What Is a Convertible Bond? How Conversion Works, With 17 Years of Fund Data
Michael Whitman
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Trading since 2017, full-time since 2020
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YouTube creator, 137K subscribers
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Published 2026-09-14
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Updated
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Reviewed
Convertible bond is a bond that can be exchanged for a fixed number of the issuer's shares at the holder's choice. You accept a lower coupon than a plain bond in return for that conversion right, so the instrument behaves like debt when the shares are weak and like equity when they are strong.
A convertible bond is a corporate bond that the holder can swap for a fixed number of the issuer’s shares, so it pays like debt and can turn into equity. On 17 years of fund data it took about 70% of the S&P 500’s monthly falls and 73% of its rises, which is a smaller version of the stock market rather than a free floor under it.
A convertible bond is two things in one wrapper: a corporate bond, and a call option on the issuer’s shares. Everything it does follows from which of the two is in charge on the day you look at it.
How it works
It pays a coupon and matures like any bond. The difference is a right: you may exchange it for a set number of shares instead of taking the cash back at maturity.
The conversion ratio is that set number of shares. A $1,000 bond with a conversion ratio of 20 converts into 20 shares, so its conversion price is $1,000 divided by 20, or $50 a share. Converting only makes sense once the shares trade above $50, because below it the 20 shares are worth less than the cash.
The right is not a gift. A convertible pays less than the same company’s plain debt, and that shortfall is the price of the option, paid in installments out of income the holder would otherwise have had.
When the shares run, the option dominates. The conversion right becomes the valuable part and the price starts to track the equity. When the shares fall, the bond dominates. The option heads toward worthless and the price settles toward what the debt alone is worth, which is called the bond floor.
For a plain bond the quick yield figure is current yield, which on 23 Oct 2026 sat within a quarter point of yield to maturity on only 32 of 299 Treasuries.
The floor is not a floor
The floor is the value of the debt, and debt is only worth something if the company can pay.
The case that breaks it is a company in real trouble. The shares collapse, so the option is worth little. The company’s credit weakens, so the bond floor drops as well. Both halves fall together because both are claims on the same business, and the protection is weakest in exactly the conditions people buy it for.
Volatility works for the holder here, which is unusual for a bond. The option part is worth more when the shares swing more, so a rise in expected movement can lift a convertible even if nothing else changes. Volatility covers that measure in its own right.
A worked example
Take a hypothetical convertible paying 2% where the same company’s plain debt pays 5%, convertible into shares that currently trade 30% below the conversion price.
On a $1,000 bond that is $20 a year instead of $50, so the holder gives up $30 a year. Over a five-year life that is $150, or 15% of face value, paid for an option that starts out of the money.
The shares must rise more than 30% before the option is worth exercising at all, and further than that before the total beats simply owning the plain bond. Neither threshold appears on the front of an offering document, and both are arithmetic anyone can redo from the coupon and the conversion price.
The option can stay worthless for years. When the S&P 500 fell 56.8% from October 2007 to March 2009, it did not close above its 2007 peak again until 28 March 2013, about 5.5 years in which a conversion right struck above the old price had nothing to convert into while the lower coupon kept being paid.
The original data
Sample: monthly total returns, from dividend-adjusted month-end closes, for the SPDR Bloomberg Convertible Securities ETF (CWB), May 2009 to September 2026, 209 months. Each month is set against SPY, an S&P 500 fund, and LQD, an investment-grade corporate bond fund. SPY fell in 65 of those months and rose in 143 (in November 2010 its total return was zero). A fund holds many convertibles, so this measures the asset class, not any single bond.
The finding: convertibles gave up almost as much of the upside as they avoided of the downside. In SPY’s 65 falling months the average was -3.46% for SPY and -2.43% for CWB, a down capture of 70.32%. In its 143 rising months SPY averaged +3.44% and CWB +2.52%, an up capture of 73.28%. CWB fell less than SPY in 46 of the 65 falling months, but it fell too in 53 of them. Over the whole sample CWB returned 557.01% and SPY 1,090.44%.
