Day Count Convention30360 calculator
Calculate Day Count Convention30360 Calculator easily with our free tool. Get practical results, tips, and comparisons for everyday decisions.
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer
Day Count Convention30360 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.
References
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer · Editorial policy
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