Day Count Convention Calculator
Enter an accrual start date and end date to compare common day count conventions. You can also solve for the earliest end date that reaches a target year fraction.
Actual/Actual ISDA year fraction0.5
- Actual calendar days
- 181
- Convention day count
- 181
- Date-adjustment trace
- Start day 31; end day 31. No day-of-month adjustment.
Convention comparison
| Convention | Day count | Year fraction | Date-adjustment trace |
|---|---|---|---|
| Actual/Actual ISDA | 181 | 0.5 | Start day 31; end day 31. No day-of-month adjustment. |
| Actual/360 | 181 | 0.5 | Start day 31; end day 31. No day-of-month adjustment. |
| Actual/365 Fixed | 181 | 0.5 | Start day 31; end day 31. No day-of-month adjustment. |
| 30U/360 (SIFMA) | 180 | 0.5 | Start day 30; end day 30. 30U/360 changes a start day of 31 to 30. 30U/360 changes an end day of 31 to 30 when the adjusted start day is 30. |
| 30E/360 | 180 | 0.5 | Start day 30; end day 30. 30E/360 caps a start day of 31 at 30. 30E/360 caps an end day of 31 at 30. |
| 30E/360 ISDA | 180 | 0.5 | Start day 30; end day 30. 30E/360 ISDA treats a month-end start date as day 30. 30E/360 ISDA treats a month-end end date as day 30. |
The start date is included and the end date is excluded for actual-day counts.
30/360 rows show the convention numerator after applying month-end and February rules.
How to use this calculator
- Choose whether to compare two dates or solve for an end date.
- Enter the start date and, in compare mode, the end date.
- Choose one convention or compare all supported conventions.
- Mark February maturity when the 30E/360 ISDA final-period exception applies.
- In solve mode, enter the target year fraction and read the achieved fraction and overshoot.
How the year fraction is calculated
A day count convention turns a date interval into the year fraction used for accrual. This calculator treats the interval as half-open: the start date is included and the end date is excluded for actual-day conventions. If the dates are equal, the actual-day count and year fraction are zero.
For Actual/360 and Actual/365 Fixed, the numerator is the number of Gregorian calendar days in the interval:
year fraction = actual days ÷ 360 or actual days ÷ 365
For Actual/Actual ISDA, the interval is split at each January 1. Days that fall in a leap year are divided by 366, and days that fall in a common year are divided by 365. Century years follow the Gregorian rule: a century year is a leap year only when it is divisible by 400.
The 30/360 methods first adjust the start and end day-of-month values, then use:
day count = 360 × (Y2 − Y1) + 30 × (M2 − M1) + (D2' − D1')
year fraction = day count ÷ 360
Month-end and February rules
30E/360 caps both day values at 30. A January 31 to July 31 period therefore has a convention day count of 180 even though it has 181 actual calendar days in 2026.
30E/360 ISDA treats any month-end start date as day 30 and treats a month-end end date as day 30 unless the end date is a contractual maturity date in February. That exception matters for final coupon periods ending on February 28 or February 29.
30U/360 applies the SIFMA-style February and day-31 adjustments. A last-February start becomes day 30. A last-February end becomes day 30 only when the start is also the last day of February. A start day of 31 becomes 30, and an end day of 31 becomes 30 when the adjusted start day is 30.
Reverse solving
In solve mode, the calculator searches supported Gregorian dates after the start date and returns the earliest end date whose convention year fraction is at least the target. Adjacent dates can sometimes produce the same 30/360 value, so returning the earliest qualifying date is important. The achieved fraction and overshoot are shown because a discrete date may not land exactly on the requested fraction.
What this calculator leaves out
The calculation is limited to accrual fractions. It does not adjust for weekends, holidays, payment calendars, business-day conventions, ex-coupon periods, coupon frequency, security prices, settlement amounts, or interest amounts. Actual/Actual ICMA is not included because it requires reference coupon-period information that is not present in a simple two-date input.
Worked example
Suppose the accrual period runs from January 31, 2026, through but not including July 31, 2026. The actual calendar day count is 181. Actual/360 gives 181 ÷ 360 = 0.5027777778, while Actual/365 Fixed gives 181 ÷ 365 = 0.4958904110.
Actual/Actual ISDA stays within the common year 2026, so it also uses 181 ÷ 365 = 0.4958904110. Under 30E/360, both day values are capped at 30, so the convention day count is 360 × 0 + 30 × 6 + (30 − 30) = 180 and the year fraction is 0.5.
Common questions
What is a day count convention?
A day count convention is a rule for turning a start date and end date into an accrual fraction of a year. Loans, bonds, swaps, and deposits use these fractions to calculate interest or coupon accruals.
What is the difference between Actual/360 and Actual/365 Fixed?
Both count the actual number of calendar days in the interval. Actual/360 divides that count by 360, while Actual/365 Fixed divides it by 365, so Actual/360 produces a larger year fraction for the same dates.
How does Actual/Actual ISDA handle leap years?
Actual/Actual ISDA splits the interval by calendar year. Days in a leap year are divided by 366, and days in a common year are divided by 365, then the pieces are added.
Why can 30/360 differ from the actual number of days?
30/360 conventions replace certain real day-of-month values with adjusted values before applying a 360-day-year formula. Month-end and February rules can make two periods with different calendar lengths produce the same convention day count.
How does the February maturity exception work under 30E/360 ISDA?
A month-end end date is normally adjusted to day 30 under 30E/360 ISDA. If the end date is the contractual maturity date and falls in February, the calculator keeps the actual February day instead.
Why does the reverse calculation slightly exceed my target year fraction?
The solver can choose only whole Gregorian end dates. If no date gives the exact target fraction, it returns the earliest date that reaches or exceeds the target and reports the overshoot.