The worst month is where the floor was supposed to show, and it did not. March 2020 was SPY’s worst month in the sample at -12.49%. CWB lost 13.10% and ICVT 13.81%, both more than the stock fund, while LQD, the plain corporate bond fund, lost 6.31%. Measured daily, from 19 February to 23 March 2020 CWB fell 32.06% from its high against SPY’s 33.72%. From its February 2021 high, CWB’s fall reached 29.86% by 16 June 2022; SPY’s fall from its January 2022 high reached 24.50% by 12 October.
The check. A second fund that tracks a different convertible index, iShares Convertible Bond ETF (ICVT), gives the same picture over its own history from July 2015: 64.05% of SPY’s falling months and 72.41% of its rising months, across 135 months. That check covers a second fund manager and a second index (both indexes are Bloomberg’s), not a second index provider or a second data feed for the returns. For the price feed, Nasdaq’s daily closes for CWB and ICVT matched Yahoo’s on all 2,513 shared sessions from 27 October 2016 to within $0.01. Every month is in the convertible fund return file.
What it cannot show. Fund returns include fees and the fund’s own mix of issuers, and 17 years is one long stretch of a mostly rising market. It says nothing about any single convertible.
Why companies issue them
A convertible lets a company borrow more cheaply than its credit alone would allow. The option sweetens the deal, so the coupon can be set below what plain debt would cost.
It is also equity sold later and at a higher price. If the shares perform, the bonds convert and the debt disappears, replaced by shares effectively sold above today’s price. That appeals to a company that thinks its shares are cheap.
Issuers are often growing companies that want equity-like funding without selling shares at the current price, and the structure tells you something about who is on the other side of it.
Existing shareholders are diluted if it converts, which is the cost the company accepted for the cheaper coupon.
How it connects to options
The conversion price works like a strike price. The shares have to trade above it before converting beats taking the cash.
Equity sensitivity is measured as delta. A convertible far above its conversion price moves almost one for one with the shares; one far below barely responds.
Convertible arbitrage is a classic hedge fund trade: buy the bond, sell the shares short in proportion to delta and keep adjusting. That hedging can lift short interest in a stock after a new issue without anybody holding a negative view of the company.
And the bond half is valued like any other debt, off its yield and duration, so rising rates can drag the floor down even when the company is healthy.
When it fails
The characteristic failure is buying it as a safer way to own the shares. In ordinary down months the funds above did fall less than the stock market. In the month that mattered most, March 2020, they fell more, because the option and the credit weakened together.
A second failure is ignoring how far out of the money the conversion is. The shares often have to rise a long way before the right is worth anything.
A third is forgetting it can be callable too. Many convertibles let the issuer force conversion once the shares clear a level, which caps the upside the holder paid for.
A fourth is treating the lower coupon as a bargain. It is the option premium, charged in installments.
And a fifth is holding one in size in a single name, which stacks credit risk and equity risk in the same position, the concentration leverage makes expensive elsewhere.
Questions people ask
Is a convertible bond debt or equity?
Debt until it converts. It sits on the balance sheet as a liability and pays interest, and only becomes shares if the holder converts. Its price, though, can behave much more like the shares when they trade well above the conversion price.
What is a conversion ratio?
The number of shares one bond converts into. A $1,000 bond with a ratio of 20 converts into 20 shares, so the conversion price is $50.
Are convertible bonds safer than stocks?
They have been less volatile, not safer in every month. From May 2009 to September 2026 CWB caught 70.32% of SPY’s average falling month, but in March 2020 it lost 13.10% to SPY’s 12.49%.
What happens to a convertible bond if the stock falls?
The conversion right loses value and the price drifts toward the bond floor, the value of the debt alone. If the fall reflects trouble at the company, the floor itself drops as the credit weakens.
Related
Corporate bond covers the debt half and the credit risk underneath it. Volatility covers the variable that prices the option half. And leverage covers what concentration in a single issuer does to an account.
A convertible is the mirror image of a callable bond. There you sold an option and were paid in extra coupon; here you bought one and paid in reduced coupon. Same mechanism, opposite direction — and knowing which side of that trade you are on tells you most of what you need about any bond with a feature bolted to it.
— Michael Whitman
This page is educational, not financial advice. Test every idea on your own charts before risking money.