Inflation Calculation Methodology, Rounding & Data Gaps

Historical source

BLS series CUUR0000SA0: CPI-U U.S. city average all items, not seasonally adjusted, index 1982–84=100. Twelve disjoint public API windows produced 1,364 valid monthly observations. The raw unavailable marker for October 2025 is retained in metadata; no inferred value is used. BLS public API limits

Amounts and exact arithmetic

Original amounts accept plain nonnegative decimals with up to two decimal places, at most 1 billion. Salary/initial-investment denominators must be positive; new pay/final value may be zero. CPI decimals become integer millionths. Monetary ratios use BigInt and round half-up once to cents. Any computed amount beyond 9 quadrillion cents is rejected before showing a result.

Historical formulas

Basket equivalent = original amount × end CPI / start CPI. Unchanged cash in initial-month dollars = original amount × start CPI / current CPI. Salary real change = (new/old)/(end CPI/start CPI)−1. Investment real change uses the same ratio under the no-cashflow condition. Percentages use unrounded ratios; displayed values have up to six decimal places.

Time convention

End must be on or after start. Both must be observed months in this snapshot. Annualized CPI change = (end CPI/start CPI)^(12/month intervals)−1; for the same month it is not applicable. This is a geometric summary, not a forecast. No annual-average series is generated.

Assumption-only scenario

A constant entered annual rate from −20% to 50%, up to two decimals, compounds for 0–100 whole years. Future basket cost and unchanged-cash purchasing power use exact rational powers; a display factor uses floating point. The currency selector changes formatting only. Amounts can exceed the supported range even when individual inputs are valid; those runs reject.

Backup, export and print

Results CSV includes the inputs, currency, basis, metrics and path. JSON up to 32 KB stores validated inputs and a snapshot identifier, not trusted result totals. Restoring recomputes and requires the same tool and data snapshot. Nothing is automatically stored. Print includes summaries, chart and assumptions; open the period table before printing if you want every row.

Scope of verification

Independent numerical fixtures, browser flows, exports, responsive layouts and SEO are checked before publication. No personal-inflation estimate, tax advice, external account authentication, live feed, legal escalation or real investment-performance certification is implied.