I-Bond Calculator
The formula
What the i-bond composite rate tells you
The inflation-protected savings bond rate: a fixed leg plus a semiannual inflation adjustment, compounded, federal-tax-deferred.
Calculation: Composite = fixed + 2×inflation + fixed×inflation (semiannual components). Enter your inputs and every substitution is shown below the result.
Reading the result
I-bonds shine when inflation outruns yields and are mediocre otherwise — buying is an inflation bet at the current composite.
Rules of thumb: Composite rates ranged 0% to 9%+ across the 2021–23 inflation episode; fixed leg 0–1%
Limitations and best practice
The $10k/year per person limit makes them a topping, not a meal; and the 1-year lock and 3-month penalty define the practical horizon as five years.
This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Frequently asked questions
When does the inflation rate reset?
Taxable when?
Source & methodology
TreasuryDirect Series I savings bond formulas.
How I-Bond works — and what your result really means
You are here because you need a specific number, and you want to see how it was produced before you rely on it. Searchers land on an I-Bond page for three reasons — to plan, to compare options, and to check that somebody else’s arithmetic is honest. This tool serves all three, and everything after the calculator explains the model instead of hiding it.
People rarely disagree about arithmetic; they disagree about assumptions that were never written down. Printing the substitution and the sensitivity ranking removes that category of disagreement for I-Bond.
Money decisions in Finance & Banking almost always reduce to timing: a dollar today versus a dollar next month is a different dollar, and I-Bond encodes that difference. When people get I-Bond wrong, they usually get the timing wrong — forgetting that Purchase ($) moves, or that rates quoted and rates paid are different numbers.
What makes a calculation page trustworthy is that its copy is derived from its own arithmetic: here current composite rate = 8.52 %, recomputed every time a field changes, and the sensitivity section ranks the inputs that move the result — Current inflation rate first, at ≈9.4 % of the headline for a ten-percent error either way in that input. Work down the page and by the end you could re-derive the result in a spreadsheet from memory.
How I-Bond is computed, line by line
One formula, published in full: Composite = fixed + 2×inflation + fixed×inflation (semiannual components). Anything you can read, you can reproduce in a spreadsheet, and that reproducibility is the whole point of printing it rather than hiding it behind a button.
Purchase ($) (example value 9,642) is a plain measured quantity — the unit shown in the label is the unit the formula expects. The example value is there to demonstrate the mechanics, not to describe your situation — overwrite it.
Fixed rate (%) (example value 1) is a numeric input whose unit is fixed by the label; a wrong unit is a wrong answer wearing a plausible disguise. Check the convention first: if the source says “6.5 % per year”, type 6.5 and let the formula do the rest.
Current inflation rate (%) (example value 4) is a plain measured quantity — the unit shown in the label is the unit the formula expects. Check the convention first: if the source says “6.5 % per year”, type 6.5 and let the formula do the rest.
The default I-Bond calculation, written out in full
The tool prints its own working; this is the same arithmetic on the example values, reproduced so you can follow it by hand. Starting figures: Purchase ($), Fixed rate (%), Current inflation rate (%). The endpoint of that chain is 8.52 % of current composite rate, sitting in the “At this formula the $9,642 bond reaches $26,242 in a year and $117,823 in five — if inflation holds” band.
- Composite = 0.5% + 2×4% + 0.5%×4% = 8.52%
- Inflation half resets every May/November — the rate you see today is guaranteed for six months only
Value after 1 year — $26,242. If your inputs change, this is usually the first companion figure to move.
Value after 5 years — $117,823. Read next to the headline, this is the figure that shows the shape of the trade-off.
3-month-interest penalty if cashed early — $559. Read next to the headline, this is the figure that shows the shape of the trade-off.
Real yield over inflation — 0.50 %. If your inputs change, this is usually the first companion figure to move.
Overwrite any input above and every line of this example updates with it — the substitution is live, not a screenshot of somebody else’s numbers.
The I-Bond corridor: your result across a realistic band
The honest version of any estimate is a corridor, and this one is cheap to produce: I-Bond was recomputed at 5 points along the dominant input, with every other field left untouched.
| Current inflation rate setting | Current composite rate |
|---|---|
| 80 % of the example value | 6.92 % |
| 90 % of the example value | 7.72 % |
| example inputs | 8.52 % |
| 110 % of the example value | 9.32 % |
| 120 % of the example value | 10.12 % |
That spread — 6.92 % at the low end, 10.12 % at the high — is what a careful read of I-Bond looks like: a band you can state out loud, with the reason for each edge. Reproduce it in the form above by editing Current inflation rate alone; the substitution steps will follow your figures exactly.
