I Bonds vs CDs: Two Ways to Protect Principal and Earn Interest

Direct Answer

A Series I savings bond is a non-marketable U.S. Treasury security bought and redeemed directly through TreasuryDirect, paying a fixed rate that never changes plus an inflation rate reset every six months, floored so the combined rate never goes below zero. A CD is a fixed-term deposit at a bank or credit union, paying one rate set when the account opens, protected by FDIC or NCUA insurance up to the standard coverage limit. An I bond cannot be touched for 12 months and forfeits three months of interest if redeemed within five years; a CD can generally be broken early for a disclosed penalty. Neither one is generally the better choice; they suit different combinations of amount, horizon, and tax situation.

Why this matters

I bonds and CDs both get grouped into the same mental bucket of "safe places to park cash," and search results and casual comparisons often reduce the decision to whichever one currently quotes the higher rate. That comparison skips the parts that matter more once real money is involved: how much can actually be committed, what it costs to change your mind, and how the interest shows up on a tax return. Two savers with identical amounts and identical horizons can reach different conclusions once the purchase cap, the tax treatment, or the redemption rules are weighed against their own situation.

How a Series I Bond Works

An I bond is a savings bond, which is a different category of Treasury security from a marketable Treasury bond, note, or bill. It is a direct, non-transferable obligation between the buyer and the U.S. Treasury: it is purchased through a TreasuryDirect account, it accrues value according to published rules rather than a market price, and it is redeemed with the Treasury rather than sold to another investor. Since January 1, 2025, TreasuryDirect states that I bonds are only available electronically; the older option of buying paper bonds with a portion of a federal tax refund no longer exists.

The rate an I bond earns has two parts. The fixed rate is set by the Treasury when the bond is issued and never changes for the life of that specific bond; TreasuryDirect announces a new fixed rate every May 1 and November 1, and that rate applies to every bond issued during the following six months. The inflation rate also resets every six months, based on the change in the non-seasonally adjusted Consumer Price Index for All Urban Consumers. TreasuryDirect combines the two into what it calls the composite rate using a published formula: the fixed rate, plus twice the semiannual inflation rate, plus the fixed rate multiplied by the semiannual inflation rate. If that arithmetic would produce a rate below zero, TreasuryDirect states the combined rate is held at zero instead, so the bond's value cannot fall from deflation.

Two structural details follow from being a direct Treasury obligation rather than a security bought and sold in a market. First, there is no interest rate risk in the usual sense, because there is no price to reprice; the redemption value is calculated by formula, not quoted by a dealer. Second, there is a purchase cap: TreasuryDirect limits electronic I bond purchases to a set dollar amount per calendar year, per Social Security Number or Employer Identification Number, a cap set by regulation and published on TreasuryDirect's own site rather than reprinted here, which makes I bonds a multi-year accumulation vehicle rather than a place to move an unlimited lump sum. Swoopr's Series I and EE Savings Bonds guide covers the full rate formula, the related Series EE bond, and the doubling guarantee that applies only to EE bonds, in more depth than fits here.

How a CD Works

A certificate of deposit is a time deposit: the depositor agrees to leave a fixed amount with a bank or credit union for a fixed term, in exchange for a stated interest rate that is typically higher than an ordinary savings account. Unlike an I bond, a CD's rate is entirely set by the issuing institution at the moment the account is opened. There is no formula and no government-published reset schedule; one bank's 12-month CD rate can differ meaningfully from another bank's rate for the identical term on the same day, and that rate is locked for the full term regardless of what happens to rates afterward.

A CD is opened directly with a bank or credit union, and it can also be bought as a brokered CD through a brokerage account, which is issued by a bank but held and, in many cases, later tradable through the brokerage rather than the issuing institution directly. The FDIC's own consumer guidance warns that CDs purchased through a deposit broker carry an added risk: the FDIC does not license or register deposit brokers, and if a broker fails to actually place the funds at an FDIC-insured institution, the deposit is not insured. That risk sits alongside the mechanics of the CD itself and is worth checking before assuming every product labeled "CD" carries the same protection.

