Title 26, executed. Part 1 of a series: the Internal Revenue Code read as a program, and what it produces compared with what is published. Part 2 applies the 2025 tax law's amendments to the Code.
We rebuilt all 295 amounts published in section 4 of Rev. Proc. 2025-32, the IRS table of figures for 2026, from the text of the Internal Revenue Code and the price index the Code names, stating each convention the text leaves open. 216 of them match the published figures numerically, 213 without qualification. Two cannot result from the statute's own rounding rule, whatever index value is used. 57 raise the question of which year's index the law specifies. The 2027 amounts are computed from the same text and sealed by hash before the IRS publishes them.
Two numbers
The head-of-household standard deduction for 2026 is published as $24,150. The base amount in the statute is $23,625. The difference is $525.
The statute, §63(c)(7)(B)(ii) of the Code as amended by P.L. 119-21, says how the base is to be increased: by "an amount equal to— (I) such dollar amount, multiplied by (II) the cost-of-living adjustment" for the year, and then: "If any increase under this clause is not a multiple of $50, such increase shall be rounded to the next lowest multiple of $50."
An increase of $525 is not a multiple of $50. No value of the price index turns the base into $24,150 by that rule, because the rule rounds the increase, and every increase it produces is a multiple of $50. That is the whole of the first finding, and it needs no index data to check. What the index data add is the figure the rule does produce: $24,175.
What we compared, and on what
Rev. Proc. 2025-32 sets out 63 items of inflation-adjusted figures for 2026, for Code provisions "as in effect on October 9, 2025", in its own words. We took the date literally: the official 2024 edition of Title 26 from govinfo, with P.L. 119-21 (the One Big Beautiful Bill Act, OBBBA) applied. The later 2025 public laws were checked and do not touch the indexing provisions.
Each of the 295 published amounts in section 4 of the revenue procedure received an indexing record: base amount, base year, year of the adjustment, rounding rule, and the quoted statutory text that says so. The cost-of-living adjustment follows §1(f)(3)–(7): the ratio of two twelve-month averages of the Chained Consumer Price Index for All Urban Consumers (C-CPI-U), each ending on August 31. The index values are those the Bureau of Labor Statistics had published when it first released the August 2025 figure, on September 11, 2025, because §1(f)(6)(A) says so. The arithmetic is decimal, at 50 digits. A few conventions the statute leaves unstated, such as how the maximum earned income credit is rounded, were inferred from the published figures and labelled as inferred.
The document we compared with is the version posted on October 17, 2025, which replaced an October 9 version at the same address. The repository holds both.
The computation in full
The two twelve-month averages of the C-CPI-U, as available on September 11, 2025, are 177.113750 for the period ending August 2025 and 173.015833 for the period ending August 2024. Their ratio is 1.0236852; the statute directs that 2024 be the base year, so the cost-of-living adjustment for 2026 is 0.0236852.
Increase: 23,625 × 0.0236852 = 559.56. Rounded to the next lowest multiple of $50: 550. Amount: 23,625 + 550 = $24,175.
The published $24,150 is consistent with a different convention: multiply the base, 23,625 × 1.0236852 = 24,184.56, and round the whole amount down to a multiple of $50. The two conventions agree for most amounts in the table. They disagree here.
Head-of-household standard deduction for 2026. Base $23,625 multiplied by 1.0236852 gives $24,184.56. The statute rounds the increase of $559.56 down to $550, so the amount is $24,175. The published figure, $24,150, is consistent with rounding the whole amount of $24,184.56 down to a multiple of $50. The published increase, $525, is not a multiple of $50.
The single deduction from the same clause, base $15,750, is $16,100 under both conventions, and the joint deduction, $32,200, is twice that. Only the head-of-household increase falls on the other side of a $50 step.
