Calculate interest in 28, 29, 30 and 31-day months
See why Actual/365 daily interest stays constant on unchanged principal while the monthly total varies, and how calendar-month and leap-year methods differ.
Check Interest method first
Open the loan rules. Actual days / 365, Actual days / year, Calendar month, 30/360 and weekly conventions produce different answers. Do not infer the method from the monthly collection date.
Calculate Actual/365 interest
Interest equals the applicable principal multiplied by the annual percentage divided by 100, multiplied by elapsed days divided by 365. On unchanged principal the unrounded daily amount is the same in every month; longer months have more elapsed days.
Compare the full-period examples
The table assumes INR 100,000 unchanged principal, 12% per year, simple interest and no charges. Each Actual/365 amount is calculated for the whole period and then rounded. Do not round the daily amount before multiplying by days.
Understand calendar-month interest
For INR 100,000 at 1% per calendar month, one complete anchored month earns INR 1,000 whether that month has 28, 29, 30 or 31 days. A partial anchored month uses the actual length of that particular anchored month, which may run between anniversary dates rather than calendar-month boundaries.
Handle leap years and other methods explicitly
Actual/actual ISDA uses 366 for the portion in a leap year and 365 for the portion in a non-leap year. Actual/365 keeps 365 even in a leap year. The named 30/360 and weekly methods apply their own rules; the app does not switch methods automatically.
Recalculate when principal changes
A repayment, further advance, rate change or permitted capitalisation can split the period into segments. Calculate each segment using its effective terms. Collection-period rounding means a long statement can differ from a single rounded full-year calculation.
| Period length | Actual/365 interest (INR) | One full calendar month at 1% (INR) |
|---|---|---|
| 28 days | 920.55 | 1,000.00 |
| 29 days | 953.42 | 1,000.00 |
| 30 days | 986.30 | 1,000.00 |
| 31 days | 1,019.18 | 1,000.00 |
Check your result
- Check the loan method, principal basis, dates and rate unit.
- Use actual elapsed days with an exclusive end date.
- Compare amounts after period rounding, not a rounded daily shortcut.