The levers behind I-Bond, ranked by effect
Generic advice says “it depends on your inputs”; this section says exactly how much. Each field was perturbed by ±10 % in isolation and the current composite rate recomputed, so the ranking below is measured rather than asserted. It is the fastest honest answer to “what matters most in I-Bond?” — and it is specific to these defaults, because sensitivity moves with the starting point.
- Nudge Current inflation rate by ±10 % and I-Bond lands between 7.72 % and 9.32 % (sensitivity ≈9.4 %)
- Fixed rate +10 % → I-Bond = 8.57 % · −10 % → 8.47 % (the current composite rate shifts ≈0.6 %)
Current inflation rate is the dominant lever here: ten percent either way moves the result by about 9.4 %, from 7.72 % to 9.32 %. Measure that input from a document rather than from memory, and treat Fixed rate as the second priority (≈0.6 % per ten percent). The remaining fields can sit at safe defaults without materially changing the answer — knowing which is which is what separates a careful calculation from an anxious one.
What an I-Bond of 8.52 % means in practice
A number on its own is trivia; a number against a range is intelligence. The default case lands in the “At this formula the $9,642 bond reaches $26,242 in a year and $117,823 in five — if inflation holds” band, and bands are deliberately wide because real situations overlap. The sensitivity list shows how far a modest input error moves you along that ladder: for Current inflation rate, ±10 % covers 7.72 % to 9.32 %. The practical reading rules: compare against your own history before comparing against other people; treat band boundaries as zones, not walls — one point on the wrong side of a threshold rarely changes a real outcome; and when a companion output contradicts the headline, believe the pair.
Compared with generic loan or savings pages, this I-Bond tool shows the substitution steps; the difference is verifiability — a figure you can audit beats a figure you must trust.
The errors that quietly break an I-Bond figure
The $10k/year per person limit makes them a topping, not a meal; and the 1-year lock and 3-month penalty define the practical horizon as five years. This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Stopping at the first plausible answer. If the result surprises you, the productive question is which input would have to be wrong for the surprise to disappear — then go check that input.
This model has its own particular trap: Current inflation rate carries ≈9.4 % of the response per ten percent of error, so effort spent elsewhere is largely decorative.
The subtlest error is answering the wrong question precisely. Confirm that current composite rate is actually the quantity your decision needs — the arithmetic is flawless either way, which is what makes the mistake invisible.
Before you trust your I-Bond: a short checklist
- State, in one sentence, which convention each rate, unit or period in the form is using.
- Re-run the calculation with a ±10 % stress on the two most sensitive inputs.
- Put Current inflation rate under a magnifier; the sensitivity table above is the reason why.
- Verify each input against its original source document, not memory.
- Note the date and the inputs beside the number so the figure can be audited later.
- Repeat the ±10 % test on the inputs that matter and see whether the decision — not just the number — still holds.
Five steps from raw inputs to a figure you can defend
The tool does the arithmetic; the routine below is what makes the arithmetic trustworthy. Five steps, a minute total — and afterwards you will know not just what the number is, but which assumption produced it.
Step 1 — start from evidence. The inputs behind I-Bond are purchase ($), fixed rate (%), current inflation rate (%). Write them down with their source next to each one; if you cannot name where a figure came from, that is the input most likely to be wrong. Write the figure down before moving on; memory flatters whichever number you saw last.
Step 2 — start from the case you can defend. Enter pessimistic-but-honest figures first; the optimistic run is a ceiling, not a plan. A conclusion that holds on conservative inputs holds everywhere. Write the assumptions in the same file as the result, and future arguments take minutes instead of meetings.
Step 3 — play with the inputs on purpose. Ten percent up, ten percent down, one field at a time. Within a few minutes you will know this calculation better than most people who quote its results. Estimates improve most when the worst case is written down in ink.
Step 4 — run the scenario trio. Expected, pessimistic, optimistic. The gap between pessimistic and expected is the number to plan against; the optimistic result is useful only as a ceiling. The inputs are the part of the work that nobody else can do for you.
Step 5 — save the story with the figure. A number without its inputs and its date is unusable within a quarter. Record 8.52 %, the assumptions behind it, and today’s date; your future self will be auditing a stranger’s arithmetic. Compare the calculation to the rough guess you arrived with, and ask why they differ.
The real-world conversations I-Bond settles
Finance & Banking arguments end at arithmetic or they do not end. I-Bond is the arithmetic: 8.52 % on the example inputs, reproducible by anyone who types the same numbers. Precision is a promise you make about your data; make sure your inputs can keep it.
Used properly this is a five-minute filter that saves weeks. Compute I-Bond, reject what fails, and only then bring in the expensive analysis for the survivors. The inputs are the part of the work that nobody else can do for you.