The same pattern appears in §6334(d)(4), the amount of wages exempt from levy for each dependent. §6334(d)(4)(C) increases a base of $4,150 by the adjustment from 2017 and says: "If any increase determined under the preceding sentence is not a multiple of $100, such increase shall be rounded to the next lowest multiple of $100." For 2026 the increase is 1,167.12, rounded down to 1,100: $5,250. Published: $5,300, which is 4,150 + 1,150, and 1,150 is not a multiple of $100. Rounding the whole amount, 5,317.12, down to $100 gives 5,300. For 2025 the same two conventions gave $5,150 under the text and $5,100 as published in Rev. Proc. 2024-40. The convention is stable; the sign of the difference changes from year to year. The qualified disability trust deduction in §642(b)(2)(C) borrows the §6334 rule and shows the same $5,300.
The scale is small: $25 of deduction for a head of household, a few dollars of tax; $50 a year per dependent in the levy tables, about a dollar a week for a person whose wages are under levy. The point is the method, and whether the published figures can be verified from the statute.
Findings by kind
Every amount that did not match falls into one of the kinds below. The row-by-row table, with the reconstructed figure, the rule and a note for each of the 295 rows, is in the repository.
| Where | What | Status | Check |
|---|---|---|---|
| §63(c)(7)(B)(ii), item 4.14(1) | Head-of-household deduction: statute $24,175, published $24,150 | Published figure cannot result from the statute's increase-rounding rule; consistent with rounding the whole amount | Rev. Proc. 2025-32 §4.14(1); 26 U.S.C. §63(c)(7) |
| §6334(d)(4)(C), items 4.50 and 4.35 | Per-dependent levy amount and qualified disability trust deduction: statute $5,250, published $5,300 (2025: $5,150 and $5,100) | Same | Rev. Proc. 2025-32 §4.50, §4.35; 26 U.S.C. §6334(d)(4) |
| §213(d)(10)(B), §7702B(d)(5), items 4.27 and 4.62 | Long-term care premium limits and per diem. Read literally, the clause mixes two index scales (the medical component of the C-CPI-U against the medical component of the CPI-U for August 1996, scaled by the all-items ratio). The published figures match a link through the medical component itself; two of them ($930, $430) also match whole-amount rounding where the text rounds the increase ($935, $435) | Question of basis. The clause directs the Secretary to prescribe an adjustment after consultation with HHS; we have not located a published act defining the adjustment applied | 26 U.S.C. §213(d)(10)(B)(ii) |
| §§6651(j), 6652(c)(7), 6695(h), 6698(e), 6699(e), 6721(f), 6722(f), 6726(d), items 4.52–4.59 | Penalties for returns and statements due in 2027. The statute indexes "for the calendar year" of the filing; all 75 published rows are consistent with the 2026 adjustment. Under the filing-year reading, computed with the eleven-month mean that omits October 2025, 57 rows would differ ($555 rather than $535 for the §6651(a) minimum) | Question of reading; the filing-year figures depend on index data published after the revenue procedure | 26 U.S.C. §6651(j); Rev. Proc. 2025-32 §4.52–4.59 |
| §1(j)(3)(B), item 4.01, Table 5 | Estates and trusts: one amount both ends the 10% bracket (base year 2016) and starts the 24% bracket (base year 2017). Published $3,300 uses 2016; 2017 gives $3,250, and three tax amounts follow | Text ambiguous; the published figures take one reading | 26 U.S.C. §1(j)(3)(B)(i) as amended by P.L. 119-21 §70101(b) |
| §45R(d)(3)(B), §512(d)(2), §513(h)(2)(C), §6039F(d) | $34,057.75 published as $34,100; $211.58 as $212; $13.878 as $13.90; $20,573.24 as $20,573 | Published figures are rounded; the statute states no rounding rule for these amounts | the cited sections |
| §59(j), item 4.11 | $9,750 published for the kiddie-tax AMT exemption cap; §55(d)(4)(A)(iii) says §59(j) "shall not apply", and P.L. 119-21 made that permanent | Computed correctly, for a provision that does not apply; also published in Rev. Proc. 2024-40 | 26 U.S.C. §55(d)(4)(A)(iii) |
| §642(b)(2)(C)(iii)(II); §877(a)(2) | A cross-reference to "the $4,150 amount in subparagraph (A)" where the amount sits in clause (iii)(I); a substitution of "2003" for "1992" in a subparagraph that has not contained "1992" since 2017 ($211,000 as intended and published; $161,000 read literally) | Drafting issues in the Code; no effect on the published figures | 26 U.S.C. §642(b)(2)(C), §877(a)(2) |
| §4611, item 4.45 | $0.27 per barrel published; the Oil Spill Liability Trust Fund rate expired after December 31, 2025, leaving $0.18 | Already corrected by Announcement 2026-2; our computation gives $0.18 | Announcement 2026-2 |
