Inflation-Linked Bond (KTBi) Real Return Calculator

Estimate the after-tax real return of a Korean inflation-linked treasury bond (KTBi) with CPI-linked principal, principal-increase tax, and breakeven inflation.

Korea-related planning note

This English page is a planning estimate. If the topic involves Korean taxes, pensions, insurance, loans, or public programs, confirm the current Korean rules, limits, and eligibility before acting.

Scenario inputs

Adjust the assumptions to compare gross value, cost drag, tax drag, and downside value.

Net ending value

$33,538

Gross value before drag

$34,852

Estimated tax

$624

Fee and tax drag

$1,314

Total contributions

$30,000

Net gain

$3,538

11.8%

Downside value

$31,861

Scenario readout

Tax drag is included in net value

This is a Korea-based inflation-linked treasury bond (KTBi) scenario. Coupon interest and, for issues on or after 1 January 2015, the principal increase are taxed at 15.4%. Confirm the specific issue terms and current Korean tax rules before investing.

Related calculators

What is a Korean inflation-linked treasury bond (KTBi)?

KTBi (Korea Treasury Bond indexed to inflation) is a government bond whose principal is linked to the Consumer Price Index (CPI). When prices rise after you buy, the principal grows, and the fixed real coupon is paid on that inflation-adjusted principal. It is Korea’s main inflation hedge in the government bond market, comparable to US TIPS.

There is one point that is easy to miss. The fact that principal grows with inflation does not mean the whole increase is yours to keep. For KTBi issued on or after 1 January 2015, the principal increase (the inflation compensation) is taxed as interest income, so the after-tax real yield is lower than the coupon rate. This calculator shows the after-tax real return with inflation removed, not just the headline nominal number.

Korea-based rules (2026): This calculator models Korean inflation-linked treasury bonds under Korean tax rules. Coupon interest and, for post-2015 issues, the principal increase are taxed at 15.4% (14% income tax + 1.4% local surtax). Rules, coupon rates, and CPI change over time, so confirm the terms of the specific bond series and current tax law before investing.

How KTBi returns are built

1. Index ratio and principal adjustment

The principal is adjusted daily by an index ratio. The index ratio is the reference CPI on the payment date divided by the reference CPI at issuance, using a 3-month lag with interpolation. For example, if prices rise 2.5% per year for 10 years, the index ratio is about 1.28, so a principal of 10,000,000 KRW grows to roughly 12,800,000 KRW.

  • • Index ratio: reference CPI at payment ÷ reference CPI at issuance
  • • Adjusted principal: face value × index ratio
  • • Principal increase: adjusted principal − face value (inflation compensation)

2. Real coupon and principal floor at maturity

The real coupon rate is fixed at issuance and is paid semi-annually on the inflation-adjusted principal, so the cash coupon grows as principal grows. The standard maturity is 10 years. The redemption amount at maturity equals face value × max(index ratio, 1), which means the original principal is guaranteed even if deflation occurs.

In other words, when prices rise, principal and coupons rise together to hedge inflation, and when prices fall you still receive at least the face value at maturity. This deflation floor is what makes KTBi a safe-asset building block.

3. Taxing the principal increase (the key variable)

Coupon interest has always been taxed at 15.4%. The important change concerns the principal increase. Under the Enforcement Decree of the Income Tax Act (Article 22-2), KTBi issued on or after 1 January 2015 has its principal increase taxed as interest income at 15.4%. Bonds issued before 31 December 2014 had a tax-exempt principal increase, but virtually all KTBi traded in 2026 are post-2015 issues and therefore taxable.

Because the inflation compensation only preserves purchasing power rather than being a true real gain, taxing it means that the higher inflation is, the larger the tax bite and the more the after-tax real yield is eroded. That is why KTBi should never be judged by nominal yield alone.

