7.3.3 Accrued Interest and Clean vs. Dirty Price

Introduction

This section delves into two fundamental aspects of bond pricing: the calculation of accrued interest and the distinction between clean and dirty prices. Understanding these concepts is crucial for anyone involved in the trading of fixed-income securities, as they have direct implications on both the pricing and the returns on bond investments.

Accrued Interest Calculation

Accrued interest is the interest that accumulates on a bond between its scheduled coupon payment dates. Since bonds typically pay interest at periodic intervals, any sale of a bond before the next coupon date requires an adjustment to account for the interest that has accrued since the last payment.

Calculation Details

The accrued interest is calculated based on the day count convention applicable to the bond, which can be either:

• Actual/Actual: Used mainly for government bonds where each year is treated with its actual days (365 or 366 in a leap year).
• 30/360: Commonly used for corporate bonds where each month is considered to have 30 days, and the year has 360 days.

Here is a basic formula to calculate accrued interest:

Example Calculation:

Suppose a bond with a face value of $1,000, a 6% annual coupon, pays interest semi-annually, and the time since the last coupon is 90 days under the 30/360 convention. The accrued interest would be calculated as follows:

1. Semi-annual coupon payment = \(6% \times $1,000 / 2 = $30\)
2. Accrued interest = \($30 \times \left( \frac{90}{180} \right) = $15\)

Clean Versus Dirty Price

The pricing of bonds in the secondary market reflects two terminologies: clean price and dirty price.

Clean Price

The clean price of a bond is the price quoted without considering any interest that has accrued since the last coupon payment. It represents the bond’s value based solely on its yield and market factors, excluding accrued interest.

Dirty Price

The dirty price, also known as the full price or invoice price, is the total price that a buyer pays for a bond. It includes the clean price plus any accrued interest up to the date of the transaction. This is the price at which bonds are typically traded and reflects the true economic outflow from the buyer to the seller.

Mermaid Diagram Example:

    graph TB;
	    A[Start] --> B[Clean Price Determined by Market Conditions];
	    B --> C[Accrued Interest Calculated Based on Days Since Last Coupon];
	    C --> D[Add to Clean Price to Get Dirty Price];
	    D --> E[Dirty Price Equals Transaction Price in Market];

Practical Implications

• Investors: Understanding whether prices are quoted as clean or dirty is vital when buying or selling bonds, as it affects the transaction’s cash settlement amount.
• Bond Market Dynamics: It allows transparency in the bond market by clearly separating earned interest from market-driven price changes.

Glossary

• Accrued Interest: Interest that accumulates on a bond from the last payment date until the purchase or sale date.
• Clean Price: Bond price excluding any accrued interest. The quoted price.
• Dirty Price: Bond price including accrued interest; reflects the actual transaction value.

Additional Resources

• CFA Institute Insights on Fixed Income
• Investopedia on Bond Pricing
• Canadian Securities Administrators (CSA) website for regulatory insights

Summary

In this section, we explored the computation of accrued interest and clarified the difference between clean and dirty bond prices. These fundamental bond pricing properties are essential for accurate valuation and trading of fixed-income securities. Understanding these concepts allows investors and analysts to gauge the true cost and return of bond transactions, thereby making more informed investment decisions.