Calculation Validation & Test Evidence
This page documents how the reviewed calculators are tested, the formula and data version each one runs, deterministic boundary tests for the highest-risk logic, and a log of material corrections. It is a companion to our Methodology.
Scope note: the calculators are educational estimators, not tax-preparation software. Independent professional (CPA / Enrolled Agent) sign-off on the tax formulas is not yet in place; where that matters, each page carries a disclaimer and links to the primary IRS source. The tax boundary tests below verify the safe-harbor decision logic, which is deterministic and independent of the annual rate dataset.
Formula & data versions (published calculators)
| Calculator | Formula version | Data source | Last reviewed |
|---|---|---|---|
| Quarterly Estimated Tax | v2.2 | IRS Rev. Proc. 2025-32 rate tables; IRS Form 1040-ES; published TY2026 dataset | 2026-09-07 |
| Self-Employment Tax | v1.1 | IRS Schedule SE; IRS Form 8959; SSA wage base; TY2026 dataset | 2026-09-07 |
| Home Office Deduction | v1.0 | IRS Simplified Method | 2026-09-07 |
| Etsy Profit | v1.1 | Etsy Fees & Payments Policy, including the Offsite Ads base and cap | 2026-09-07 |
| Amazon FBA Profit Estimator | v1.3 | Amazon FBA fee schedule (representative tiers) | 2026-09-07 |
| Freelancer Fee & Tax-Reserve Estimator | v1.1 | Platform fee pages; user-entered reserve % | 2026-09-07 |
| VAT Calculator | v1.2 | Standard VAT arithmetic (user-set rate, 0-100%) | 2026-09-07 |
Quarterly estimated tax - safe-harbor boundary tests
These cases verify the rule that decides the prior-year safe-harbor percentage and the smaller-of logic. They are deterministic (independent of the annual bracket dataset) and match the implementation exactly.
| # | Scenario | Expected behavior |
|---|---|---|
| 1 | Prior-year AGI $149,999, not MFS | Safe-harbor rate = 100% |
| 2 | Prior-year AGI $150,000, not MFS | Safe-harbor rate = 100% (threshold is "above") |
| 3 | Prior-year AGI $150,001, not MFS | Safe-harbor rate = 110% |
| 4 | Married Filing Separately, prior-year AGI $74,999 | Safe-harbor rate = 100% |
| 5 | Married Filing Separately, prior-year AGI $75,000 | Safe-harbor rate = 100% |
| 6 | Married Filing Separately, prior-year AGI $75,001 | Safe-harbor rate = 110% |
| 7 | Current-year income $200,000 but prior-year AGI $90,000 | Rate stays 100% (decided by prior-year AGI, not current income) |
| 8 | Current-year income $90,000 but prior-year AGI $200,000 | Rate = 110% |
| 9 | Prior-year tax $0, prior year was a full 12-month year | No-prior-liability exception: $0 required, no penalty |
| 10 | 90% of current-year tax is smaller than the prior-year target | Required = the 90% current-year figure (smaller-of) |
| 11 | Balance due after withholding = $999 ($1,000 general rule) | $0 required (general-rule exemption, decided in the engine) |
| 12 | Balance due after withholding = $1,000 | Proceed through the safe-harbor worksheet |
| 13 | Balance due after withholding = $1,001 | Proceed through the safe-harbor worksheet |
| 14 | Single, $5,000 net SE income only (total tax $706, no withholding) | $0 required under the general rule (total $706 < $1,000) |
Formula v2.1 added the $1,000 general-rule gate to the engine (cases 11-14). Verified in-browser 2026-07-15.
