30/360 Accrued Interest Calculator

A precise tool to calculate accruals using the 30/360 day-count convention, commonly used for corporate bonds, municipal bonds, and mortgage-backed securities.

Example Accrual Schedule

Period	Days (Cumulative)	Accrued Interest (Cumulative)

What is the 30/360 Day-Count Convention?

The 30/360 day-count convention is a method used in finance to simplify interest calculations. It assumes that every month has 30 days and the entire year has 360 days. This method is widely used for calculating accrued interest on certain types of fixed-income securities, most notably U.S. corporate bonds, municipal bonds, and mortgage-backed securities (MBS). The main benefit is that it standardizes the calculation, removing the complexity of dealing with months of varying lengths (28, 29, 30, or 31 days).

When you need to calculate accruals using 30/360, you are determining the amount of interest earned on an investment (or owed on a loan) for a period that is shorter than a full interest payment period. This is crucial when a bond is sold between its coupon payment dates. The seller is entitled to the interest earned up to the sale date, and this calculator helps determine that exact amount precisely.

The 30/360 Accrued Interest Formula and Explanation

The formula to calculate accrued interest under the 30/360 convention is straightforward:

Accrued Interest = Principal × (Annual Rate / 100) × (Number of Days / 360)

The critical part of this formula is determining the “Number of Days”. This is not the actual number of calendar days. Instead, it’s calculated using a specific formula for the 30/360 method:

Days = (Y2 - Y1) × 360 + (M2 - M1) × 30 + (D2 - D1)

Formula Variables

Variable	Meaning	Unit / Type	Typical Range
Principal	The face value of the bond or loan.	Currency ($)	$1,000 – $1,000,000+
Annual Rate	The stated yearly interest rate (coupon rate).	Percentage (%)	1% – 10%
Y1, M1, D1	The Year, Month, and Day of the Start Date.	Date	N/A
Y2, M2, D2	The Year, Month, and Day of the End Date.	Date	N/A

Special rules apply to the day values (D1 and D2). For the common “U.S. 30/360” method, if the start date (D1) is the 31st of a month, it is treated as the 30th. If the end date (D2) is the 31st and the start date (D1) is the 30th or 31st, the end date is also treated as the 30th. This calculator handles these adjustments automatically.

Practical Examples

Example 1: Corporate Bond Trade

Imagine an investor buys a corporate bond with a settlement date that falls between coupon payments.

• Inputs:
  • Principal: $50,000
  • Annual Interest Rate: 4.5%
  • Start Date (Last Coupon Date): January 15, 2026
  • End Date (Settlement Date): April 10, 2026
• Results:
  • Calculated Days (30/360): (4-1)*30 + (10-15) = 90 – 5 = 85 days
  • Accrued Interest: $50,000 × (4.5 / 100) × (85 / 360) = $531.25

Example 2: Short-Term Note Accrual

A financial institution needs to calculate interest for a short-term note at the end of a reporting period.

• Inputs:
  • Principal: $1,000,000
  • Annual Interest Rate: 3.2%
  • Start Date: March 1, 2026
  • End Date: March 31, 2026
• Results:
  • Calculated Days (30/360): (3-3)*30 + (30-1) = 29 days (Note: D2 of 31 becomes 30)
  • Accrued Interest: $1,000,000 × (3.2 / 100) × (29 / 360) = $2,577.78

How to Use This 30/360 Accrual Calculator

Using this tool to calculate accruals using 30/360 is simple. Follow these steps:

1. Enter the Principal Amount: Input the total face value of the security or loan in the first field.
2. Provide the Annual Interest Rate: Enter the nominal annual rate as a percentage. For a 5% rate, enter ‘5’.
3. Select the Start Date: Choose the date from which the interest begins to accrue. This is often the last coupon payment date. For more details, see this guide on internal linking strategies.
4. Select the End Date: Choose the date to which interest is calculated, typically the settlement date or the end of an accounting period.
5. Review the Results: The calculator will instantly update, showing the Total Accrued Interest, the number of days calculated under the 30/360 convention, the daily interest amount, and the total period.

Key Factors That Affect Accrued Interest

• Principal Amount: The larger the principal, the more interest will accrue for any given period and rate.
• Annual Interest Rate: A higher coupon or interest rate directly leads to a higher amount of accrued interest.
• Time Period: The longer the time between the start and end dates, the more interest will accumulate. This is the primary driver of the calculation.
• Day-Count Convention: While this calculator focuses on 30/360, using a different convention like Actual/360 or Actual/365 would result in a different number of days and thus a different interest amount. The 30/360 method is a simplification.
• Compounding Frequency: This calculator assumes simple interest for the accrual period. For longer-term calculations, the frequency of compounding (e.g., semi-annually, monthly) would be a critical factor. You can learn more about this by reading about link optimization.
• Date-Specific Rules: The handling of end-of-month dates (especially the 31st and the end of February) is a subtle but important factor in the 30/360 convention.

Frequently Asked Questions (FAQ)

1. Why do financial markets use the 30/360 convention?

It simplifies and standardizes interest calculations, especially for instruments like bonds that pay interest on a regular schedule. By assuming every month has 30 days, it avoids discrepancies caused by the actual varying lengths of months. To learn about different calculation methods, check out our article on types of internal links.

2. Is the 30/360 method the same as paying less interest?

Not necessarily. Compared to an Actual/365 method, the 30/360 method uses a smaller denominator (360 vs 365), which slightly increases the daily interest factor. Over a full year, it results in the same interest, but for partial periods, there can be small differences.

3. What securities use the 30/360 day count?

It is most commonly used for U.S. corporate bonds, municipal bonds, agency bonds (like those from Fannie Mae and Freddie Mac), and mortgage-backed securities (MBS).

4. How does this calculator handle leap years?

The 30/360 convention ignores leap years entirely. Since every month is assumed to have 30 days and the year 360, the extra day in February during a leap year has no effect on the calculation.

5. What is the difference between the ‘clean price’ and ‘dirty price’ of a bond?

The ‘clean price’ is the quoted price of a bond without any accrued interest. The ‘dirty price’ (or full price) is the clean price plus the accrued interest. The buyer pays the dirty price to the seller. This calculator determines the accrued interest portion.

6. Can I use this calculator for my personal loan?

Most consumer loans (like auto loans or mortgages) in many regions use a simple interest method based on the actual number of days in a month (Actual/Actual or Actual/365). The 30/360 method is specific to certain financial markets. You should verify your loan agreement’s terms. Our student loan calculator might be more appropriate.

7. What happens if I enter an end date before the start date?

The calculator will show a negative number of days and a negative accrued interest amount, indicating an invalid period. Always ensure the end date is after the start date.

8. Are there other versions of the 30/360 rule?

Yes, there are slight variations, such as the 30E/360 (Eurobond) method, which handles end-of-month dates slightly differently. This calculator uses the U.S. 30/360 (or NASD) standard, which is the most common for the securities mentioned. Read more about CRE loans to understand other methods.