the original principal only — it never compounds. If you borrow $10,000 at 5% simple interest for 3 years, you pay exactly $1,500 in interest ($10,000 × 0.05 × 3). With compound interest at the same rate compounded monthly, you would pay $1,614 — about 7.6% more. The difference grows dramatically over longer periods. Over 10 years, simple interest on $10,000 at 5% totals $5,000, while monthly compound interest reaches $6,470. Simple interest is straightforward and predictable, which is why it is used for short-term loans, auto loans (in some structures), and certain bond calculations where transparency matters.

Where Simple Interest Is Actually Used

Many consumer loans are marketed as "simple interest" loans, which means interest accrues daily on the outstanding principal balance. When you make a payment, a portion goes to accrued interest and the rest reduces principal. Paying early reduces total interest because the principal drops faster, reducing the daily interest charge. This is fundamentally different from precomputed interest loans, where the total interest is calculated upfront and added to the balance — early payments on precomputed loans do not save you money.

The Simple Interest Formula Explained

The formula I = P × r × t has three components: P (principal, the original amount), r (annual interest rate as a decimal), and t (time in years). For periods shorter than a year, convert to fractions: 6 months = 0.5, 90 days = 90/365 = 0.2466. A $5,000 loan at 8% for 90 days costs $5,000 × 0.08 × (90/365) = $98.63. This simplicity makes it easy to compare short-term borrowing costs across different products. For annualized comparisons, always convert to an annual rate — a "1% monthly" charge is actually 12% annual simple interest, not 1%.

Simple Interest and Loan Payoff Strategy

On simple interest loans, payment timing directly affects total cost. Paying biweekly instead of monthly results in 26 half-payments per year — equivalent to 13 monthly payments instead of 12. On a $20,000 auto loan at 6% for 48 months, biweekly payments save approximately $310 in interest and pay off the loan 4 months early. Even paying a few days early each month reduces total interest because the daily accrual calculation uses a slightly lower balance. Set up automatic payments for the day after your paycheck hits to minimize the number of days interest accrues on the higher balance. Use our Payoff Calculator to model accelerated payment strategies.

Simple Interest in Investing

While compound interest dominates long-term investing, simple interest applies to several common instruments. Treasury bills, the safest short-term investment, use simple interest through a discount mechanism — you buy a $10,000 T-bill for $9,750 and receive $10,000 at maturity, earning $250 in simple interest. Certificates of deposit technically pay simple interest per compounding period, though they are often quoted with an Annual Percentage Yield (APY) that reflects compounding. When comparing short-term investment options, convert all rates to the same basis — either simple annual rate or APY — to make accurate comparisons.

Calculating Daily Interest Accrual

Many simple interest loans accrue interest daily using the formula: daily interest = principal balance × (annual rate ÷ 365). On a $15,000 loan at 7%, daily interest is $15,000 × 0.07 ÷ 365 = $2.88 per day. This means every day you delay a payment costs you $2.88, and every day you pay early saves $2.88. Over the course of a 4-year loan, the cumulative effect of paying consistently on time versus a few days late can mean $200–$400 in additional interest. Some lenders use a 360-day year convention instead of 365, which slightly increases the daily rate — always check your loan agreement.

Simple Interest for Business Cash Flow

Business owners frequently encounter simple interest on lines of credit, merchant cash advances, and short-term working capital loans. A $50,000 line of credit at 8% simple interest costs $10.96 per day when fully drawn. If you use $30,000 for 45 days to cover a payroll gap, the cost is $30,000 × 0.08 × (45/365) = $295.89. This makes simple interest credit lines highly transparent for short-term borrowing — you pay only for what you use, for exactly as long as you use it. Compare this to merchant cash advances, which often quote a "factor rate" (e.g., 1.3×) rather than an interest rate, obscuring the true annual cost which may exceed 50–100% APR.

Common Mistakes with Simple Interest Calculations

The most frequent error is confusing the time period. If a problem states "6% for 18 months," you must convert 18 months to 1.5 years before multiplying: I = P × 0.06 × 1.5. Using 18 produces a wildly incorrect answer. Another common mistake is forgetting that the "rate" in the formula must be in decimal form — 6% becomes 0.06, not 6. Finally, be careful with "add-on" interest, which some lenders call "simple interest" but calculate differently. True simple interest accrues on the declining balance; add-on interest calculates total interest on the original principal and adds it to the loan amount upfront, resulting in a much higher effective rate. A $10,000 add-on loan at 6% for 3 years charges $1,800 regardless of payments, while true simple interest on a declining balance charges roughly $950. See our Interest Rate Calculator to find the true rate on any loan.

Simple vs. Compound: When the Gap Matters Most

Over short periods (1–3 years), the difference between simple and compound interest is marginal. Over decades, compounding creates exponential divergence. This is why simple interest is reasonable for short-term loans but would be deeply unfavorable for long-term savings — and why compound interest is both the investor's best friend and the borrower's greatest cost over time.

Regulatory Context and Consumer Protections

The Truth in Lending Act (TILA) requires lenders to disclose the APR on all consumer loans, which converts any interest structure — including simple interest — into an annualized rate that includes fees and other costs. This allows consumers to compare loans with different fee structures on equal footing. For simple interest loans, the stated rate and APR are often identical if no origination fees are charged. When fees are added, the APR rises above the stated rate. A 6% simple interest loan with a 2% origination fee on a 3-year term has an effective APR of approximately 7.3%. Always compare APRs rather than stated rates when shopping for loans, as the APR reflects the total cost of borrowing more accurately.

How to Use This Calculator

1. Enter principal — The starting amount of money.
2. Set interest rate — Annual percentage rate.
3. Choose time period — Duration in years, months, or days.

Tips and Best Practices

See also: Compound Interest · Interest Rate · Loan Calculator · Savings Goal