Quarterly golden case (exact engine outputs)
Single filer, $80,000 net SE income, standard deduction, no withholding, prior-year tax $10,000, prior-year AGI $90,000 (full year). These are the calculator's exact outputs on the current published 2026 dataset (formula v2.1), verified in-browser 2026-07-15:
| Step | Exact value |
|---|---|
| SE tax base (92.35% × $80,000) | $73,880 |
| Self-employment tax | $11,304 |
| Federal income tax (after standard deduction, 2026 single brackets) | $7,527 |
| Total estimated tax | $18,830 |
| 90% current-year path | $16,947 |
| Prior-year safe harbor (100% × $10,000) | $10,000 |
| Required (smaller of the two) | $10,000 |
| Quarterly (÷ 4) | $2,500 |
Independently derived reference cases
The tables below this one are snapshots: they record what the engine currently produces, so they catch a change but cannot prove the engine is right. The cases in this table are different. The expected column was worked out from the primary source first, and the engine was then required to match it. An external audit on 2026-09-07 found four defects this way; all four are corrected and re-tested below.
| Case | Inputs | Expected, from the source | Live output |
|---|---|---|---|
| Income tax, Married Filing Separately | Taxable income $500,000 (W-2 $516,100 less the $16,100 standard deduction) | $146,082 Rev. Proc. 2025-32 s4.01 Table 4: 37% starts above $384,350, not $640,600 | $146,082 |
| Income tax, Head of Household | Taxable income $100,000 (W-2 $124,150 less the $24,150 standard deduction) | $14,901 Rev. Proc. 2025-32 s4.01 Table 2: 17,700 / 67,450 / 105,700 / 201,750 / 256,200 / 640,600 | $14,901 |
| SE tax deduction above the Additional Medicare threshold | $300,000 net SE profit, no W-2 wages, Single | $15,456 Schedule SE lines 10-13: half of ($22,878 Social Security + $8,034.45 Medicare). The $693.45 Additional Medicare Tax is figured on Form 8959 and is not part of that half. | $15,456 Schedule SE tax $30,912, Additional Medicare $693 shown on its own line |
| SE tax below the $400 floor | $100 net SE profit | $0 Schedule SE line 4c: net earnings of $92 are below $400, so no SE tax is due | $0, with the reason stated on the page |
| Etsy Offsite Ads fee base | $35 item + $5 shipping charged, 15% rate | $6.00 fee, $16.25 profit Etsy charges the fee on the order total, shipping included: 15% of $40 | $6.00 fee, $16.25 profit |
| Etsy Offsite Ads $100 cap | $1,000 item + $5 shipping charged, 15% rate | $100.00 fee, $795.58 profit 15% of $1,005 is $150.75, above the $100 per-order cap | $100.00 fee (labelled as capped), $795.58 profit |
| Amazon loss keeps its sign | $15 sale price, $20 product cost, other inputs at their defaults | -$11.38 per unit A loss must read as a loss in every place the number appears | -$11.38 in the headline, breakdown, mobile summary and export; -$1,137.62 monthly |
| Sensitivity keeps a fractional amount | VAT extract mode, gross $120.60, rate 20% | Base row 120.60, results 80.40 / 90.45 / 100.50 / 110.55 / 120.60 The five scenario amounts are 96.48, 108.54, 120.60, 132.66 and 144.72, each divided by 1.20. The base row must equal the calculator's own result. | Base 120.60 and $100.50, matching the main result exactly |
| Export states the calculation mode | VAT, $120.60 at 20%, run once in add mode and once in extract mode | Two different results from identical inputs $144.72 adding VAT, $100.50 extracting it. The saved file has to name the mode, because the numbers alone do not. | Mode recorded as "add (net to gross)" and "extract (gross to net)", with the result labelled accordingly |
| VAT rejects an unsupported rate | Extract mode, $120 gross, rate -100% | No result Dividing by (1 + rate) is undefined at -100%; the calculator must refuse rather than print a number | Results blank with an explanation; no Infinity |
Live outputs above were captured in a browser on the canonical URLs on 2026-09-07, on release ych-r11-2026-09-07.
