Day-Count Interest Calculator
Compare simple interest under Actual/365, Actual/360, 30/360 (US) and 30E/360 for the same dates.
Quick answer
A day-count convention sets how days are counted and how many make a year. Simple interest = principal x rate x day count / year basis, compared under Actual/365, Actual/360, 30/360 (US) and 30E/360.
Day-Count Interest Calculator
Simple Interest (selected)
—
Day Count
—
Year Fraction
—
Highest Convention
—
Spread, Highest − Lowest
—
Interest by convention
Convention Comparison
| Convention | Day count | Days in year | Year fraction | Interest (₹) | vs Actual/365 (₹) |
|---|
Assumptions & method
- Interest = principal × annual rate × year fraction, where year fraction = day count ÷ days in year. This is simple interest with no compounding.
- Actual/365 and Actual/360: day count = end date − start date (the start date counts, the end date does not); denominators 365 and 360.
- 30/360 (US): day count = 360 × Δyears + 30 × Δmonths + Δdays after setting D1 = 30 if it is 31 or the last day of February, D2 = 30 if it is the last day of February when D1 is too, and D2 = 30 if D2 = 31 and D1 ≥ 30.
- 30E/360: any day-of-month of 31 is set to 30 for both dates; no February rules.
- Leap years (Actual/365 uses 365 even in a leap year), holiday adjustments, rate changes, fees and compounding are not included. Check your agreement for the convention that applies.
Results are estimates based on the figures you enter. They are not guarantees and not financial advice.
How day-count conventions change interest
Interest is rate × time, and time is measured as a fraction of a year. A day-count convention defines both parts of that fraction: how the days between two dates are counted and how many days make up a year. Actual conventions use the real calendar, while 30/360 treats every month as 30 days. Over one period the differences are small, but on a large principal or a long period they add up. This calculator takes a principal, annual rate and two dates, then shows the day count, year fraction and simple interest for each of the four common conventions side by side.