I Bond Calculator
Calculate Series I savings bond interest from fixed rate, inflation rate, and holding period. Enter values for instant results with step-by-step formulas.
Calculator
Adjust values & calculateYear-by-Year Growth
Formula
The composite rate combines the fixed rate (set at purchase, lasts 30 years) with the semiannual inflation rate (updated every 6 months based on CPI). Interest compounds semiannually. A 3-month interest penalty applies if redeemed before 5 years.
Last reviewed: January 2026
Worked Examples
Example 1: Standard I Bond Purchase Held to Maturity
Example 2: Early Redemption with Penalty
Background & Theory
The I Bond Calculator applies the following established principles and formulas. Finance and investing rest on the foundational concept of the time value of money: a dollar received today is worth more than a dollar received in the future, because present funds can be deployed to earn a return. This principle underlies virtually every valuation technique in modern finance. The future value of a present sum P growing at rate r over n periods is expressed as FV = P(1 + r)^n, while the present value of a future cash flow FV is PV = FV / (1 + r)^n. Compound growth amplifies returns significantly over long horizons, a dynamic often described as the eighth wonder of the world. Net Present Value (NPV) extends these mechanics to evaluate investment projects by summing the present values of all expected cash flows minus the initial outlay: NPV = sum[CF_t / (1 + r)^t] - C_0. A positive NPV indicates the project creates value above the required return. The Internal Rate of Return (IRR) is the discount rate that sets NPV to zero, providing a single percentage benchmark for project comparison. The risk-return tradeoff is the central tension of investment theory. Higher expected returns generally require accepting greater uncertainty. Harry Markowitz formalized this in Modern Portfolio Theory by demonstrating that portfolio variance can be reduced through diversification when assets are imperfectly correlated. The efficient frontier represents the set of portfolios offering the maximum return for a given level of risk. The Capital Asset Pricing Model (CAPM) extends this by introducing the market portfolio as a reference, defining expected return as E(r) = r_f + beta * (E(r_m) - r_f), where beta measures an asset's sensitivity to systematic market risk. Asset classes — equities, fixed income, real assets, and alternatives — differ in their return profiles, liquidity, and correlations. Strategic asset allocation determines long-run target weights based on investor objectives and risk tolerance, while tactical allocation permits short-run deviations to exploit perceived mispricings. Discount rates used in valuation models must reflect the cost of capital appropriate to the risk of the cash flows being discounted, a point stressed in corporate finance texts from Brealey, Myers, and Allen through to Damodaran.
History
The history behind the I Bond Calculator traces back through the following developments. The formal practice of lending at interest dates to ancient Mesopotamia, where the Code of Hammurabi around 1750 BCE regulated interest rates on grain and silver loans. Banking as an institutional activity took root in medieval Italy, with merchant bankers in Florence and Venice financing trade across Europe through instruments such as bills of exchange. The Medici family operated one of the most sophisticated banking networks of the fifteenth century, pioneering double-entry bookkeeping and correspondent banking relationships. Organized equity markets emerged in the early seventeenth century. The Dutch East India Company (VOC), chartered in 1602, issued shares to the public and created the Amsterdam Stock Exchange — widely regarded as the world's first formal stock exchange. The VOC allowed investors to buy and sell shares freely, establishing the template for the joint-stock company. The period also produced the Dutch tulip mania of 1636 to 1637, one of history's first recorded speculative bubbles, in which tulip bulb futures contracts reached extraordinary prices before collapsing. England's financial revolution followed in the late seventeenth century with the founding of the Bank of England in 1694 and the development of government bond markets. The South Sea Bubble of 1720 illustrated the dangers of speculative excess and contributed to early securities regulation. Throughout the eighteenth and nineteenth centuries, industrialization created enormous demand for capital, fueling the expansion of stock exchanges in London, Paris, New York, and beyond. The New York Stock Exchange, formalized in 1817, became the world's dominant equities market by the twentieth century. The Great Crash of 1929 and subsequent Great Depression prompted the US Securities Act of 1933 and Securities Exchange Act of 1934, establishing the SEC and mandatory disclosure requirements. Harry Markowitz published his landmark portfolio selection paper in 1952, launching quantitative finance. The CAPM emerged in the 1960s through work by Sharpe, Lintner, and Mossin. John Bogle launched the first retail index fund in 1976, democratizing diversified investing and challenging active management orthodoxy.