Golden cases - other published calculators (default inputs)
These are engine snapshots, not independent expectations: each calculator's default inputs produce a deterministic result, and a change to any of these values signals that something moved. They were captured in-browser and re-checked on release ych-r11-2026-09-07 (2026-09-07).
| Calculator | Input | Exact output |
|---|---|---|
| Self-Employment Tax | $50,000 net SE income | $7,065 Schedule SE tax (92.35% → 15.3%); deductible half $3,532 |
| Home Office Deduction | 250 sq ft (Simplified Method) | $1,250 ($5/sq ft); caps at $1,500 for 300+ sq ft |
| VAT | $100 net, 20% rate | $20 VAT, $120 gross |
| Etsy Profit | $35 item + $5 shipping, $8 COGS, $4.50 ship, $1 packaging, Offsite Ads off | $4.25 Etsy fees → $22.25 net profit/item |
| Amazon FBA Profit Estimator | $15 sale, 15% referral, small-standard tier, $4 COGS, $0.75 inbound | $5.63 Amazon fees → $4.62 net/unit |
| Freelancer Fee & Tax-Reserve Estimator | $50/hr × 120 billable, 10% + 2.75% fees, 22% + 15.3% reserve, 25 unpaid hrs | $3,282 monthly cash; $22.64 effective/hr |
Material corrections log
| Date | Page | Change |
|---|---|---|
| 2026-09-08 | All published calculators | The scenario and sensitivity block rounded every amount to a whole number, including the row that is supposed to mirror the calculator itself. Entering $120.60 in the VAT calculator produced a base row of 121 and a result of $100.83 next to a main result of $100.50. The base row now uses the exact value entered, and the other scenarios keep the precision of the control, so a rate typed as 52.50 stays 52.50 and an area typed as 175 sq ft stays a whole number. |
| 2026-09-08 | VAT Calculator | Formula v1.2: the downloadable summary did not record which direction the calculation ran, so the same amount and rate could produce two different results with nothing in the file to tell them apart. The export now names the mode and labels the result net or gross to match. The mobile summary heading also still read "Gross Amount" while showing the net figure in extract mode. |
| 2026-09-08 | Downloadable summaries | The saved file now cites the page's canonical address rather than whatever address the browser happened to be on, and keeps the blank lines that separate its sections. |
| 2026-09-08 | Home Office Deduction | The headline had been relabelled a ceiling, but the explanation under it still called the figure a deduction and the comparison table did the same. Both now carry the qualification, and the explanation states the two limits the area calculation does not model: the space must have been in regular and exclusive business use, and the deduction cannot exceed the income that business produced after its other expenses. |
| 2026-09-08 | Site-wide analytics | Google Analytics had been loading with no consent signal at all. It now runs under Google Consent Mode with every storage type denied by default for visitors in the EEA, the UK and Switzerland, so no analytics cookie or local-storage entry is created there; measurement continues without an identifier that persists between visits. Advertising and personalization storage are denied everywhere. The privacy, cookie and privacy-choices pages were rewritten to describe that behaviour instead of a consent banner the site does not show. |
| 2026-09-08 | Review dates | The seven published calculators still showed a calculation-logic date of April 2026 after their logic changed. Each now shows the date its logic actually changed, and the sitemap dates were moved to match. |
| 2026-09-07 | Quarterly Estimated Tax | Formula v2.2: corrected the Tax Year 2026 rate tables. The Head of Household table carried four wrong bracket boundaries, and no Married Filing Separately table existed at all, so that status silently used the Single table and reached 37% at $640,600 instead of $384,350. Both are now taken from IRS Rev. Proc. 2025-32 section 4.01 Tables 2 and 4, along with a Qualifying Surviving Spouse table and the MFS and QSS standard deductions. On the two cases an external audit reported, the income tax component was understating by $2,313 (MFS at $500,000 taxable) and overstating by $78 (Head of Household at $100,000 taxable). The audited tables are now pinned in the calculator code, so a dataset value is used only when it matches them. The Schedule SE $400 floor was also added to this calculator's SE component. |