| Five amounts in four items changed between the October 9 and October 17, 2025 versions | EIC completed phaseout (joint, three or more children) $70,224 → $70,244; tax at the 24% bracket (married filing separately) $17,996 → $17,966; head-of-household 24% ceiling $201,775 → $201,750; low-income housing credit ceiling $3.05 / $3,530,000 → $3.416 / $3,953,600 | Already corrected; our computation equals the October 17 value in each case | both PDFs in the repository, with hashes |
The last two rows are the control that matters most. The IRS has itself corrected six of these amounts: five, in four items, between the two versions of its document, and the oil spill rate by a later announcement. The reconstruction from the text produces the corrected value every time.
Why the date of the index matters
§1(f)(6)(A) fixes which values of the index count: "the latest values so published as of the date on which such Bureau publishes the initial value of the Chained Consumer Price Index for All Urban Consumers for the month of August for the preceding calendar year." The C-CPI-U is published first as a preliminary value and revised afterwards, so the values available on September 11, 2025 are not the values on the BLS site today.
Of the 213 unqualified matches, 53 hold only on the September 11, 2025 values. On the revised values they would differ: the joint-return boundary between the 12% and 22% brackets would be $100,850 rather than the published $100,800; the 35%/37% boundary $769,100 rather than $768,700; the head-of-household 24%/32% boundary $201,850 rather than $201,750. A recomputation with fresh index data gives different figures and settles nothing. The index values have to be the ones the statute names, as of the date the statute names.
What open-source models do
Tax-Calculator and PolicyEngine-US, two widely used open-source models of the US individual income tax, both hold $24,150 for the 2026 head-of-household deduction, taken from Rev. Proc. 2025-32. We asked each project to document its convention. The Tax-Calculator maintainer answered the same day (issue #3191, October 21, 2026): the new revenue procedure supplies the indexed values each November, they are checked against the IRS forms early the following year, and the model stays consistent with other US income tax simulation models. The PolicyEngine-US issue (#10348) is open.
Following the published figures is a reasonable choice for a model that reproduces what the IRS administers. It also means the check compares one IRS document with another: the forms take their figures from the revenue procedure, so a difference between the statute and the published figure cannot appear there. A check against the statutory text is a separate step. This is that step.
2027, before the IRS
The August 2026 value of the C-CPI-U was first published on September 11, 2026. With it we computed all 307 amounts: the 295 rows above and 12 amounts whose base P.L. 119-21 set or reset, among them the $15,000,000 estate tax basic exclusion amount and the information-reporting threshold of §6041(a). One input is missing, as explained below, so every 2027 figure is stated with its treatment of that month.
The result is a file, forecast-2027.json, whose SHA-256 (beginning 07119073d7e96fad; the full value is in PIN.sha256) was recorded on October 21, 2026 at 11:53 UTC; the file was published the same day in the repository and submitted to OpenTimestamps. Before publishing we checked that no general 2027 revenue procedure had appeared on irs.gov; as of October 22, 2026, none had.
Every 2027 figure carries a condition, because of one missing number. BLS could not collect October 2025 prices during the lapse in appropriations, published no all-items C-CPI-U for that month, and has stated that "the October 2025 final C-CPI-U index will not be published." The statute's twelve-month average for 2027 runs from September 2025 to August 2026, and §1(f) does not say what to do when a month is missing. We computed four treatments: a mean of the eleven published months, geometric and linear interpolation of October, and carrying September forward. 163 of the 307 amounts depend on the choice. The joint-return 12%/22% boundary, for instance, is $104,050 with the eleven-month mean and $103,900 with linear interpolation. 67 amounts are the same under both readings and all four treatments we modelled; the single standard deduction, $16,600, and the joint deduction, $33,200, are among them.
