Day count calculator

The same rate on the same notional produces different interest depending on how the agreement counts days. This calculator shows the accrual days and year fraction for one period under ACT/360, ACT/365 Fixed, 30/360, 30E/360 and ACT/ACT (ISDA), applies a rate to a notional under each, and reports how far each basis sits from ACT/360. It is meant for anyone checking an interest clause, a coupon or an accrual entry.

Features

• Five bases in one tableACT/360, ACT/365 Fixed, 30/360, 30E/360 and ACT/ACT (ISDA) for the same dates, with the day count, denominator, year fraction to eight decimals and the interest each produces.
• Gap versus ACT/360Each basis is measured against ACT/360 in dollars and percent, because SOFR is compounded and quoted on actual days over 360, so ACT/360 is the basis that matches the benchmark.
• The arithmetic shownA trace under the table prints how the 30-day bases adjusted the 31st and how ACT/ACT split the period across calendar years, so you can reproduce every figure by hand.
• Edge-case presetsOne click loads month-end, February and leap-year periods where the conventions disagree, including the Jan 31 to Mar 31 and Feb 28 to Mar 31 cases that trip up spreadsheets.
• SOFR as the rateFill the rate with the latest published SOFR, or with compounded SOFR for your exact dates computed from the New York Fed's SOFR Index.
• CSV exportDownload the inputs and all five rows for a tie-out against a system or a rate notice.

How to use the day count calculator

1. Enter the accrual start and end dates as the agreement defines the period. The start date earns interest and the end date does not, so use the period end or payment date as written, not the day before it.
2. Enter the notional and the annual rate. If the rate is SOFR, the two SOFR buttons fill it in; the compounded option needs both dates to be days with a published SOFR Index value.
3. Read the table. The day count and year fraction depend only on the dates; the interest and the gap versus ACT/360 scale with the notional and rate.
4. Check the trace line for the 30-day bases and ACT/ACT. If either date is the last day of February, a note shows what 30E/360 (ISDA), which treats that day as day 30, would count instead.

The five conventions, rule by rule

A day-count convention has two parts: how many days the period contains, and how many days make a year. The year fraction is the first divided by the second, and interest is simply:

The definitions below follow Section 4.16 of the 2006 ISDA Definitions, which names each basis and gives its formula. For new interest rate derivatives, ISDA's 2021 Interest Rate Derivatives Definitions replaced the 2006 book as the standard from the weekend of October 2/3, 2021 (ISDA's introduction to the 2021 Definitions), so check which book a confirmation incorporates. Loan agreements and bond documents often use the same names but set out the basis in their own text, and that text is what counts. In every case the period includes the first day and excludes the last.

ACT/360 and ACT/365 Fixed

Both count actual calendar days. ACT/360 divides by 360 and ACT/365 Fixed by 365, regardless of leap years. Because the numerator is the same, the ratio between them never changes: ACT/360 interest is always 365/360 of ACT/365 Fixed interest, or 1.39% more, and ACT/365 Fixed is always 1.37% less than ACT/360.

ACT/360 is the money-market basis and the one SOFR uses. The New York Fed compounds SOFR on actual calendar days over a 360-day year for its SOFR Averages and SOFR Index, so a loan at compounded SOFR plus a margin on ACT/360 keeps the rate and the accrual on the same footing. Over a full non-leap year the ACT/360 year fraction is 365/360 = 1.01388889.

30/360 (bond basis) and 30E/360 (Eurobond basis)

Both treat every month as 30 days and the year as 360. With Y, M and D the year, month and day of the start (1) and end (2) dates:

The two differ only when the end date is the 31st and the start date is before the 30th. From January 29 to March 31, 2026, 30/360 keeps D2 = 31 and counts 62 days; 30E/360 caps it at 30 and counts 61. Actual days are 61.

In the US, 30/360 is the corporate bond convention: FINRA's Rule 11620 computes interest on bonds traded between members "on the basis of a 360-day year", with "every calendar month" counted as 1/12 of 360 days, and its examples match the 31st-day rules above (30th or 31st to the 31st of the next month is 30 days). 30E/360 carries the name Eurobond Basis in the ISDA definitions.

The February rules this calculator does not apply

Neither ISDA's 30/360 nor its 30E/360 adjusts February. The calculator implements them exactly as written, so the last day of February is day 28 or 29, not day 30. That produces results that look odd until you see the arithmetic:

Look at a monthly schedule paid on the last day of the month. January 31 to February 28 counts 28 days and February 28 to March 31 counts 33, a total of 61, while January 31 to March 31 in one step counts 60. A semiannual coupon from August 31 to February 28 comes to 178 days instead of the 180 a reader might expect from "half a year".

