Day Count Fraction: Conventions, Costs, and Disclosure Rules

A day count fraction is the formula a lender, bond issuer, or derivatives counterparty uses to translate an annual interest rate into the interest owed for a specific period. It has two parts: a numerator that counts the days in the interest period and a denominator that stands in for a full year. Different markets assign those numbers differently, and the convention written into your contract changes the dollar amount of every interest payment.

How the Calculation Works

Interest for a period comes from three inputs: the principal balance, the annual interest rate, and a fraction representing how much of the year has passed. Multiply the rate by the fraction, then by the principal, and you have the interest due.

The fraction is where the disagreement lives. Some conventions count every calendar day as it falls. Others pretend every month has 30 days. Others count real days on top but divide by a fictional 360-day year. These choices are not academic; they move the amount of money that changes hands. The International Swaps and Derivatives Association publishes standardized definitions so counterparties on opposite sides of a trade agree on exactly how interest accrues.¹

The Four Main Conventions

30/360

The 30/360 convention treats every month as 30 days and every year as 360. A period from January 15 to February 15 counts as 30 days no matter how many days January actually contains. Twelve identical monthly periods produce twelve identical interest amounts. The simplicity predates automated calculation, and 30/360 remains the standard for most U.S. corporate and municipal bonds.

Actual/360

Actual/360 counts real calendar days in the numerator and keeps the denominator fixed at 360. Because a real year has 365 or 366 days, the borrower pays interest on five or six extra days each year at the daily rate. This is the standard in U.S. dollar money markets, including commercial paper and short-term lending, and it is the recommended convention for SOFR-based loans that now dominate commercial lending after the transition away from LIBOR.²

Actual/365 (Fixed)

Actual/365 counts real calendar days in the numerator and divides by a fixed 365, ignoring leap years entirely. The denominator does not shift in a leap year. Some institutions prefer this because it avoids the small recalibration a leap year would otherwise require. It appears in certain sterling-denominated markets and in bilateral loan agreements where the parties want a consistent denominator across years.

Actual/Actual

Actual/Actual counts real days in both the numerator and the denominator. In a leap year, the denominator shifts to 366. This produces the closest alignment between interest charges and the calendar. U.S. Treasury notes, bonds, and bills use this method; all Treasury yields are based on actual day counts with a 365- or 366-day year.³ The ISDA 2021 Definitions include an Actual/Actual (ICMA) variant used for bond-style derivatives, with specific rules for how payment dates falling on non-business days affect the fraction.⁴

What the Difference Actually Costs

The gap between conventions looks small on paper and grows with principal and time. Take a $1,000,000 interest-only loan at a stated rate of 6%.

Under Actual/365, the daily accrual is $1,000,000 × 0.06 ÷ 365 = $164.38. Over a 365-day year, total interest is $60,000, matching the stated 6%.
Under Actual/360, the daily accrual is $1,000,000 × 0.06 ÷ 360 = $166.67. Over 365 real days, total interest is $60,833. The effective annual rate is roughly 6.08%.

An $833 gap on a single million-dollar loan may not sound dramatic. Scale it to $10 million and the annual difference is over $8,000. Over a ten-year term, the Actual/360 borrower pays tens of thousands more than someone with the same stated rate under Actual/365. The 30/360 method, by contrast, produces exactly $60,000 in annual interest because 360/360 reduces to 1, returning a true 6% effective rate. That is why experienced borrowers read the convention buried in their documents rather than comparing stated rates alone.

Which Convention Applies Where

Convention choice is rarely negotiated fresh. Each market segment has a default, and departing from it takes explicit agreement.

Corporate and municipal bonds in the United States use 30/360. The predictable monthly accrual simplifies portfolio accounting for institutional investors.
U.S. money markets and commercial lending use Actual/360. The Alternative Reference Rates Committee recommended this convention for SOFR-based syndicated and bilateral business loans, noting that it is the existing standard in U.S. money markets.⁵
U.S. Treasury securities use Actual/Actual. Treasury yields are never computed on a 30/360 basis.³
Interest rate swaps and derivatives depend on the floating rate benchmark and currency. ISDA’s 2021 Definitions specify which fraction applies to each rate, and any individual trade confirmation identifies the chosen convention.¹

The transition from LIBOR to SOFR preserved Actual/360 as the dominant convention for dollar-denominated loans, so most borrowers did not see the day count method change when their benchmark rate did. Sterling markets typically use Actual/365, and the ARRC acknowledged this divergence in its published recommendations.²

When Payment Dates Fall on Weekends or Holidays

Day count fractions assume payments happen on specific dates, but those dates sometimes land on a Saturday, Sunday, or bank holiday. Business day conventions handle that by shifting the payment, which in turn changes the length of the interest period and the amount of interest due.