| 2026-09-07 | Self-Employment Tax | Formula v1.1: the above-the-line deduction was taking half of the Social Security, Medicare and Additional Medicare taxes together. The Additional Medicare Tax is figured on Form 8959, is not part of the Schedule SE tax, and is not deductible, so it inflated the deduction by half of itself. At $300,000 of net profit the deduction showed $15,803 instead of $15,456. The page now shows the Schedule SE subtotal and the Form 8959 amount on separate lines and deducts only half of the former. Also added the Schedule SE line 4c $400 floor, and fixed a bug where changing income back to zero left the previous scenario's numbers in the breakdown. |
| 2026-09-07 | Etsy Profit | Formula v1.1: the Offsite Ads fee was charged on the item price alone and had no cap. Etsy charges it on the order total including the shipping you charge, and caps it at $100 per attributed order. A $35 item with $5 shipping was billed $5.25 instead of $6.00; a $1,005 order was billed $150.75 instead of $100. The methodology block had described the correct base since publication, so the code, not the documentation, was wrong. |
| 2026-09-07 | Amazon FBA Profit Estimator | Formula v1.3: a loss lost its minus sign in the headline figure, the breakdown and the mobile summary, so a $11.38 per-unit loss read as a $11.38 profit next to a correctly negative monthly figure. All result formatting is now signed. Separately, the default example rendered on the server still showed pre-surcharge numbers ($4.74 profit, $5.51 fees) while the browser computed $4.62 and $5.63; both are now generated from one set of constants. |
| 2026-09-07 | All published calculators | The scenario comparison and sensitivity block only watched the one primary input, so changing a cost, a filing status, a toggle or a preset left the "Current" row showing a stale result. It now follows every control on the page, and running a scenario snapshots and restores the whole form instead of one field. The downloadable summary now records selects, checkboxes and calculator mode alongside the numbers, plus the page URL, formula version and a machine-readable copy of the same run. |
| 2026-09-07 | VAT Calculator | Formula v1.1: a rate outside 0-100% is now refused at the calculation boundary. In extract mode a rate of -100% divides by zero and the page had displayed Infinity. |
| 2026-09-07 | Home Office Deduction | Removed an unsourced claim that the Simplified Method carries lower audit risk, and relabelled the result as the ceiling implied by the office area, before the qualifying-use and business-income limits. The same unsourced framing was removed from the companion guide. |
| 2026-09-07 | Quarterly Estimated Tax, Etsy guide, 1099-K guide | Copy corrections: the quarterly Limitations paragraph still said only the prior-year safe harbor was calculated, which had not been true since v2.0; the Etsy pricing rule of thumb treated flat fees as a percentage and described a divisor change in the wrong direction, and is replaced by a defined break-even formula with a worked example; and a cited IRS Schedule 1 page that now returns 404 was replaced with the live Form 1040 instructions. |
| 2026-07-15 | Quarterly Estimated Tax | Formula v2.1: implemented the IRS $1,000 general rule in the calculation engine (was previously only explained in text) - the calculator now returns $0 when the expected balance due after withholding is under $1,000. Reconciled the page copy (formula box, FAQ, worked example) to this single specification and replaced the illustrative worked example with exact deterministic outputs. |
| 2026-07-14 | Quarterly Estimated Tax | Formula v2.0: safe-harbor percentage now decided by prior-year AGI (was current-year income); added MFS $75,000 threshold, Head of Household / MFS / Qualifying Surviving Spouse statuses, standard/itemized deduction control, refundable-credit input, smaller-of (90% current / prior-year) logic, and the no-prior-liability exception. |
| 2026-07-14 | Freelancer estimator | Rescoped to a fee and tax-reserve estimator: tax percentages relabeled as user-entered reserves, not actual tax owed; keep-rate benchmarks removed. |
| 2026-07-14 | Amazon FBA estimator | Renamed to "Profit Estimator"; added a "what this estimate does not include" panel listing omitted Amazon fees. |
| 2026-07-14 | Site-wide copy | Replaced categorical profit and take-home claims with scoped estimate language across every calculator, hub card, and guide. |
| 2026-06-19 | Freelancer / Upwork | Upwork fee corrected from a flat 10% to the 0%-15% per-contract range. |
| 2026-06-19 | Amazon FBA | Added the 3.5% fuel & logistics surcharge (effective 2026-04-17) and an exact-fee override input. |
Found a discrepancy between a result and its stated method? Please report it with the calculator name and your inputs.