📏 Simple Interest Calculator

Key concepts

The simple interest formula is the cleanest in finance. I = P × r × t: interest equals principal times annual rate times time in years. Three inputs, one output, no compounding, no edge cases. A $10,000 deposit at 5% for 10 years earns exactly $5,000 in interest — $500 every year, on the dot, regardless of payout cadence. The total amount returned to the holder at maturity is P + I = P × (1 + r × t). The reason this formula matters in 2026 isn’t that anyone actually invests in simple-interest savings instruments anymore (banks all compound), but because the dominant consumer-credit products in the US, UK, and SA — auto loans, federal student loans, most personal loans — are simple-interest at the product level. Understanding the formula tells you exactly how prepayment, refinancing, and the math of late-fee calculations work.

Where simple interest actually shows up in real life. First, auto loans: every major US lender uses simple-interest amortisation, where daily interest accrues on the outstanding principal and your monthly payment is split first to accrued interest and second to principal reduction. Pay early in the month and a smaller fraction of that payment goes to interest because fewer days have accrued; pay late and the inverse is true. Second, federal student loans (Stafford, PLUS, Direct loans): simple-interest accrual using the actuarial method. Third, short-term consumer credit: payday loans, pawnshop loans, some buy-now-pay-later products. Fourth, certain bond conventions (Treasury bills are quoted at a simple-interest discount rate, then converted to a bond-equivalent yield for comparison purposes). Fifth, late-fee math on most consumer contracts. Saving instruments, by contrast, are essentially always compound — checking, savings, money market, CDs, ISAs, TFSAs, money-market funds — because the institution wants the disclosure-friendly APY number.

The Rule of 72 connects simple and compound intuitively. The Rule of 72 says: 72 ÷ annual interest rate ≈ years to double under compound interest. So at 7% compound your money doubles in about 10.3 years; at 12% in 6 years. The simple-interest equivalent uses 100: 100 ÷ rate = years to double under simple interest (1 = r × t when interest equals principal). At 7% simple your money takes 14.3 years to double, not 10.3. That 4-year gap is the value of compounding on a single doubling. Stretch the horizon: in 30 years at 7%, compound returns 7.6× the principal ($1,000 → $7,612); simple returns just 3.1× ($1,000 + $2,100 = $3,100). The dollar gap is $4,512 on the exact same starting principal and rate — that’s why we say compound interest is the long game and simple interest is the short.

Why compound wins the long game. The mechanism is exponential vs linear growth. Simple interest grows the balance linearly: each year adds the same fixed dollar amount (P × r). The graph is a straight line. Compound interest grows the balance exponentially: each year adds interest on the new larger balance, so the dollar amount per year grows year-over-year. The graph is a J-curve. In year 1 the two are nearly identical; the gap appears slowly through years 5-10 and becomes dramatic by year 20-30. This calculator’s spread column shows the headline dollar difference at any horizon you choose — experiment with longer horizons to see the J-curve in action. The compounding effect is exactly the same on the borrowing side: a credit card at 22% APR compounding daily can turn a $5,000 balance into $50,000+ over a decade if you only pay minimums.

Mistakes people make with the formula. The single most common error is using the rate as a percent (5) instead of a decimal (0.05) in the formula — this calculator handles the conversion automatically, but if you’re computing by hand, divide the percent by 100 first. The second error is using months for t while keeping rate as the annual rate — t must be in years, so 6 months is t = 0.5, not t = 6. The third error is assuming the payout cadence (monthly vs quarterly vs annual) changes the simple-interest total — it does not. The fourth error is applying simple-interest math to a compound product like a savings account or a mortgage — both are compound, and the simple-interest total understates the true cost or earnings. Always confirm in the product’s contract or the regulator’s disclosure whether you’re looking at a simple- or compound-interest product.

The takeaway for savers and borrowers. For savers: simple-interest products are essentially extinct in retail banking — if a bank is quoting you simple interest, walk away. The dominance of compound is so total that “5%” from a US bank almost certainly means 5% APY (compound), not 5% nominal simple. For borrowers: simple-interest loan products are usually preferable to compound-interest loan products, because prepayment instantly reduces future interest accrual on a simple-interest loan. Auto loans, federal student loans, and most personal loans are simple — pay extra and you save real money fast. Credit cards and HELOCs are compound — pay down aggressively because daily compounding makes carrying a balance painful. The mental model: I = P × r × t is the simplest formula in finance, but compound interest is the most powerful force.