Simple interest calculated over a number of days is computed with the formula I = P × (R ÷ 100) × (D ÷ 365), where P is the principal, R is the annual interest rate as a percentage, and D is the number of days the money is borrowed or deposited. The total you owe or earn is then P + I. Lenders use this version of the simple interest formula when a loan term is short — anything from a single month to under a year — because measuring time in days is more precise than rounding to whole years. Treasury bills, short-term personal loans, bridge financing, and some auto and store credit promotions are quoted this way. Because simple interest never compounds, the per-day interest stays fixed: $1,000 borrowed at 6% produces roughly $0.16 of interest per day for the entire term, whether the loan runs for 30 days or 180 days. The Simple Interest Calculator applies this same linear behavior to give an instant answer once you supply the principal, the annual rate, and the time span.
The Day-Based Simple Interest Formula
Simple interest calculated over a number of days uses the formula I = P × (R ÷ 100) × (D ÷ 365). The first variable, P, is the principal — the loan amount or starting deposit in dollars. R is the annual interest rate expressed as a percentage, so a 6% loan means you plug in 6, not 0.06. D is the number of days the money is in use, and 365 is the length of the year assumed for the conversion. The interest I you compute is the dollar amount charged or earned over that span; the total balance at the end of the term is P + I.
The same formula also rearranges to I = P × r × (D ÷ 365), where r is the rate written as a decimal. Both forms produce the same number; pick whichever feels more natural. If you prefer to think in terms of years, just note that D ÷ 365 plays the role of t — the time, expressed in years — that the standard formula I = P × r × t expects.
Quick sanity check: $1,000 at 5% annual simple interest is $50 per year, or about $50 ÷ 365 ≈ $0.137 per day. That per-day figure stays constant for the entire term because simple interest never compounds. Whether the loan runs for 30 days or 300 days, the daily interest is the same number, and doubling the day count exactly doubles the total interest charged.
Where Day-Based Simple Interest Appears
Lenders and issuers reach for the day-count version of the simple interest formula whenever the term is shorter than a year and a precise figure matters more than a rounded "X% annual" sticker. A few of the most common settings are listed below.
| Use case | Typical day range | How days enter the picture |
|---|---|---|
| U.S. Treasury bills | 4 to 52 weeks | Quoted as a yield over a specific number of days to maturity |
| Short-term personal loans | 30 to 180 days | Term matches one billing cycle or a few |
| Bridge loans | 30 to 365 days | Short horizon, priced by the day |
| Promotional store credit | 30 to 180 days | Interest-free period measured in days |
| Money-market instruments | Overnight to several months | Often use a 360-day year convention |
Each of these scenarios can be checked against the Simple Interest Calculator once you translate the day count into a fraction of a year. Because the underlying math is linear, the result scales cleanly: a 90-day term yields roughly a quarter of the annual interest, a 180-day term roughly half, and a 365-day term the full annual amount.
The 365 vs. 360 Day-Count Convention
The day-count formula above assumes 365 days in a year, which is the convention for most consumer loans, U.S. Treasury bills, and many everyday deposits. Under that convention, every day of the year is weighted equally, and the annual interest is divided by exactly 365 to get the per-day rate. Wikipedia's overview of interest (see the Interest entry) treats this as the textbook default.
Some money-market quotes, certain bank deposit products, and a handful of corporate borrowing arrangements use a 360-day convention instead. The formula is identical except 365 is replaced by 360 in the denominator. Because the denominator is smaller, the per-day rate is larger, and the total interest over the same number of days comes out a bit higher. Real-world examples of the 360-day convention show up in some U.S. money-market funds, eurodollar deposits, and certain commercial paper pricing, though you will rarely encounter it as a consumer. When in doubt, ask which day-count convention the quote is using — it can shift the bottom line noticeably on large balances held for long stretches.
Calculate Simple Interest for Days Using the Calculator
The Simple Interest Calculator accepts the principal, the annual rate, and the time expressed in years. To produce a day-count answer, convert the day span to a fraction of a year and type that fraction into the time field. The calculator then applies I = P × r × t internally and displays both the interest earned and the total balance instantly. Everything runs locally in your browser, so no numbers you type are uploaded.
- Write the day count as a fraction of a year. Divide the number of days by 365. For 90 days, that's 90 ÷ 365 = 0.2466. For 180 days, it is 180 ÷ 365 = 0.4932. For 365 days exactly, it is simply 1.
- Type the principal. Enter the loan or deposit amount in dollars — for example, 2500.
- Type the annual rate as a percentage. Enter 5 for 5%, 6.5 for 6.5%, and so on. Do not divide by 100 yourself; the calculator converts the percentage to a decimal internally.
- Enter the converted time value. Put the fraction from step 1 into the time field. The calculator accepts whole numbers like 1 or 2 and fractions like 0.25, 0.5, or 1.5.
- Read the interest and the total. The two output fields update instantly: one shows the simple interest charged or earned over the day count, the other shows principal plus that interest. Set the rate or time to zero to confirm the interest drops to $0 and the total equals the principal — a useful edge case when a promotional period is interest-free.
If a calendar-based lender quotes interest between two specific dates rather than as a plain day count, the same day-counting conversion applies at the end — see how to calculate simple interest between two dates for the date-counting variant.
Worked Example: $2,500 at 6.5% for 180 Days
Suppose you borrow $2,500 at an annual rate of 6.5%, and the loan runs for 180 days. Here is the calculation worked out by hand before plugging anything into a tool.
Step 1 — Convert days to years: 180 ÷ 365 = 0.49315, so the time the calculator wants is roughly 0.4932.
Step 2 — Convert the percentage to a decimal: 6.5% = 0.065.
Step 3 — Apply the simple interest formula: I = P × r × t = 2500 × 0.065 × 0.49315. Multiplying through gives 2500 × 0.065 = 162.5, then 162.5 × 0.49315 = 80.14 (rounded to the nearest cent).
Step 4 — Add the principal to find the total repaid: 2500 + 80.14 = 2580.14.
So $2,500 at 6.5% simple interest for 180 days produces about $80.14 of interest, and the total repayment is $2,580.14. Type 2500 into the principal, 6.5 into the rate, and 0.4932 into the time field of the Simple Interest Calculator — the screen displays those same two figures. Because the underlying math is linear, doubling the day count to 360 days roughly doubles the interest, and halving the rate to 3.25% roughly halves it.
Simple Interest vs. Compound Interest Over the Same Day Count
Day-count simple interest and day-count compound interest use the same starting ingredients, but they diverge almost immediately. With simple interest, the per-day interest stays constant for every day of the term, so the total interest scales in a straight line with the day count. With compound interest, each day's interest is added to the balance before the next day's interest is computed, so the effective daily earnings grow — albeit slowly — over the life of the loan or deposit.
For a 180-day window, the difference is small but real. At 6.5% over half a year, simple interest on $2,500 comes out to about $80, while daily compounding produces a slightly higher figure. Over longer periods the gap widens; that is why simple interest is rarely used for multi-year savings products but is common for short, fixed-term loans. When you want interest that itself earns interest — savings accounts, certificates of deposit, most mortgages, and credit-card balances — the Compound Interest Calculator models the exponential growth that this approach deliberately leaves out. For the linear, non-compounding version covered in this article, the Simple Interest Calculator does the job.
Numbers shown here are estimates for general information only and are not financial advice.