Methodology & editorial standards
Every calculator here shows its formula. This page shows where the numbers come from, who checks them, and what happens when something is wrong.
How the tools are built
Each tool is a static page: HTML, CSS, and plain JavaScript. The calculation runs in your browser, so the inputs never leave your device and the result is instant. There is no account, no server-side processing, and no model-generated answer — the output is deterministic arithmetic from the formulas listed on the page.
Anything that changes by year or jurisdiction is stored as an explicit constant (for example, the Social Security wage base) so it can be updated and cited, rather than buried inside a formula.
Sources used
| Tool | Primary sources |
|---|---|
| 1099 self-employment tax | IRS Schedule SE instructions; IRS Topic 554; SSA wage-base announcements; IRS Additional Medicare Tax guidance |
| Mortgage early payoff | Standard amortization formula; CFPB mortgage servicing guidance; lender prepayment disclosures |
| Mortgage interest, property tax, capital gains | IRS Publication 936 (mortgage interest); IRS Publication 530 (tax information for homeowners); IRS Publication 523 (selling your home); Rev. Proc. 2025-32 (2026 standard deduction and bracket thresholds) |
| Home office, rental, and 1099 income | IRS Publication 587 (business use of your home); IRS Publication 527 (residential rental property); Schedule E instructions; 27.5-year residential recovery period under MACRS |
| Homeowners insurance | NAIC homeowners policy guidance; insurer coinsurance clauses (80% requirement); replacement-cost vs. market-value distinction |
| Concrete volume and bag counts | Bag yields printed on Quikrete and Sakrete bags; ACI 332 / ACI 301 thickness guidance; NRMCA ready-mix ordering guidance |
| Concrete cost and weight | NRMCA ready-mix price reporting; regional ready-mix plant quotes; ASTM C33 aggregate grading; 4,050 lb per cubic yard for normal-weight mix |
| Footings and reinforcement | ACI 318-19 §7.7.2.3 (flexural spacing) and §24.4.3.3 (shrinkage and temperature reinforcement, 0.0018); ACI 332 residential footing minimums; CRSI placing tolerances; local frost-depth tables |
| Lumber and hardwood | NIST PS 20-20 (American Softwood Lumber Standard) for nominal vs. actual dimensions; NHLA grading rules; supplier pricing sheets |
| AC tonnage, BTU, and load | ACCA Manual J (8th ed.) block-load method; ACCA Manual S for equipment selection; ASHRAE Fundamentals 1% cooling and 99% heating design temperatures; IECC climate zones 1–7; ASHRAE 62.2 ventilation rates |
| CFM and duct sizing | Sensible heat equation 1.08 × CFM × ΔT at sea level; ACCA Manual D friction-rate method; ASHRAE duct velocity limits for supply, return, and branch runs |
| SEER2, heating cost, and heat pump vs. furnace | DOE 2023 energy conservation standards (13.4/14.3 SEER2 split, 7.5 HSPF2 heat pumps); AHRI directories for rated capacity; EIA residential electricity and natural gas price series; equivalent full-load hours by climate zone |
| Installed HVAC cost | Contractor price surveys by region; state and local permit fee schedules; EPA Technology Transitions rule (GWP ≤700 from 2025-01-01, R-454B and R-32); DOE refrigerant handling requirements |
| GPA calculator | Common US 4.0 and 4.3 scales as published by university registrars; honors/AP weighting policies vary by district |
| Word game helpers | ENABLE word list; the public Wordle accepted-guess list; standard English Scrabble tile values |
Review and update policy
- Year-dependent constants (tax brackets, wage bases, bag yields) are checked each January and whenever an agency publishes a change.
- Every tool page carries a “last reviewed” date and a short list of the sources it relies on.
- Corrections take priority over new tools. A confirmed error is fixed on the page and recorded below.
- No page is published without a worked example that a reader can reproduce by hand.
Who writes this
Every number on this site is checked against the primary source named at the bottom of the page — an IRS publication, an ACI provision, an ACCA manual, a DOE standard, or a manufacturer's printed yield. Those citations are links, so you can open the source and compare it to what the page claims.
The formula shown in the prose is the formula the script actually runs. Where a page states a constant — the 2026 Social Security wage base, the $40,400 SALT cap, the 13.4 SEER2 minimum — that constant exists once in the code as a named value, so a change breaks something visible instead of drifting silently out of the prose.
Money pages are re-checked against the current IRS publications before each filing season. Construction pages are checked against manufacturer bag yields and ACI provisions. Heating and cooling pages are checked against ACCA Manual J procedures and the efficiency standards in force at the time of writing.
The correction log below is the record. Report an error and it gets fixed; corrections come before new tools.
Known limitations
- Tax tools estimate self-employment tax only; they do not compute federal or state income tax.
- Mortgage math assumes a fixed rate, no prepayment penalty, and extra payments applied to principal.
- Concrete volumes assume the bag yields printed by major manufacturers; always confirm the figure on the bag you buy.
- Heating and cooling loads follow ACCA block-load principles with a simplified envelope model. A Manual J produced in a contractor's software will differ in the details, especially for unusual construction — use these pages to sanity-check a bid, not to replace a load calculation.
- Installed HVAC costs are regional ranges, not quotes. Within one market, contractor pricing varies more than it does between markets.
- Word lists are public dictionaries, not official tournament lexicons.
Corrections log
| Date | Page | Change |
|---|---|---|
| 2026-10-30 | 1099 self-employment tax | Worked-example table corrected for the 2026 wage base ($184,500). |
| 2026-10-30 | Mortgage early payoff | Example text aligned with the calculator's own default output. |
| 2026-10-30 | Concrete calculator | Bag count example corrected (one cubic yard ≈ 45 × 80 lb bags). |
| 2026-10-30 | SEER2 savings | Payback in the worked example recomputed from the calculator's own default output; the earlier text overstated annual savings and therefore understated payback. |
| 2026-10-30 | Duct sizing | Friction-rate example corrected to agree with the page's own formula — a 10 in. round trunk carries about 370 CFM at 0.08 in. w.g., not 290. |
| 2026-10-30 | Heat pump vs. furnace | Auxiliary-heat share now weighted by part-load hours instead of scaling linearly with capacity shortfall, which roughly halved the calculated cold-climate penalty. |
| 2026-10-30 | Heating cost comparison | Break-even gas price against an 80% furnace corrected to about $1.50 per therm at the default electricity price; fuel ranking in the prose reordered to match the calculated order. |
Contact
Report an error or suggest a tool at the contact page.