The most common rule is the modified following business day convention. If a scheduled payment date falls on a non-business day, the payment rolls forward to the next business day. The exception: if rolling forward would push the payment into the next calendar month, the date rolls backward to the preceding business day instead.⁶ That prevents a January 31 payment falling on a Saturday from becoming a February 2 payment and distorting the accrual across month boundaries.

For Actual/Actual (ICMA) calculations in bond-style derivatives, ISDA clarified in its 2021 Definitions that when a period end date is adjusted for a non-business day, the extra days are added to both the numerator and denominator, so the accrual rate stays the same even though the payment shifts by a day or two.⁴

Finding the Convention in Your Documents

Every loan agreement, bond prospectus, and swap confirmation should identify the day count convention somewhere in its definitions or interest provisions. In corporate credit facilities and syndicated loans, look for a defined term like “Day Count Fraction,” “Interest Calculation,” or “Applicable Day Count” in the section that governs how interest accrues. Bond offering documents state it in the description of the securities. Swap confirmations governed by ISDA documentation reference the fraction by its ISDA-defined name.

If the convention isn’t spelled out, that’s a red flag. Ambiguity has led to litigation, particularly when one party assumed 30/360 and the other expected Actual/360. Some well-drafted agreements include a worked calculation for a hypothetical period, which removes any room for interpretation. Before signing, run the math on at least one payment period using the stated convention and confirm the numbers match what you expect.

Federal Rules on Disclosure and Accuracy

Regulation Z, which implements the Truth in Lending Act for consumer credit, does not require lenders to disclose which day count convention they use. What it does require is accurate disclosure of the annual percentage rate and finance charges. Creditors are permitted to simplify by treating all months as equal and by ignoring leap years.⁷ A lender may use a 360-day year internally for calculation tools even if it collects interest based on a 365-day year, provided the disclosed APR falls within tolerance.

The tolerance is not to the penny. For most transactions, the disclosed APR is considered accurate if it is within one-eighth of one percentage point of the mathematically precise figure. For irregular transactions, the tolerance widens to one-quarter of a percentage point.⁸ Regulation Z does not, however, let creditors ignore the effect of applying a 1/360 daily rate factor to 365 days. A lender using Actual/360 cannot pretend the effective rate equals the stated rate when disclosing APR.⁷ Where disclosures fall outside tolerance, borrowers can recover actual damages plus statutory damages under 15 U.S.C. § 1640, along with attorney’s fees and costs.⁹

Tax Reporting Uses a Different Method

If you buy a bond at a discount from face value, you may owe tax on original issue discount (OID) each year, and the IRS does not use market day count conventions to figure it. Instead of 30/360 or Actual/360, the IRS requires a constant yield method that allocates OID across accrual periods and then divides it equally among the days within each period.¹⁰

Daily OID for an accrual period is the adjusted issue price at the start of the period, times the yield to maturity divided by the number of accrual periods per year, minus any qualified stated interest for the period, then divided by the number of days in the period. If a bond changes hands mid-month, the month’s OID is split between buyer and seller by the days each held it.¹⁰ The practical point is that the interest income reported on your taxes may not match the interest figures produced by whatever convention your bond’s prospectus specifies. Brokers report OID on Form 1099-OID; if you hold bonds outside a brokerage account or want to verify a figure, IRS Publication 1212 walks through the calculation.

1
International Swaps and Derivatives Association. 2021 ISDA Interest Rate Derivatives Definitions
2
Federal Reserve Bank of New York. SOFR In Arrears Conventions for Syndicated Business Loans
3
U.S. Department of the Treasury. Interest Rates – Frequently Asked Questions
4
International Swaps and Derivatives Association. Key Changes in the 2021 ISDA Interest Rate Derivatives Definitions
5
Federal Reserve Bank of New York. Forward Looking Term SOFR and SOFR Averages Conventions for Syndicated and Bilateral Business Loans
6
U.S. Securities and Exchange Commission. Terms and Conditions of the Notes
7
Consumer Financial Protection Bureau. 12 CFR 1026.17 – General Disclosure Requirements
8
Consumer Financial Protection Bureau. 12 CFR 1026.22 – Determination of Annual Percentage Rate
9
Office of the Law Revision Counsel. 15 USC 1640 – Civil Liability
10
Internal Revenue Service. Publication 1212, Guide to Original Issue Discount (OID) Instruments