A bank-direct CD has no federal purchase cap and no purchase minimum set by the government; the institution sets its own minimum deposit, which varies by bank and by term and is sometimes tiered so a larger deposit earns a higher rate. Swoopr's Certificates of Deposit guide covers penalty schedules, no-penalty CDs, and maturity mechanics in full; the CD laddering guide covers the standard technique for managing rate-direction uncertainty across several CDs at once.

How the Rate-Setting Mechanism Differs

The single biggest structural difference between these two instruments is where the rate comes from. A CD's rate is a promise: the bank states a number, the depositor accepts it, and that number does not move for the length of the term, in either direction. If market rates rise sharply the month after opening a five-year CD, the depositor is contractually stuck at the lower rate until maturity or until paying the early withdrawal penalty. If market rates fall instead, the same lock becomes an advantage, and the CD continues paying its original, now above-market rate.

An I bond's rate is a formula, not a promise about a fixed number. The fixed-rate component behaves like a CD's locked rate in one sense, once bought it is permanent for that bond, but it is only half the equation. The inflation component moves with measured inflation every six months for as long as the bond is held, which means an I bond bought during a period of high inflation can see its combined rate fall sharply if inflation cools, and an I bond bought during low inflation can see its combined rate rise if inflation later increases. Over any single six-month window, an I bond's return is not something either the buyer or the Treasury knows in advance; it depends on inflation data that has not been published yet.

This is also why the two are not really substitutes for the same job. A CD is a tool for locking in a known nominal return for a known period. An I bond is a tool for keeping pace with inflation over an unknown period, at the cost of an unknown near-term nominal return. Swoopr's TIPS guide covers the other major inflation-linked federal instrument, which behaves differently again because it is marketable and carries a market price.

Safety and the Backing Mechanism

An I bond's safety comes from the U.S. government's taxing and borrowing authority, commonly described as full faith and credit backing, with no dollar limit stated anywhere in that guarantee. There is no insurer, no coverage tier, and no ownership-category rule to track, because the obligation runs directly from the Treasury to the bondholder.

A CD's safety instead comes from deposit insurance. A CD issued by an FDIC-member bank is insured up to the FDIC's standard coverage limit, a figure set by regulation and published on the FDIC's own site, covering principal and any interest already credited, calculated dollar for dollar, per depositor, per insured bank, per ownership category. A CD issued by a federally insured credit union carries the equivalent NCUA share insurance instead, at the same standard limit. Swoopr's FDIC Deposit Insurance guide explains how the ownership-category rules work across joint accounts, trusts, and retirement accounts; because deposit insurance limits are set by regulation and have changed historically, confirm the current figure at the FDIC source cited below before assuming any specific number applies to a large balance.

A saver holding CDs at one bank that add up to more than the insured limit, across multiple certificates, can have uninsured balances even though each individual CD looked fully protected on its own. An I bond of any size carries no equivalent concentration risk, since the backing is not capped per instrument or per holder.

Getting the Money Out Early

An I bond's liquidity rules are fixed by regulation and identical for every bond of that series: TreasuryDirect states that a bond cannot be redeemed at all in the first 12 months, with no exceptions and no discounted early sale, because there is no secondary market for a non-marketable security. Once the 12-month mark passes, the bond can be redeemed at any time, but redeeming before five years forfeits the three most recent months of interest. After five years, the bond can be redeemed with no penalty at all, and it continues earning interest for up to 30 years in total if left alone.

A CD's liquidity rules are set individually by each issuer and disclosed at account opening, most often as a penalty equal to a stated number of months of interest, for example three months of interest on a one-year CD or six months on a five-year CD. The FDIC notes that most fixed-rate CD terms allow paying a fee to redeem before maturity, though some CD types, including certain market-linked CDs, may not allow early redemption at all. Because the penalty is defined as a number of months of interest rather than a share of interest actually earned, a CD closed very soon after opening can owe more in penalty than it has accrued, with the shortfall coming out of principal. A brokered CD works differently again: instead of paying a penalty, it can often be sold on a secondary market before maturity, at a price that moves with prevailing interest rates rather than a fixed forfeiture.

