Bond Duration Explained Simply
Maturity tells you when a bond ends. Duration tells you how sensitive it is to interest rate changes before it ends. Two bonds can have identical maturities but very different price behaviour when rates move — because of how their cash flows are structured. Duration is the tool that captures this difference. It sounds technical, but the core idea is simple.
The problem with using maturity alone
Imagine two 5-year bonds, both at face value:
- Bond A: Pays a 10% annual coupon. You receive ₹100/year for 4 years, then ₹1,100 in year 5.
- Bond B: Zero-coupon bond. Pays nothing for 4 years, then ₹1,610 at maturity (the equivalent in present value terms).
Both mature in 5 years. But Bond A returns most of your money through coupons — you get 40% of your total cash back in the first 4 years. Bond B gives you nothing until year 5.
If rates rise by 1% today, which bond falls more in price? Bond B — by a lot. All its value sits at year 5, maximally exposed to the new discount rate. Bond A has already returned much of its value early, so it is less exposed.
Maturity says they are equal. Duration correctly says they are not.
Macaulay duration: the weighted average wait
Macaulay duration is the weighted average time to receive all of a bond’s cash flows, where the weights are the present value of each cash flow as a proportion of the total bond price.
For a 3-year bond at ₹1,000 face value, 8% annual coupon, and a market yield of 8% (so price = ₹1,000):
| Year | Cash flow | PV at 8% | Weight (PV ÷ price) | Year × weight |
|---|---|---|---|---|
| 1 | ₹80 | ₹74.07 | 7.41% | 0.074 |
| 2 | ₹80 | ₹68.59 | 6.86% | 0.137 |
| 3 | ₹1,080 | ₹857.34 | 85.73% | 2.572 |
| Macaulay duration | 2.783 years | |||
The Macaulay duration is 2.78 years — less than the 3-year maturity, because you start receiving cash flows before maturity. For a zero-coupon bond, Macaulay duration always equals maturity exactly, because all cash flows arrive on the last day.
Modified duration: the price sensitivity number
Modified duration converts Macaulay duration into a direct price sensitivity estimate:
Modified duration = Macaulay duration ÷ (1 + yield)
For our bond: 2.783 ÷ 1.08 = 2.58
This means: for every 1% (100 basis point) rise in market yield, this bond’s price will fall by approximately 2.58%. For a 1% rate fall, the price rises by approximately 2.58%.
Modified duration is the number most practitioners use because it directly answers the question: "how much does my bond move for a given rate change?"
Duration across different bond types
| Bond type | Approximate modified duration | Price impact of 1% rate rise | Price impact of 1% rate fall |
|---|---|---|---|
| 91-day T-Bill | ~0.24 | −0.24% | +0.24% |
| 1-year T-Bill | ~0.95 | −0.95% | +0.95% |
| 3-year NCD (8% coupon) | ~2.6 | −2.6% | +2.6% |
| 5-year G-Sec (7% coupon) | ~4.2 | −4.2% | +4.2% |
| 10-year G-Sec (7% coupon) | ~7.1 | −7.1% | +7.1% |
| 30-year G-Sec (7% coupon) | ~13.5 | −13.5% | +13.5% |
The 30-year G-Sec loses more than 13% of its price on a 1% rate rise. That is equity-like volatility from a "safe" government bond. This is why long-duration bond funds can have very turbulent periods when central banks are hiking rates aggressively.
DV01: the rupee value of a basis point
DV01 (Dollar Value of 01, or in our context, Rupee Value of 1 basis point) is a close sibling of modified duration. It tells you exactly how many rupees you gain or lose for a 0.01% (1 basis point) move in yield:
DV01 = Modified duration × Bond price × 0.0001
For a ₹10 lakh position in a bond with modified duration 5 and price ₹100:
DV01 = 5 × ₹10,00,000 × 0.0001 = ₹500 per basis point
So a 0.25% (25 bps) rise in rates costs this position ₹500 × 25 = ₹12,500. DV01 is especially useful for understanding the monetary impact of rate moves on a specific position size.
What duration is not
Duration only captures interest rate risk. It says nothing about:
- Credit risk: The issuer could default regardless of what rates do. A short-duration junk bond can be far riskier than a long-duration G-Sec.
- Liquidity risk: A bond might have low duration but be nearly impossible to sell at a fair price in a stressed market.
- Convexity: Modified duration is a linear approximation. For large rate moves (>2%), a more precise measure called convexity is needed. For retail investors holding bonds to maturity, this is mostly a technical footnote.
Using duration to match your investment horizon
The most practical use of duration for a retail investor is horizon matching: choosing a bond whose duration approximately matches your investment horizon.
- If you need the money in 2 years, a bond with duration ~2 means you are largely insulated from rate changes — price moves and reinvestment risk roughly offset each other over that period.
- If you are investing for 7 years, a 5-year bond with duration ~4 means you face reinvestment risk (you must reinvest when the bond matures in 5 years, at whatever rate then prevails).
- If you are investing for 1 year, T-Bills (duration <1) are the natural match.
This does not mean you must match exactly. But being aware of the mismatch — and whether you can afford the mark-to-market volatility if you need to exit early — is what duration helps you reason about.
Duration and bond funds
Debt mutual fund categories are partly defined by their duration targets. Long-duration funds hold bonds with duration >7 years; short-duration funds target 1–3 years; liquid funds stay below 91 days.
In a rate-hiking cycle, long-duration bond funds will show NAV declines even though the underlying bonds will eventually pay out fully. Investors who panic-sell during these declines crystallise losses they could have avoided by staying invested. Understanding duration helps you hold through volatility that is temporary for a buy-and-hold investor.
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