What a 30-year government bond is — why price falls when yield rises
A 30-year government bond is an IOU under which the state repays the principal in 30 years and pays fixed interest in the meantime. When market rates rise, existing bonds fall in price, because newly issued bonds pay more and the old ones must be discounted to compete
The three lines
- Structure — principal repaid at a 30-year maturity, with fixed interest paid along the way
- Price and yield — they move in opposite directions, by arithmetic rather than sentiment
- Maturity effect — the same rate move shifts a 30-year bond's price far more than a 2-year's
Key questions
- Why do bond prices fall when yields rise?
- Because newly issued bonds offer better terms. Suppose you paid $1,000 for a bond paying 4%, or $40 a year. Market rates then rise to 5%, so a fresh $1,000 bond pays $50. Nobody will pay $1,000 for your $40. You must discount it until the buyer's return matches 5% — roughly $800. The yield rose and your price fell; these are two descriptions of one event.
- What is the difference between a 30-year and a 2-year bond?
- The time until repayment. Longer waits carry more uncertainty about inflation and government finances, so long bonds usually pay more. They also move far more in price for the same change in rates, because a rate change applies to thirty years of interest payments instead of two. Bond markets call this sensitivity duration.
- Do rising US yields raise Korean borrowing costs?
- Not by direct linkage, but through transmission. When US long yields rise, capital moves toward US bonds, which narrows the room for other central banks to cut rates. Korean long-dated government bond yields tend to move in the same direction, and fixed-rate mortgages are priced off those. The effect arrives with a lag rather than immediately.
On August 17, 2026 the US 30-year Treasury yield reached 5.31%, the highest since 2007.
There is something odd in that sentence. The yield rose — so why is it described as a bond selloff? Isn't a higher yield good news?
Both descriptions refer to the same event. Here is why.
1. What the instrument is
A 30-year government bond is an IOU issued by the state. Three terms define it.
| Term | Detail |
|---|---|
| Principal | The face value is repaid at maturity, 30 years out |
| Interest | A fixed rate (the coupon) is paid periodically until then |
| Trading | It can be bought and sold at any time before maturity |
The third line is the crucial one. You do not have to hold it for 30 years. It can be sold at any point, which means it has a price that moves daily.
2. Why price and yield move in opposite directions
One number makes it obvious.
Suppose you buy a $1,000 bond paying 4%. Every year you receive $40.
Market rates then rise to 5%. Newly issued government bonds pay $50 a year on the same $1,000.
| Option | Annual interest | Cost |
|---|---|---|
| Your existing bond | $40 | You paid $1,000 |
| A newly issued bond | $50 | $1,000 |
Will anyone pay $1,000 for yours? No — the same money buys more next door.
To sell, you must discount it. By how much? Until the buyer's return matches 5%. Roughly $800: pay $800, collect $40 a year, and the return is 5%.
As the rate moved from 4% to 5%, your bond's price fell from $1,000 to about $800.
That is why "yields rose," "there was a bond selloff," and "bond prices fell" all describe one event. The causation runs both ways: if enough holders sell and prices drop, buyers get the same coupon for less money, and the yield rises.
The example above is simplified. Real pricing accounts for coupon frequency, remaining maturity and accrued interest.
3. Why long bonds swing harder
The same one-point rate move produces a much bigger price change at longer maturities.
The reason is straightforward: the number of interest payments the rate change applies to differs.
| Maturity | Remaining payments (annual) | Effect of a 1-point rate rise |
|---|---|---|
| 2-year | 2 | Small price decline |
| 10-year | 10 | Moderate |
| 30-year | 30 | Large price decline |
A holder of two-year paper endures a below-market coupon for two years and then gets the principal back. A holder of 30-year paper endures it for three decades, and the market prices all thirty years of that shortfall today.
Bond markets call this sensitivity duration. The term sounds technical; the meaning is simply "how much the price moves when rates move by one percent."
4. Where 5.31% actually lands
Government interest costs. Fortune reported on August 13 that the US is set to pay the most for 30-year debt in a quarter of a century. Each new issue carries a higher coupon, that interest widens the deficit, the deficit requires more issuance, and more issuance pushes prices down again.
Fixed-rate mortgages. A 30-year fixed mortgage prices off the 30-year government benchmark, because it is funding of matched maturity.
Pensions and insurers. Institutions owing money decades from now are the natural buyers of long bonds. Rising yields cut the value of what they hold and improve the return on what they buy next — a loss on the existing book, a gain on the future one.
The relative appeal of equities. If a risk-free asset pays above 5%, the return demanded from riskier assets rises with it.
5. Why yields are rising now
The reporting reviewed here identifies three causes. All three concentrate at the long end.
| Driver | Why it bites harder at long maturities |
|---|---|
| Rising government spending | The further off repayment is, the more doubt attaches to it |
| Heavy long-dated issuance | More supply means lower prices, and lower prices mean higher yields |
| Five years of above-target inflation | Money due in 30 years is eroded far more than money due in two |
The third is specifically a long-bond problem. That inflation has exceeded target for five years is a passing inconvenience to a two-year holder and a baseline assumption to a thirty-year one.
6. How this reaches Korea
Not by direct linkage. Three channels.
- Capital flows — US long paper above 5% attracts money, narrowing the room for the Bank of Korea to cut rates.
- Korean long-dated bonds — domestic 30-year government bond yields tend to move in the same direction.
- Lending rates — fixed-rate mortgages price off those domestic long bonds.
August 17 was a substitute public holiday in South Korea, and both equity and bond markets were closed. How far Korean long bonds follow this move is only observable from the August 18 session.
7. What remains unconfirmed
The pricing example is simplified, as noted above.
The Korean 30-year yield could not be confirmed for August 17, because the market was shut.
Transmission lags were not measured. The precise delay and magnitude with which US long yields reach Korean lending rates has not been quantified by this page.
Related coverage: "US 30-year Treasury yield hits 5.31%," "S&P 500 closes at 7,745.06," and "What deleveraging is."
Sources
- Bloomberg — US Bond Selloff Drives 30-Year Yields to Highest Since 2007
- Fortune — US set to pay most for 30-year debt in quarter of a century
- FRED (St. Louis Fed) — Market Yield on U.S. Treasury Securities at 30-Year Constant Maturity (DGS30)
- Axios — What rising Treasury yields are telling us
- CNBC — Treasury yields rise, hurt by rising oil prices as traders focus on Middle East threats