What is Average Maturity, Macaulay Duration, and Modified Duration of Debt Funds?

Not all debt funds carry the same interest rate risk, even if they invest in similar bonds. Usually, this difference comes from:

• What is the remaining maturity of bonds held by the fund
• When cash flows are received, and 
• How sensitive is the portfolio to changes in interest rates

Answers to these questions are usually obtained by analysing three different portfolio metrics: Average Maturity, Macaulay Duration, and Modified Duration. They allow investors to evaluate the fund's maturity profile and estimate its response to interest rate movements.

Read this article till the end to learn what each metric means and how they differ.

Average maturity of debt funds is the “weighted” average time to maturity of all the bonds held in a debt fund’s portfolio. The weights are assigned based on each bond’s proportion in the fund’s portfolio. 

Several investors analyse average maturity to judge how a debt fund is likely to behave under different market conditions. Generally, it gives an idea of the fund's interest rate risk and return potential. Also, this metric is widely used to compare different debt fund investments.

Let’s understand in detail: 

Macaulay Duration is the weighted average time it takes for an investor to recover the price paid for a bond through the present value (PV) of its expected cash flows, which includes both coupon payments and principal repayment. 

 It helps investors to estimate:

• How a debt fund may respond to interest rate changes
• Whether the fund suits their investment period, and 
• How the fund manager manages interest rate risk within the portfolio.

Let’s understand in detail:

Modified Duration is the approximate “percentage change” in a debt fund’s NAV (Net Asset Value) for a 1% (100 basis points) change in interest rates (YTM), assuming all other factors remain the same. Using it, investors assess how sensitive a debt fund is to interest rate movements.

Modified Duration is calculated using Macaulay Duration. Mathematically, the relationship is shown by the following formula:

Where,

• YTM = Yield to Maturity (annualised)

• f = coupon frequency (e.g., 2 for semi-annual, 1 for annual)

Let’s study an example to better understand:

Suppose your debt fund investment has a Macaulay Duration of 5 years and a Yield to Maturity (YTM) of 8% p.a. Assume that the portfolio of the fund holds semi-annual coupon bonds. 

Now,

*

Interpretation? This means the fund's NAV is expected to change by about 4.81% for every 1% change in market interest rates.

• If market interest rates rise by 1%, the fund's NAV will fall by about 4.81%.

• If market interest rates fall by 1%, the fund's NAV will rise by about 4.81%.

*This estimate assumes all other factors remain unchanged. The Modified Duration is calculated in “years” but interpreted as a % change.

Below is a detailed comparison of the three debt-fund metrics - Average Maturity, Macaulay Duration, and Modified Duration:

So, now you know about the Average Maturity, Macaulay Duration, and Modified Duration of a debt fund investment. These are three different measures used by investors to evaluate a debt fund from different perspectives. 

If we were to revise:

• Average Maturity represents the weighted average time left until the bonds within the portfolio matures.
• Macaulay Duration is the weighted average time it takes to recover your investment through the present value of the fund's expected cash flows.
• Modified Duration shows how much the fund's NAV is expected to change for every 1% change in market interest rates.

Rather than relying on historical or past performance alone, investors may use these measures to make potentially better fund selection decisions.

For more information, you can visit ww.tatamutualfund.com/deshkarenivesh. The Investor Service Centre of Tata Asset Management Pvt. Ltd. is located at Mulla House, Ground Floor, 51, M.G. Road, Near Flora Fountain, Mumbai – 400 001, Maharashtra. The office hours are Monday to Friday, 9:00 AM to 5:30 PM. For assistance, you can also call (022) 6282 7777 from Monday to Saturday, 9:00 AM to 5:30 PM, or email service@tataamc.com

1. How is “Macaulay Duration” different from the “Average Maturity” of debt funds?

Average Maturity only considers when each bond will mature. Whereas, Macaulay Duration determines when the investor recover the investment through the present value of all expected cash flows It considers both:

• Periodic interest (coupon) payments, and
• The final principal repayment.

Since investors receive part of their money through coupon payments before maturity, Macaulay Duration is usually shorter than the average maturity of debt funds (except for zero-coupon bonds where both Average Maturity & Macaulay Duration is same).

2. Does a higher Modified Duration always mean better performance?

Realise that a higher Modified Duration indicates greater sensitivity to interest rate changes. It does not represent the performance of a debt mutual fund.

3. Can two debt funds have the same Average Maturity but different Macaulay Duration?

Yes, two funds may have similar Average Maturity but different Macaulay or Modified Duration due to differences in coupon rates, Yield to Maturity (YTM), and the mix of bonds in their portfolios.