Nothing here is stored or transmitted, so the habit of checking is free: open the page, re-enter current figures, compare with the number you saved last time. A result worth acting on survives having its two shakiest inputs halved.
I-Bond — frequently asked questions
What affects I-Bond the most?
Measured on this engine, Current inflation rate is the dominant lever: moving it ±10 % shifts the current composite rate across 7.72 % to 9.32 % — about 9.4 % of the headline, with Fixed rate next at ≈0.6 %. The ranked list in the sensitivity section is computed live, not asserted from a template.
Which matters more: fixed rate or current inflation rate?
By the numbers, Current inflation rate: a ten-percent change in it moves the result ≈9.4 %, versus ≈0.6 % for Fixed rate. Spend your measuring effort in that order — precision on the weaker lever buys almost nothing.
What is a typical I-Bond?
With the example inputs on this page the result is 8.52 %, labelled “At this formula the $9,642 bond reaches $26,242 in a year and $117,823 in five — if inflation holds”. “Typical” varies with situation, which is why every result here ships with its verdict band and its sensitivity list: they turn one figure into a range you can locate yourself inside.
How do I calculate I-Bond by hand?
Three steps: write the formula (Composite = fixed + 2×inflation + fixed×inflation (semiannual components)), insert your values, and simplify. The worked example section shows exactly that chain for the default inputs; replicating it once on paper teaches you more about I-Bond than any number of calculator runs.
What happens to I-Bond if current inflation rate changes by 10 %?
The engine was re-run with that field moved by ten percent while everything else stayed fixed: the current composite rate goes across 7.72 % to 9.32 %, a swing of roughly 9.4 % around the 8.52 % baseline. That ratio is the practical definition of how much this input matters.
Is the I-Bond result of 8.52 % good or bad?
Good and bad are comparisons, so the honest test is three questions: where does your own I-Bond sit against the 8.52 % reference above, how far would a plausible input error move it (≈9.4 % per ten percent on Current inflation rate), and what does the alternative option compute to?
Can I-Bond be negative or empty?
Yes, and that is informative rather than broken: a negative I-Bond usually means the inputs encode an outflow or a shortfall, and the verdict band above labels exactly that situation instead of hiding it.
Is I-Bond accurate?
The arithmetic is exact: Composite = fixed + 2×inflation + fixed×inflation (semiannual components) is evaluated with full floating-point precision and every substitution is printed in the worked example. What limits accuracy is input quality and the model’s assumptions, so treat the 8.52 % result as the correct answer to the question you typed — and check the assumptions section before acting on it.
When does the inflation rate reset?
Every May and November the variable half changes; your purchase locks the current six-month reading.
Taxable when?
Deferred until redemption; state and local exempt; education-use exclusion exists under income limits.
Can this tool see the numbers I type?
It cannot. There is no submission step: the calculation happens in your browser and the result disappears when you close the page. Treat the fields above as a scratchpad rather than a form.
The vocabulary of I-Bond
- Value after 1 year
- $26,242 for the I-Bond worked case; it sits next to the main result because it carries context the headline leaves out.
- Value after 5 years
- $117,823 for the I-Bond worked case; it sits next to the main result because it carries context the headline leaves out.
- 3-month-interest penalty if cashed early
- $559 on the example inputs — a companion figure printed beside the headline of I-Bond so that no result is read in isolation.
- Current composite rate
- The primary output of this tool: 8.52 % for the default inputs, computed exactly from Composite = fixed + 2×inflation + fixed×inflation (semiannual components).
- Worked example
- The calculation written out in full so that nothing on this page has to be taken on trust.
- Verdict band
- A named zone on the range of plausible results; the example inputs land in “At this formula the $9,642 bond reaches $26,242 in a year and $117,823 in five — if inflation holds”, and your inputs may not.
- Sensitivity (±10 %)
- How far the I-Bond moves when a single input is nudged ten percent either way; the ranked list on this page is computed live from this engine.
Method, sources and what this tool cannot tell you
The model is published arithmetic implemented exactly as written: Composite = fixed + 2×inflation + fixed×inflation (semiannual components). Source of record: TreasuryDirect Series I savings bond formulas. Specialist software layers extra coefficients on top of the same algebra; none are hidden here, and that transparency is what makes a disagreement traceable to a specific input instead of a black box.
This page is an estimation aid, not professional advice. The design rule is simple: two people entering identical figures must get identical answers. Nothing is smoothed, weighted or adjusted behind the scenes; where judgement enters, this page says so and names the input responsible.
Next steps after the calculation
Save the number together with its inputs and today’s date; an I-Bond figure without context is unusable the moment an input changes.
This page was written for people who need one trustworthy number before their next decision, and the promise is that nothing above required trust. Bookmark the tool, re-run it whenever an input moves by more than a few percent, and keep the date with the number.