Some documents adjust February. The variant ISDA defines is 30E/360 (ISDA), set out separately in Section 4.16(h) of the 2006 ISDA Definitions. It sets D1 to 30 when the start date is the last day of February or the 31st, and sets D2 to 30 when the end date is the 31st or the last day of February, unless that end date is the termination date of the trade. Under it, February 28 to March 31, 2026 counts 30 days, and August 31, 2025 to February 28, 2026 counts 180, provided February 28 is not the final termination date. Other documents write their own end-of-month rule, so read the clause itself.

If your document adjusts February, the figures here will differ on some periods that start or end on the last day of February. The calculator flags those dates and shows the 30E/360 (ISDA) count beside the unadjusted ones.

ACT/ACT (ISDA), and why Treasuries use ACT/ACT (ICMA)

ACT/ACT (ISDA) splits the period at each January 1. Days falling in a leap year are divided by 366, days in other years by 365, and the pieces are added. A period entirely inside 2026 therefore gives the same result as ACT/365 Fixed; the two separate only when part of the period falls in a leap year.

US Treasury notes and bonds use a different ACT/ACT. Appendix B to 31 CFR Part 356 computes accrued interest by spreading the half-year coupon evenly over the actual number of days in that half-year period. Market convention calls that Actual/Actual (ICMA), and ISDA 4.16(c) defines it by reference to the ICMA rules. Under ICMA, every full regular semiannual period counts as exactly half a year whether it has 181 or 184 days. This calculator implements ACT/ACT (ISDA), not ICMA, so do not use its ACT/ACT row to check Treasury accrued interest.

Worked example: a period across a leap-year boundary

The calculator's default is $10,000,000 at compounded SOFR from November 30, 2023 to May 31, 2024. The New York Fed's SOFR Index was 1.10983366 on November 30, 2023 and 1.14024578 on May 31, 2024, so compounded SOFR over the 183 days was (1.14024578 / 1.10983366 − 1) × 360 / 183 = 5.39064%, rounded to five decimals.

Had this been a Treasury note paying coupons on November 30 and May 31, ACT/ACT (ICMA) would count the period as exactly 0.5 years, not the 0.50023954 that the ISDA split produces. The SOFR Index calculator reproduces the 5.39064% rate, and the SOFR interest calculator adds a spread to it.

When to use it, and its limits

Who it is for

Use it to find which basis produced a figure: an interest notice slightly above expectations, a coupon that does not match notional × rate ÷ 2, or a spreadsheet that quietly switched to ACT/365 Fixed. Running the dates through all five bases usually identifies the convention. For SOFR compounding with a lookback or observation shift, go to the compounded SOFR calculator; for a schedule of several periods, the floating-rate loan calculator.

Caveats

The calculator does not roll dates for weekends or holidays. Enter the adjusted accrual dates if your document adjusts them, and the unadjusted ones if it says the period is unadjusted. It also applies one rate to the whole period as simple interest; compounding within the period is a separate question from the day count.

Frequently asked questions

Why do SOFR loans use ACT/360 rather than ACT/365?

Because SOFR itself is an actual/360 rate. The New York Fed compounds daily SOFR on actual days over 360 when it produces the SOFR Averages and SOFR Index, so accruing a SOFR-based rate on ACT/360 keeps the loan consistent with the benchmark. Applying an ACT/360 rate on ACT/365 Fixed would cut interest by about 1.37%.

Does a one-year period always have a year fraction of 1 under ACT/ACT (ISDA)?

No. January 1, 2024 to January 1, 2025 gives 366/366 = 1 exactly, but December 31, 2023 to December 31, 2024 gives 1/365 + 365/366 = 1.00000749, because one day falls in a 365-day year and the rest in a 366-day year. ACT/365 Fixed gives 366/365 = 1.00273973 for either period.

Why does 30/360 count January 31 to February 28 as 28 days?

D1 = 31 becomes 30, D2 stays 28, so 30 × 1 + (28 − 30) = 28. The rule only adjusts the 31st. 30E/360 (ISDA) would count 30, unless February 28 is the termination date.

When do 30/360 and 30E/360 give different answers?

Only when the end date is the 31st and the start date is the 1st through the 29th of its month. 30/360 leaves D2 at 31 in that case and 30E/360 caps it at 30, so 30/360 counts one more day.

Which convention do US Treasuries use, and can I check one here?

Treasury notes and bonds accrue on actual days over the actual days in the semiannual coupon period, the convention known as ACT/ACT (ICMA). This calculator's ACT/ACT row is the ISDA version, which splits by calendar year instead, so it will not match Treasury accrued interest except by coincidence.

How large is the gap between bases on a typical quarter?

Between ACT/360 and ACT/365 Fixed it is always 1.37% of the ACT/360 interest. The 30-day bases depend on the dates: a quarter with 92 actual days counts 90 days on 30/360, about 2.2% less interest than ACT/360, while a 90-day quarter from January 1 to April 1 in a non-leap year counts 90 on both.

Privacy and data

Every calculation runs in your browser. The two SOFR buttons read a dated snapshot of Federal Reserve Bank of New York data, through October 8, 2026, from this site's own server; nothing you type is sent anywhere or stored.