Nominal return vs real return

The nominal return ignores inflation, while the real return removes inflation to show the true increase in purchasing power. They are connected by the Fisher relation.

real return = (1 + nominal return) ÷ (1 + inflation) − 1

The pre-tax real yield of KTBi converges toward the real coupon rate, because principal grows with inflation and offsets it. Once tax applies to both the coupon and the principal increase, however, the after-tax real yield falls below the coupon rate, and when inflation is very high the after-tax real yield can even turn negative. The calculator shows pre-tax and after-tax nominal returns alongside pre-tax and after-tax real returns so the gap is obvious at a glance.

How to use this calculator

Step 1: Holding period and principal

Pick a 5, 10, 20, or 30-year preset or enter your own, then enter the amount invested at face value on the purchase date.

Step 2: Real coupon rate and inflation

The coupon rate is the real coupon of KTBi and varies by issue series. The expected annual inflation rate can be negative (deflation) so you can stress-test scenarios.

Step 3: Principal-increase tax toggle

Choose taxable for post-2015 issues or exempt for pre-2015 issues, and see immediately how the after-tax real yield changes.

Step 4: Real yield and breakeven inflation

Review the after-tax real CAGR and after-tax real purchasing power, then enter a comparable nominal government bond rate to see the breakeven inflation (BEI). If actual inflation exceeds BEI, KTBi wins; if it is lower, the plain nominal bond wins.

Scenarios

Hedging inflation in a high-price era

If you expect prices to rise quickly, KTBi grows principal and coupons together to protect purchasing power. But higher inflation also means a larger tax on the principal increase, so judge with after-tax real yield rather than nominal yield. Change the inflation assumption to see how the after-tax real return responds.

Defending retirement purchasing power

For money that must last many years, the biggest risk is losing real value to inflation. KTBi is a government-guaranteed safe asset that reflects inflation in the principal, so it suits long-term purchasing-power defense. Check whether the after-tax real purchasing power (in today’s won) stays at or above your principal before relying on it for retirement planning.

Comparing with a plain government bond (BEI)

The breakeven inflation rate (BEI) is the nominal yield of a plain government bond minus the real coupon of KTBi. If average inflation is higher than BEI, KTBi is the better choice; if lower, the nominal bond wins. For example, a 3.5% nominal bond and a 1.5% KTBi coupon give a BEI of 2.0%, so expecting inflation above that level is a reason to favor KTBi.

Frequently asked questions

Q. Why is the real yield different from the coupon rate?

A. The pre-tax real yield converges toward the real coupon rate, but once 15.4% tax applies to the coupon and the principal increase, the after-tax real yield falls below the coupon rate.

Q. Is the principal increase really taxed?

A. Yes. For KTBi issued on or after 1 January 2015 the principal increase is taxed as interest income at 15.4%. Pre-2015 issues were exempt, but current KTBi are effectively all taxable.

Q. Do I lose principal if deflation occurs?

A. No. The maturity redemption equals face value × max(index ratio, 1), so the original principal is guaranteed even if prices fall.

Q. What is breakeven inflation (BEI)?

A. It is the nominal government bond yield minus the KTBi real coupon. If actual inflation is above BEI, KTBi wins; if below, the nominal bond wins.

Q. Can I rely on this result directly?

A. This is an educational estimate that uses an annual approximation. Real coupons are paid semi-annually and the index ratio is computed daily, so confirm the terms of the specific issue and current Korean tax rules before acting.

Before you invest

  • Judge by real yield. The after-tax real return with inflation removed is the true increase in purchasing power.
  • Account for the principal-increase tax. Higher inflation means a larger tax on the inflation compensation, eroding real yield.
  • Compare against BEI. Actual inflation must exceed the breakeven inflation for KTBi to beat a plain bond.
  • Rates and inflation are assumptions. Coupon and inflation vary by series and market, so update them with real values.
  • Watch aggregate financial income. If coupon and principal increase combine with other financial income above 20 million KRW per year, comprehensive taxation may apply.