Frequently Asked Questions
Formula
Composite Rate = Fixed Rate + (2 x Semiannual Inflation Rate) + (Fixed Rate x Semiannual Inflation Rate)
The composite rate combines the fixed rate (set at purchase, lasts 30 years) with the semiannual inflation rate (updated every 6 months based on CPI). Interest compounds semiannually. A 3-month interest penalty applies if redeemed before 5 years.
Worked Examples
Example 1: Standard I Bond Purchase Held to Maturity
Problem: You purchase $10,000 in I Bonds with a 1.3% fixed rate and 2.96% current inflation rate. You hold for 5 years. Federal tax rate is 24%.
Solution: Composite rate = 1.3% + (2 x 1.48%) + (1.3% x 1.48%) = 4.28%\nMonthly rate = 4.28% / 12 = 0.357%\nValue after 5 years = $10,000 x (1.0428/2)^10 = $12,350\nTotal interest = $2,350\nNo early redemption penalty (held 5+ years)\nFederal tax on interest = $2,350 x 24% = $564\nAfter-tax return = $2,350 - $564 = $1,786\nNo state tax owed.
Result: Value: $12,350 | Interest: $2,350 | After-tax interest: $1,786
Example 2: Early Redemption with Penalty
Problem: You need to redeem your $10,000 I Bond after 18 months. Fixed rate is 1.3%, inflation rate is 2.96%.
Solution: Composite rate = 4.28%\nValue after 18 months = $10,000 x (1 + 0.0428/12)^18 = $10,656\nEarly redemption penalty = last 3 months interest\nPenalty = value - value/(1 + 0.0428/12)^3 = $10,656 - $10,543 = $113\nRedeemable value = $10,656 - $113 = $10,543\nEffective interest earned = $543 on $10,000 over 18 months\nAnnualized return after penalty = approximately 3.57%
Result: Gross value: $10,656 | Penalty: $113 | Redeemable: $10,543
Frequently Asked Questions
How is the I Bond composite interest rate calculated?
The composite rate uses a specific formula that combines the fixed rate and the semiannual inflation rate. The formula is: Composite Rate = [Fixed Rate + (2 x Semiannual Inflation Rate) + (Fixed Rate x Semiannual Inflation Rate)]. This means the fixed rate and inflation rate work together rather than being simply added. For example, with a 1.3% fixed rate and a 2.96% annualized inflation rate (1.48% semiannual), the composite rate equals 0.013 + (2 x 0.0148) + (0.013 x 0.0148) = 4.28%. The inflation component is updated every May and November based on CPI-U data, while the fixed rate is set at purchase and never changes.
How are I Bond interest earnings taxed?
I Bond interest is subject to federal income tax but exempt from state and local income taxes, giving them a tax advantage over many other fixed-income investments. You have two options for reporting interest: the cash method (default) where you report interest in the year you redeem or the bond matures, or the accrual method where you report interest annually. Most people choose the cash method to defer taxes. If used for qualified higher education expenses at eligible institutions, the interest may be completely tax-free if your income is below certain thresholds. This education exclusion applies to tuition and fees but not room and board.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.
How accurate are the results from I Bond Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.
How do I verify I Bond Calculator's result independently?
The Formula section on this page shows the equation used. You can reproduce the calculation manually or in a spreadsheet using those steps. Compare your answer against the worked examples in the Examples section, which use known reference values so you can confirm the calculator is behaving as expected.
References
Reviewed by Sahil, Senior Finance & Tax Editor · Editorial policy