Put plainly, an I bond's early-access cost is a fixed rule that applies identically to every bond: no access for a year, then a flat three-month interest forfeiture for four more years. A CD's early-access cost varies by issuer and term, is disclosed in advance, and can in the worst case reduce the payout below the original deposit.

How Each Is Purchased

An I bond is bought exclusively through a TreasuryDirect account, opened directly with the Treasury, with a stated minimum purchase and an annual electronic purchase cap per Social Security Number or Employer Identification Number; TreasuryDirect publishes the current cap rather than it being fixed by this guide. There is no way to buy an I bond through a brokerage, a bank, or any intermediary; the account relationship is always directly with the Treasury.

A CD is opened directly with the issuing bank or credit union at a rate posted before the deposit is made, with the account created as part of the same transaction, or it is bought as a brokered CD through an existing brokerage account. Minimums are set by the institution rather than by any federal rule, and there is no annual cap on how many CDs, or how much total money, a saver can place across institutions, which is the structural reason CD laddering across several banks is a common technique for savers with more to place than any single insured limit or I bond cap would otherwise allow.

What Happens at the End of the Term

An I bond has no fixed maturity date in the way a CD does. It simply continues accruing interest under the fixed-rate-plus-inflation formula for as long as it is held, up to a maximum of 30 years, at which point it stops earning interest entirely and TreasuryDirect will have already flagged it for redemption. The holder decides when, within that window, to redeem, and nothing forces a decision on a specific calendar date.

A CD matures on a specific date chosen at purchase. Most institutions provide a short grace period immediately after maturity, commonly a window of several days stated in the account agreement, during which the funds can be withdrawn or moved without penalty. If the depositor takes no action during that window, the CD typically renews automatically into a new CD of the same term at the institution's then-current rate, which may be well below the rate the maturing CD had been earning, and the renewed CD is then subject to a fresh early withdrawal penalty if the mismatch is caught late.

Tax Treatment

TreasuryDirect states that I bond interest is subject to federal income tax but exempt from state and local income tax, while federal estate, gift, and excise taxes, along with state estate or inheritance taxes, can still apply. The reporting timing is where an I bond differs most from ordinary interest: a holder may defer reporting the interest until filing the federal return for the year the bond is redeemed or matures, rather than paying tax on it year by year as it accrues. TreasuryDirect also notes that using the proceeds for qualified higher education expenses may allow some or all of the interest to be excluded from federal income tax, subject to eligibility rules that should be checked before relying on the exclusion.

CD interest is taxed as ordinary income at the federal level and, unlike I bond interest, generally at the state and local level as well, with no comparable state exemption. It is taxed in the year it is credited to the account, whether or not the depositor actually withdraws it, and the bank reports it annually on Form 1099-INT. There is no deferral option: a five-year CD's interest is taxed year by year as it is earned, not in one lump sum at maturity.

For a saver in a state with a meaningful income tax rate who does not need the interest income in the near term, the I bond's combination of a state-tax exemption and deferred federal reporting can matter more than a small difference in headline rate. For a saver who wants interest income reported and available annually, or who lives somewhere with no state income tax, that advantage narrows or disappears.

A Worked Example

The figures below are illustrative only, chosen to show how the two mechanisms behave differently, not a forecast or a quote of any currently published rate. Do not treat any rate in this section as the actual rate available from TreasuryDirect or any bank today.

Suppose a saver commits $7,500 to each instrument on the same day. The I bond carries an illustrative fixed rate of 1.20% and an illustrative semiannual inflation rate of 1.10%. Using TreasuryDirect's published formula, the composite rate works out to approximately 3.41% (0.0120 + 2 × 0.0110 + 0.0120 × 0.0110). Over the first year, that accrues roughly $256 in interest, compounding into the bond's value rather than being paid out. The CD carries an illustrative 12-month annual percentage yield of 4.00%, which would accrue roughly $300 in interest if held to maturity.

Now suppose the saver needs the funds back at month eight, before either the I bond's 12-month lockup has passed in one case or the CD's term has ended in the other. The I bond cannot be redeemed at all yet; the money is simply inaccessible until the 12-month mark, regardless of the rate. The CD, by contrast, can typically be broken early, but the bank's disclosed penalty, for example an illustrative three months of interest at the account's 4.00% rate, applies against the interest already accrued, reducing what is returned. Neither outcome is better in the abstract: one instrument enforces illiquidity by rule for a fixed window, the other allows early access at a disclosed, but real, cost that could exceed a smaller CD's accrued interest in the very earliest months.