The head-of-household deduction for 2027 is the most informative figure in the table. With the eleven-month mean, the statute's rule, the increase rounded down to $50, gives $24,925; rounding the whole amount down to $50 gives $24,950, or $24,900 with October interpolated. Whichever figure the IRS prints will show which rounding it applied: because the base ends in 625, the statute's rule always produces an amount ending in 25 or 75, and whole-amount rounding always produces a multiple of $50. The per-dependent levy amount separates the two rules as well: $5,450 under the text, $5,400 if the whole amount is rounded down to $100.
The penalty rows cannot be computed in advance under the filing-year reading, because the index for 2028 does not exist yet; the 2027 table shows them under the prior-year convention the published figures have followed.
Check it yourself in five minutes
- Open Rev. Proc. 2025-32, section 4.14(1). Heads of households: $24,150.
- Open 26 U.S.C. §63 (or the Cornell LII copy), subsection (c)(7)(B)(ii). The base is $23,625; the last sentence rounds the increase to the next lowest multiple of $50.
- 24,150 − 23,625 = 525. Divide by 50.
That takes two minutes and no index data. For the rest: the two twelve-month averages above, one division, one multiplication, one rounding. For §6334(d)(4): 5,300 − 4,150 = 1,150; divide by 100.
The repository holds the statute texts with their hashes, both versions of the revenue procedure, the index series as of the dates used, one indexing record per row quoting its rule, the scripts, and the SHA-256 of every result. Reproduction needs Python and nothing else.
What we do not claim, and whom we told
This is not legal advice, and nothing here is a recommendation to amend a return or to depart from published IRS instructions. We do not say the IRS erred: the published figures are consistent with a stable convention, and a convention may rest on a basis we have not found. We say that two amounts cannot be produced by the rounding rule as the statute words it, that one adjustment rests on a basis we could not locate in published guidance, and that one phrase, "for the calendar year", admits a reading the published figures do not follow. These are statements about text and arithmetic, and each can be checked from the citations.
On October 21, 2026 we sent these findings, with the repository, to the staff of the Joint Committee on Taxation, the Senate Finance Committee, the Government Accountability Office, the Taxpayer Advocate Service, the Treasury Inspector General for Tax Administration, the Department of Health and Human Services, the AICPA Tax Section and the National Association of Enrolled Agents, to the maintainers of Tax-Calculator, PolicyEngine-US and TAXSIM, and to the tax press. When the 2027 revenue procedure appears, the row-by-row comparison goes into the same repository. Corrections to hi@arxo.io.
Why we did this
At Arxo Law we build executable models of legal texts, and the Internal Revenue Code is one of them. An executable Code needs its indexed amounts built from the statute's own rule and the index values the statute names, not copied from a table; building them is the only way to know when table and text part ways. This year they part at $25 for heads of household and $50 per dependent for people under levy, and at a question about which year's index the penalty provisions mean. Next year the question is how a missing month is treated. The test does not change: a published figure should be derivable from the text, and where it is not, the question goes to the text or to the publication.
Rifat Jumagulov, founder, Arxo Law
Sources
- Reproduction package: github.com/arxohq/arxo-irs-inflation-adjustments: FINDINGS.md, 2026 table, 2027 forecast,
forecast-2027.json,PIN.sha256,SHA256SUMS,sources/. - IRS: Rev. Proc. 2025-32 (I.R.B. 2025-45), Rev. Proc. 2024-40, Announcement 2026-2, Publication 1494.
- Code: 26 U.S.C. §1, §63, §6334; USCODE-2024-title26 and P.L. 119-21 on govinfo.
- BLS: Handling of missing 2025 data.
- Earlier on this blog: The Law Has Unit Tests, Law as Function, Law as Data.
Originally published on blog.arxo.io on October 22, 2026.
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