30/360 Day Count Convention Calculator
Calculate day counts and accrued interest using 30/360 US, 30E/360, and 30E/360 ISDA conventions. Compare results across different day count methods.
30/360 Day Count Convention Calculator
Calculator
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Formula: Days = 360(Y2-Y1) + 30(M2-M1) + (D2-D1); Interest = Principal x Rate x (Days/360)
Worked example — 30/360: $2,750.00 | Actual/365: $2,742.47 | Difference: $7.53
Formula
Days = 360(Y2-Y1) + 30(M2-M1) + (D2-D1); Interest = Principal x Rate x (Days/360)
The 30/360 formula counts the number of days between two dates assuming 30-day months and 360-day years. The adjusted day values (D1, D2) depend on the specific convention variant. Interest is then calculated as principal multiplied by the annual rate multiplied by the year fraction.
Worked Examples
Example 1: Corporate Bond Accrued Interest
Problem:Calculate accrued interest on a $100,000 bond at 5.5% from January 15, 2024 to July 15, 2024 using 30/360 US convention.
Solution:Day count = 360(0) + 30(7-1) + (15-15) = 180 days Year fraction = 180/360 = 0.500000 Accrued interest = $100,000 x 5.5% x 0.5 = $2,750.00 Actual days = 182 (leap year) Actual/365 interest = $100,000 x 5.5% x 182/365 = $2,742.47
Result:30/360: $2,750.00 | Actual/365: $2,742.47 | Difference: $7.53
Example 2: Short Period Interest Calculation
Problem:Calculate interest on $250,000 at 4.25% from March 15 to April 30, 2024 using 30E/360.
Solution:Under 30E/360: d1=15, d2=30 (April 30 stays 30) Day count = 360(0) + 30(4-3) + (30-15) = 45 days Year fraction = 45/360 = 0.125 Interest = $250,000 x 4.25% x 0.125 = $1,328.13 Actual days = 46 Actual/365 = $250,000 x 4.25% x 46/365 = $1,338.36
Result:30E/360: $1,328.13 | Actual/365: $1,338.36 | Difference: $10.23
Frequently Asked Questions
What is the 30/360 day count convention?
The 30/360 day count convention is a method used in finance to calculate accrued interest on bonds and loans by assuming every month has exactly 30 days and every year has exactly 360 days. This simplification makes interest calculations more predictable and easier to compute than counting actual calendar days. The convention is widely used in US corporate and municipal bond markets, mortgage-backed securities, and many fixed-income instruments. Under this method, a six-month period always equals 180 days regardless of whether months have 28, 29, 30, or 31 actual days. This standardization allows consistent interest accrual across different calendar periods and simplifies settlement calculations between bond traders.
What is the difference between 30/360 US and 30E/360?
The key difference between 30/360 US (also called Bond Basis or NASD method) and 30E/360 (also called Eurobond Basis) lies in how they handle the 31st day of a month. Under 30/360 US, if the start date is the 31st it changes to the 30th, and the end date changes to the 30th only if the start date is already the 30th or 31st. Under 30E/360, both start and end dates are simply changed to the 30th if they fall on the 31st, regardless of each other. The 30E/360 ISDA variant adds additional handling for February end-of-month dates. These differences can result in slightly different day counts and therefore different interest amounts, particularly when period boundaries fall on the 31st of a month.
When is the 30/360 convention used versus Actual/365?
The 30/360 convention is standard for US corporate bonds, agency bonds, municipal bonds, and many mortgage products. It is the default for most fixed-rate instruments in the US market. The Actual/365 convention (also called Actual/Fixed) counts the real number of days between dates and divides by 365, making it common for US Treasury bonds, UK government gilts, and many money market instruments. The Actual/360 convention is standard for money markets, LIBOR-based instruments, and commercial paper. Euro-denominated bonds typically use Actual/Actual or 30E/360. The choice of convention is specified in the bond indenture or loan agreement and directly affects the amount of interest accrued and paid.
How does the 30/360 convention affect interest payments?
The 30/360 convention creates predictable interest amounts because each month is treated as exactly one-twelfth of a year. For a bond with a $100,000 face value and 6% annual coupon, the monthly accrual is exactly $500 regardless of the month length. With Actual/365, February accrues only $460.27 while a 31-day month accrues $509.59. Over full coupon periods, the differences tend to be small, but they become significant for odd first or last coupon periods, accrued interest calculations at bond settlement, and partial period interest on loans. When buying a bond between coupon dates, the accrued interest calculation using 30/360 versus actual day count can differ by tens or hundreds of dollars on large positions.
How do you handle February dates in 30/360 calculations?
February presents unique challenges for 30/360 calculations because it has only 28 or 29 days. Under the standard 30/360 US convention, February is still treated as having 30 days for counting purposes. If a period starts on January 31 (adjusted to 30) and ends on February 28, the day count is 30 times zero months plus 28 minus 30 which equals negative 2, then corrected. The 30E/360 ISDA variant specifically addresses February by changing the last day of February to the 30th when it appears as a start or end date. Different financial software implementations may handle these edge cases slightly differently, which is why it is critical to verify calculations and confirm which specific convention variant applies to your instrument or contract.
How is the 30/360 day count formula applied to a specific pair of dates?
Apply Days = 360(Y2 - Y1) + 30(M2 - M1) + (D2 - D1) after adjusting any day numbers according to the convention variant. From 15 January to 15 July of the same year the calculation is 360 times 0, plus 30 times 6, plus 0, giving 180 days and a year fraction of 180/360 = 0.5. On 100,000 of principal at a 6 percent annual rate that produces exactly 3,000 of interest.
What is accrued interest and how does a bond buyer settle it under 30/360?
Between coupon dates the seller has earned interest that the issuer will pay in full to whoever holds the bond on the next payment date. The buyer therefore pays the clean price plus accrued interest, calculated as the coupon multiplied by the 30/360 year fraction from the last coupon date to settlement. Using 30/360 makes that figure predictable, since each elapsed month contributes exactly one twelfth of an annual coupon regardless of calendar length.
Why is 30/360 called Bond Basis and where does the 360-day year come from?
The 360-day year predates computers. Twelve equal 30-day months divide cleanly by 2, 3, 4, 6, and 12, which made semiannual and quarterly coupon arithmetic straightforward with pen and paper and produced identical accruals for every period. The convention became entrenched in US corporate and municipal bond documentation, earning the name Bond Basis, and it survives today largely because outstanding indentures and long-standing market practice specify it.
What is a stub period and how does 30/360 handle a short first coupon?
A stub is an irregular first or last period that does not span a full coupon interval, which happens when a bond is issued between regular payment dates. Under 30/360 the stub coupon is the full coupon scaled by the year fraction between the dated date and the first payment date. Because the convention treats months as uniform, stub calculations are consistent across issues, but the day adjustment rules for the 31st still apply at both endpoints.
How does coupon frequency interact with the day count convention?
The day count produces the year fraction, and the frequency determines how many such fractions occur per year. Under 30/360 a semiannual bond has periods of exactly 180/360 = 0.5 and a quarterly bond exactly 90/360 = 0.25, so each payment equals the annual coupon divided evenly. Under an actual day count the periods vary in length, so consecutive payments differ slightly even though the annual total is the same.
References
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer · Editorial policy
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