Simple Interest Calculator

Simple interest is the most basic way to calculate the cost of borrowing money or the return on a deposit: interest is charged or earned only on the original principal amount, for the entire length of the loan or deposit, using the formula I = P × r × t — principal times the annual rate times the time in years. It never compounds, meaning interest earned in one period doesn't itself start earning interest in the next. That makes it easy to calculate by hand and easy to verify, which is exactly why it's still used for a meaningful share of real-world lending, even though compound interest gets more attention in personal finance content. This calculator handles the full formula plus unit conversion, so you can enter a time period in years, months, or days and get both the interest amount and the total amount due or received.

Picture someone taking out a $15,000 used car loan at a 6% simple annual interest rate over 4 years. Using the formula directly: I = $15,000 × 0.06 × 4 = $3,600 in total interest over the life of the loan, for a total repayment of $18,600. Many auto lenders use exactly this method — often called the 'add-on' interest method in that context — where the total interest is calculated upfront using simple interest and then spread evenly across the monthly payments, rather than recalculated month to month against a shrinking balance the way an amortized mortgage works. Knowing this distinction matters: two loans with the same stated rate and term can produce different effective costs depending on which method a lender uses, and simple/add-on interest loans are generally more expensive than equivalent amortized loans at the same nominal rate, since the interest is always calculated on the original principal rather than the actual remaining balance.

Simple interest also shows up on the saving side, particularly for short-term deposits. A $10,000 certificate of deposit at 4% simple annual interest held for 90 days earns: I = $10,000 × 0.04 × (90 ÷ 365) ≈ $98.63. Because the term here is measured in days rather than years, getting the day-count convention right matters more than it might seem — some financial institutions use a 360-day year for certain calculations rather than 365, which changes the result by roughly 1.4%, a small but real difference on larger balances or longer holding periods. This calculator's day/month/year toggle handles that conversion for you, so you can compare quotes from different institutions on equal footing rather than needing to redo each one by hand with the correct day-count assumption.

Simple interest is also the standard method for calculating interest on overdue payments, whether that's a late invoice between businesses, an overdue court judgment, or interest specified in a contract for a missed deadline. If an invoice for $8,000 is 45 days overdue and the contract specifies 1.5% monthly simple interest (an annual rate of 18%), the late fee comes to: I = $8,000 × 0.18 × (45 ÷ 365) ≈ $177.53. Because the calculation is simple and doesn't compound, both parties can verify the exact amount owed without needing to agree on compounding frequency or amortization assumptions — one of the practical reasons contracts and legal judgments default to simple interest rather than compound interest for this kind of calculation.

The gap between simple and compound interest is small over short periods and grows significantly over long ones. On a $10,000 balance at 5% annual interest for 1 year, simple interest gives exactly $500, and annually-compounded interest gives $500 as well — they're identical for a single compounding period. Stretch that same rate out to 20 years, though, and simple interest totals $10,000 (flat $500 per year), while compound interest grows to roughly $16,533 — more than 65% more, because each year's interest is added to a growing balance that itself earns interest. This is why simple interest tends to favor the lender on the borrowing side over long terms (the total interest owed is more predictable and generally lower) but disadvantage the saver on the deposit side (a long-term simple-interest account earns meaningfully less than an equivalent compound-interest one).

Simple interest math also explains a common but often-overlooked business practice: early payment discount terms like '2/10 net 30,' which appear on countless B2B invoices. That term means a buyer gets a 2% discount if they pay within 10 days, instead of the full amount due at 30 days — effectively a 2% return for paying 20 days earlier. Annualized, that comes out to a striking rate: roughly 2% ÷ (1 − 2%) × (365 ÷ 20) ≈ 37.2% per year, using the same time-value-of-money logic as simple interest. That's why finance teams treat early payment discounts as one of the highest-return, lowest-risk uses of available cash — turning down a 2/10 net 30 discount to keep cash on hand is effectively borrowing at nearly 40% annual interest, even though no interest rate is ever explicitly stated on the invoice.

A few practical notes when using simple interest figures in real decisions. Always confirm which day-count convention a lender or institution uses (365 vs. 360 days) before comparing quotes precisely, since it can shift the result by more than a percentage point on longer-term balances. If a loan is advertised with a monthly rate rather than an annual one, multiply by 12 first to get the annual rate this calculator expects (a '1.5% per month' rate is 18% annually, not 1.5% annually). And remember that 'simple interest' describes how interest accrues, not whether a loan has regular payments — an auto loan using simple/add-on interest still has a fixed monthly payment schedule, it's just that the total interest baked into that schedule was calculated using the simple interest formula rather than a true amortization schedule that recalculates interest against the actual remaining balance each period. This same day-count sensitivity shows up in bond and money-market pricing internationally — conventions like Actual/360, Actual/365, and 30/360 are chosen deliberately by market participants and can shift the effective yield on otherwise identical securities by a measurable amount, which is why professional fixed-income quotes always specify the convention alongside the rate.