Series I Bonds: The Odd Savings Bond Worth Knowing

Series I savings bonds are the strangest product in the savings aisle: a government bond with a moving interest rate, a one-year leash, a small annual cap, a tax quirk, and no market price to watch. They’re also, in certain stretches, the best risk-free deal in the savings world — they briefly paid above 9% annualized in 2022 when inflation spiked, and they’ve paid far less since. Both facts are true, and both matter for deciding whether they belong in your stack.

This post is the mechanics, without hype: how the rate is built, what the lock-up rules actually cost you, where they beat T-bills and CDs, and where they don’t fit at all.

How the I Bond Rate Is Built

The I bond’s rate has two parts. There’s a fixed rate set when you buy, which stays with that bond for its life, and an inflation adjustment based on CPI-U that resets every six months. The published composite rate works like this:

Composite rate = fixed rate + (2 × the most recent semiannual inflation rate)

Illustrative math, clearly labeled as an example and not a quote: if the fixed rate for your purchase window is 1.2% and the most recent semiannual inflation adjustment is 1.0%, your composite rate for the next six months is 1.2% + 2.0% = 3.2%, annualized. That composite applies for six months, then the inflation part re-sets to whatever CPI did next; the fixed part never changes for that bond.

New fixed and inflation rates are announced each May and November, so your purchase date matters — you lock that window’s fixed rate for the life of the bond. The current numbers are always on TreasuryDirect, which is the only rate source worth trusting here, because I-bond rates change twice a year and any article (including this one) is a snapshot, not a price.

The Rules That Actually Decide Whether It Fits

The rate is the sexy part; the rules are the product. Here’s the complete short list:

• Purchase cap: $10,000 per person per calendar year in electronic bonds through TreasuryDirect. Spouses, and kids with their own accounts, each have their own limit — a family can layer these limits, but nobody’s buying six figures a year of these.
• One-year lock-up: you cannot redeem at all for the first twelve months. No exceptions for emergencies. This is the rule that rules out using I bonds as a true emergency fund.
• The 3-month interest penalty: redeem in years two through five and you forfeit the most recent three months of interest. After year five, no penalty. Held the full 30 years, they just mature and stop earning.
• Tax treatment: interest is exempt from state and local income tax, and federal tax is deferred until you redeem. (There’s also an education-related federal tax exclusion under income limits — worth reading the TreasuryDirect page on if college costs are in your future.)

Quick feel for the penalty: suppose your composite rate were 3.2%. That’s about 0.27% per month, so redeeming in year two forfeits roughly 0.8% of the balance — about $80 on a $10,000 bond. Meaningful, not catastrophic. The one-year lock is the harder wall than the penalty.

The Rate History, Honestly

I bonds became famous in 2022 when the composite rate crossed 9% annualized for one stretch — driven almost entirely by the inflation component, not the fixed rate. That’s also the honest warning: the inflation component swings, and when inflation cools, the composite falls with it. What the I bond actually promises isn’t a high rate; it’s a rate that tracks inflation plus a modest fixed floor. In high-inflation years that’s a gift. In low-inflation years it’s merely fine. Anyone who bought I bonds expecting 9% forever misunderstood the product — the whole design is “you will keep pace with measured inflation,” nothing more.

Where I Bonds Sit in a Savings Stack

I think of cash in tiers, ordered by how quickly you can get at it:

Tier	Vehicle	Liquidity	Job
1	Checking cushion	Instant	One to two months of bills
2	High-yield savings	1–2 days	Emergency fund, near-term goals
3	CD ladder or T-bills	Weeks to months	Money with a known date
4	I bonds	12+ months locked	Long-horizon cash that should track inflation

Tier 4 is the whole insight: I bonds are the right home for money that is cash in purpose but long in horizon. The house down-payment fund with a five-year runway. The “someday” car fund. The part of your safety net you’ve decided can tolerate an illiquid year in exchange for inflation protection no bank account can offer. Compare that against the alternatives: a CD ladder locks in a rate for a term but pays no inflation adjustment, and T-bills are the most liquid of the three but re-price at every auction. I bonds are the only savings product where inflation is your ally by construction.

Also worth naming plainly: the tax deferral compounds quietly. Because you pay no federal tax on interest until redemption, the money that would have gone to taxes each year stays invested inside the bond. Over a decade-plus hold, that’s worth real basis points — and the state-tax exemption matters a lot if you live in a high-tax state.

When NOT to Buy Them

• You need the money inside a year. Non-starter. The lock is absolute.
• This is your emergency fund. Wrong tool. Emergencies don’t schedule themselves after your bond’s first birthday. Fund the HYSA first — the basics of why are in my HYSA explainer.
• You might need flexibility on timing. If the goal date could move earlier by a year, the lock-up turns an inconvenience into a real problem.
• You’re chasing the rate of the moment. I bonds are a hedge, not a yield lottery. The rate you’re excited about today resets in six months.

And the honest annoyance: TreasuryDirect’s interface feels like a government website from 2004, because it is one. The purchase process is clunky, transfers out are manual, and there’s no app. That friction is a real cost — it’s the price of a product that doesn’t advertise and doesn’t compete.

A Sensible Way In

The natural strategy for a family is boring: buy the annual cap for each eligible account holder every year you have long-horizon cash, treat it as tier-4 money, and don’t think about it again until the one-year marks start arriving. Each bond quietly re-prices itself with inflation twice a year while you do nothing. A decade of that at the caps builds a six-figure inflation-protected layer — slowly, boringly, and with zero drama. (Illustrative arithmetic, not advice: $10,000 a year for ten years is $100,000 of principal, per person, before any interest.)

Bottom line: I bonds are a specialty tool — an inflation-tied, illiquid, capped parking spot for cash with a long fuse. For that narrow job, they have no true competitor. For every other job, use the more liquid tiers.

One forward step: if you have money you’re certain you won’t touch for at least a year, check the current composite rate on TreasuryDirect, compare it to your best CD and T-bill yield for the same horizon, and let the liquidity difference — not just the headline rate — make the call.