01 / Publication status
The current release uses research methodology 0.1.0 and an illustrative base date of 01 January 2021. A final methodology, approved base date and named administrator are required before the index can be used for settlement.
Research values must not be treated as approved settlement fixings. Changing the base date will require a new methodology version and an explicit transition notice.
02 / The underlying data
IPU uses the U.S. Consumer Price Index for All Urban Consumers (CPI-U), U.S. city average, all items, 1982–84 = 100, not seasonally adjusted. BLS series identifier: CUUR0000SA0.
Only published monthly observations enter the calculation. Annual averages, seasonally adjusted series and predictive models are excluded. This release uses a dated, versioned BLS snapshot. BLS reports no usable October 2025 value in this dataset. Affected daily dates are omitted from charts, which show a gap; exact-date and interval requests requiring this input return 422. Historical observations may reflect corrections present at retrieval; this is not a first-release vintage archive.
03 / Daily Reference CPI
For a date in calendar month M, the first reference observation is CPI for M−3; the second is CPI for M−2. Daily interpolation uses calendar days and a UTC valuation date. On the first of a month, only the first observation is required.
Here d is the day of the month and D is the number of days in that month. For example, an April valuation interpolates between January and February CPI. Reference CPI is truncated to six decimal places and then rounded half-up to five, following the Treasury calculation convention.
04 / From CPI to IPU
IPU normalizes the daily Reference CPI to an illustrative base value of 100. The base Reference CPI is calculated using exactly the same method.
IPU uses the five-decimal Reference CPI values in the ratio, then rounds the final index half-up to five decimals. The IPU ratio does not copy Treasury’s separately rounded security-specific index ratio. Displayed headline values may use four decimals; API values retain five. No floor, cap or discretionary adjustment applies.
05 / Missing data and corrections
If an input needed for a date is unavailable, the API returns an explicit data-unavailable response for that requested date. The default dashboard shows the latest calculable date, labels stale data and never invents an observation.
This differs from Treasury’s contingency provisions: IPU research does not project a missing CPI value. A change to an existing source observation fails automated update validation and requires a documented correction review. No historical settlement is silently rewritten.
An official-series discontinuation, rebasing or material methodology change pauses publication until a successor rule and governance decision are published.
06 / Reproduce a value
Download the source observations and metadata, select the valuation date, derive its input months, interpolate Reference CPI, and divide by the base Reference CPI. The metadata endpoint supplies the dataset SHA-256 hash and method version.
The formula is a price-level reference. It creates no claim on assets, no yield entitlement, and no promise of purchasing power protection.