What it means
Most bonds repay principal in one amount at maturity, whereas a sinkable bond unwinds gradually. The indenture sets a sinking fund schedule, and the issuer retires part of the issue at each scheduled date, often at par.
The MSRB describes the municipal version directly. A portion of the principal may be subject to prepayment based on a predetermined sinking fund schedule, or based on the availability of accumulated funds set aside for that purpose.
The mechanism is a planned redemption, not an optional call. The issuer usually has two ways to satisfy the schedule.
It can call bonds at the stated redemption price, or it can buy bonds in the open market and deliver them against the requirement. When the bond trades below par, open-market purchase is the cheaper route, and that buying can support the price.
Holders trade certainty for safety. Spreading repayment over time and earmarking funds reduce the chance of a large default at maturity, which can lower the issuer's borrowing cost.
The offset is that any particular holder may be redeemed early, losing future interest and facing reinvestment at current rates. Some funds accumulate cash before redemption, while others operate as a pure repurchase obligation, and the MSRB notes that in some structures redemptions occur when the accumulated fund reaches a stated amount.
Either way, the holder should expect principal to arrive earlier than the stated maturity. Because the cash-flow timing differs from a bullet bond, investors often evaluate a sinkable bond on yield to average life rather than yield to maturity.
Average life weights each principal repayment by when it is expected to arrive, so two bonds with the same maturity can have very different average lives. Sinking fund terms also interact with market prices in both directions, since mandatory buying supports a depressed price but mandatory redemption caps the upside a holder can keep when rates fall sharply, which is why the coupon concession is usually modest rather than dramatic.
The schedule is contractual, not discretionary, so holders should read the indenture for redemption prices, selection methods and any option to double up repurchases, because those terms, not the words sinking fund alone, determine what the holder actually receives.
In practice
Real-world examples.
Example
A fictional $20 million issue retires 5% of its bonds each year through the fund. After ten years, roughly half the principal is already repaid rather than outstanding to maturity. A holder who planned for a single repayment in year twenty has to plan for partial repayments along the way.
Example
A fictional bond trades at 96. The issuer buys on the market to satisfy the sinking fund instead of calling at 100, and that steady demand cushions the price. Holders who wish to sell during this period find a more willing buyer than they might otherwise.
Example
A fictional holder's bonds are selected for redemption at par. The returned principal must be reinvested at lower current rates, cutting the holder's expected income. The holder checks the indenture to see how bonds are selected and whether any notice period applies.
Formula
Calculation
Illustrative annual retirement: scheduled repurchase = issue size x annual sinking percentage. For $20,000,000 at 5%, the issuer retires $20,000,000 x 5% = $1,000,000 of bonds per year.
Illustrative coupon saving: if the feature lowers the coupon from 6.3% to 6.0%, first-year interest saved = $20,000,000 x 0.3% = $60,000. Figures are fictional and simplified.
Illustrative open-market saving: if the issuer must retire $1,000,000 of face value and the bonds trade at 96, buying in the market costs $1,000,000 x 0.96 = $960,000, which is $40,000 less than calling at par ($1,000,000 x 1.00).Case study
Seen in the real world.
This case study is fictional and illustrative. An income investor buys a 20-year sinkable bond and assumes the cash flows match a bullet bond maturing in 20 years. In year six, part of her holding is redeemed at par under the schedule. Rates have fallen, so the returned principal now reinvests at a lower yield, and her income drops below plan.
She rebuilds the model using yield to average life and staggers maturities across issues. The sinking fund still provides safety, but her projections now reflect that repayment arrives in pieces. She also asks the issuer's trustee for the redemption schedule and keeps a calendar of dates when partial calls are possible. The investor and the bond are invented for illustration and describe no real security.
Watch out
Common mistakes.
- Evaluating a sinkable bond only on yield to maturity instead of yield to average life.
- Assuming the sinking fund guarantees repayment; it schedules repurchases but does not eliminate credit risk.
- Forgetting that early redemption returns principal that must be reinvested at prevailing rates.
Questions
People also ask.
How does a sinking fund retire bonds?
Per the indenture schedule, the issuer calls bonds at the stated redemption price or buys them in the open market and retires them against the requirement.
Is a sinkable bond safer?
Generally the scheduled repayment and reserved funds reduce default risk, which can lower the coupon the issuer must pay.
What yield measure fits a sinkable bond?
Yield to average life, because principal returns in stages rather than at one maturity date.
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