Day Count Convention Calculator
Enter two dates to see the day count and year fraction under eight common conventions, from 30/360 bond basis to Actual/Actual ICMA. Add a principal and rate to compare the accrued interest each convention produces.
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ICMA assumes the start date is a regular coupon date.
How to use the Day Count Convention Calculator
- Enter the start date (accrual start or last coupon date) and the end date (settlement or period end).
- Choose the convention named in your bond prospectus, loan agreement or swap confirmation.
- Add principal and rate if you want accrued interest; for Actual/Actual ICMA also choose the coupon frequency.
- Read the day count and year fraction, then follow the steps to see which date adjustments were made.
- Compare the table to see how much the choice of convention changes the interest.
Day count formulas
All conventions produce a year fraction, and interest is then principal × annual rate × year fraction. For the 30/360 family, write the dates as Y1-M1-D1 and Y2-M2-D2, adjust the days, then use days = 360 × (Y2 − Y1) + 30 × (M2 − M1) + (D2 − D1).
| Convention | Day adjustments / day count | Year fraction | Typical use |
|---|---|---|---|
| 30/360 US (NASD/SIA) | If D1 and D2 are both the last day of February, D2 = 30; if D1 is the last day of February, D1 = 30; if D2 = 31 and D1 ≥ 30, D2 = 30; if D1 = 31, D1 = 30 | days ÷ 360 | US corporate, agency and municipal bonds |
| 30/360 Bond Basis (ISDA) | D1 = 31 → 30; D2 = 31 → 30 only if D1 is now 30 or 31 | days ÷ 360 | Swap fixed legs, some bonds |
| 30E/360 (Eurobond) | Any 31st becomes 30 | days ÷ 360 | Eurobonds, older European bonds |
| 30E/360 ISDA | Last day of any month becomes 30, except a February maturity date | days ÷ 360 | Some EUR swaps and German-style instruments |
| Actual/360 | Actual days | days ÷ 360 | USD and EUR money markets, SOFR and many loans |
| Actual/365 Fixed | Actual days | days ÷ 365 (even in leap years) | GBP money markets, some swaps and loans |
| Actual/Actual ISDA | Actual days, split by calendar year | leap-year days ÷ 366 + other days ÷ 365 | Swaps (ISDA definitions) |
| Actual/Actual ICMA | Actual days, split by coupon period | days ÷ (frequency × days in coupon period) | US Treasuries, most government bonds |
Worked example: February 28 to August 31, 2026
There are 184 actual days between these dates, and both are month-ends, which brings out the differences between the 30/360 variants:
| Convention | Adjusted D1 / D2 | Day count | Year fraction |
|---|---|---|---|
| 30/360 US | 30 / 30 | 180 | 0.50000000 |
| 30/360 Bond Basis | 28 / 31 | 183 | 0.50833333 |
| 30E/360 | 28 / 30 | 182 | 0.50555556 |
| 30E/360 ISDA | 30 / 30 | 180 | 0.50000000 |
| Actual/360 | — | 184 | 0.51111111 |
| Actual/365 Fixed | — | 184 | 0.50410959 |
| Actual/Actual ISDA | — | 184 | 0.50410959 |
On $1,000,000 at 5%, the gap between the lowest (30/360 US, $25,000.00) and highest (Actual/360, $25,555.56) is more than $550 for one half-year, which is why the convention is always written into the contract.
Tips
- Check the end-of-month rule. For bonds paying on the last day of the month, the 30/360 US February adjustment applies; for other instruments use Bond Basis or the variant named in the documents.
- Act/Act ICMA needs coupon dates. This calculator assumes the start date is a regular coupon date and builds periods forward from it; irregular first or last coupons need the actual schedule.
- Business-day adjustments are separate. Payment dates that move to the next business day may or may not change the accrual period (“adjusted” vs “unadjusted”); enter the accrual dates your contract uses.
Frequently asked questions
What is a day count convention?
It is the rule that turns the period between two dates into a fraction of a year for interest calculations. Interest = principal × annual rate × year fraction, so the convention decides how many days count and what the year length is.
How does 30/360 work?
Every month is treated as 30 days and the year as 360. The day count is 360 × (Y2 − Y1) + 30 × (M2 − M1) + (D2 − D1), after adjusting day 31 (and sometimes the last day of February) to 30. The variants differ only in those adjustments.
What is the difference between 30/360 and 30E/360?
In 30E/360 (Eurobond basis) any 31st becomes the 30th, for both dates. In US 30/360 the end date's 31st only becomes 30 when the start date is the 30th or 31st, and the SIA version also treats the last day of February as the 30th.
Why does Actual/360 give more interest than Actual/365?
Both count actual days, but Actual/360 divides by 360, so a full year produces 365/360 ≈ 1.0139 years of interest. That is why money-market and floating-rate loans quoted on Actual/360 pay slightly more than the headline rate.
What is the difference between Actual/Actual ISDA and ICMA?
ISDA divides the days falling in leap years by 366 and the rest by 365. ICMA divides the days by the length of the coupon period times the number of coupons per year, so every full coupon period is worth exactly 1/frequency of a year. ICMA is used for most government bonds, including US Treasuries.
Which convention should I use?
Use the one in your contract. Typical defaults: US corporate and municipal bonds 30/360; Eurobonds 30E/360; US Treasuries and many government bonds Actual/Actual ICMA; USD and EUR money markets and SOFR loans Actual/360; GBP money markets Actual/365 Fixed.
Is the end date included in the day count?
Conventions count from the start date up to but not including the end date, so January 1 to January 31 is 30 actual days. Interest accrues for the start date and not for the end date.
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