Extend the same $7,500 out to year six for the I bond. By then the 12-month lockup and the five-year interest-forfeiture window have both passed, so the full accrued value, compounded twice yearly under whatever composite rates applied over that period, some higher and some lower depending on inflation during each six-month window, is available with no penalty at all. A CD held on a rolling series of one-year terms over the same six years would have reset its rate annually to whatever each bank offered at renewal, with tax due each year on the interest credited rather than deferred to a single year.

I Bond and CD, Side by Side

A Decision Framework, Not a Recommendation

A saver whose money can sit untouched for at least a year, who wants a return that tracks inflation rather than a fixed nominal number, who is holding the funds in a taxable account where deferring the tax bill has real value, and whose total contribution across the year fits within the annual electronic purchase cap, is describing circumstances where an I bond's mechanics fit well. A household with more than one Social Security Number available to it, since each person's cap is separate, can also scale the approach further than a single filer could alone.

A saver who might need the funds back before 12 months are up, who wants an amount above the I bond's annual cap placed in one instrument, who wants a specific, known maturity date to plan around, who wants interest income taxed and reported annually rather than deferred, or who is placing the funds through an existing brokerage relationship, is describing circumstances where a CD's mechanics fit well, whether opened directly with a bank or bought as a brokered CD.

Many savers reasonably hold both: an I bond for the portion of savings meant to sit untouched for years and keep pace with inflation, and a CD, or a ladder of several, for money on a more defined schedule. Neither instrument is a universally better choice; they answer different questions about the same underlying goal of protecting principal while earning something on it.

Myths and Misconceptions

• "I bonds are risk-free and CDs are risky." Both are considered very low risk. The distinction is not a risk ranking, it is where the safety comes from: direct government backing for one, deposit insurance up to a coverage limit for the other. A CD within the insured limit at a healthy institution is not meaningfully riskier for a typical saver.
• "You can still buy paper I bonds with your tax refund." TreasuryDirect states that as of January 1, 2025, I bonds are only available electronically. The paper-bond-via-tax-refund option that existed for years no longer exists, and any guidance describing it as current is outdated.
• "FDIC insurance covers everything at the bank, including investments sold there." FDIC coverage applies to deposit products specifically: checking accounts, savings accounts, money market deposit accounts, and CDs. A brokerage or investment product sold at the same physical branch, including one with a similar name, is typically not FDIC-insured at all.
• "The I bond's 'fixed rate' is the rate you'll actually earn." The fixed rate is only one input into the composite rate. The inflation component resets every six months and usually makes up most of the combined rate, so quoting the fixed rate alone understates what the bond has been earning, or overstates it, depending on the inflation environment.
• "A CD guarantees you'll never lose money, period." A CD's stated rate is guaranteed if held to maturity, but the early withdrawal penalty is a real, contractual cost. The FDIC's own guidance notes that a penalty defined as months of interest, rather than a share of interest earned, can exceed what a CD closed very early has actually accrued, drawing the shortfall from principal.

FAQ

Educational Use

This page is educational and informational. It does not tell a reader what to buy, hold, or contribute, and it does not account for an individual's objectives, taxes, legal situation, benefits, debts, time horizon, or risk tolerance. The worked example uses illustrative rates chosen to demonstrate the mechanism and is not a projection, a rate forecast, or a current market quote. Verify current I bond rates at TreasuryDirect, current CD rates and terms with a specific institution, and current insured limits at the FDIC before acting.

References

• TreasuryDirect: Series I Savings Bonds
• TreasuryDirect: I Bonds Interest Rates
• TreasuryDirect: Tax Information for EE and I Savings Bonds
• FDIC: Deposit Insurance FAQs
• FDIC: Shopping for a Certificate of Deposit

This content was reviewed by the Swoopr Editorial Team in August 2026 and reflects publicly available information at that time.