Guides

The calculators give a number; these pages explain the rules that produce it, with the statute or form cited for each one and worked figures computed by the same engine. Each guide names the documents it was written from.

• 2026 estimated tax due dates and the safe harbor rule

  The four deadlines, who has to pay, and how the 90% / 100% / 110% test sets the minimum — with figures for three levels of profit.

• What happens if you miss a quarterly payment

  The charge is interest at the IRS underpayment rate, by the day, until you pay. How Form 2210 computes it, what a typical miss costs, and the exceptions.

• Using W-2 withholding to cover 1099 tax

  Withholding counts as paid evenly through the year whenever it was withheld. How to size the extra amount on Form W-4 and when it stops being enough.

• The self-employed health insurance deduction

  Who qualifies, the earned-income cap, the month-by-month employer-plan rule, and why it lowers income tax but not self-employment tax.

• The home office deduction

  Regular and exclusive use, the simplified $5-per-square-foot method against Form 8829, the gross income limit, depreciation and recapture — and why a Schedule C deduction saves more per dollar.

• The business mileage deduction

  2026's two standard rates, what counts as business driving and why commuting does not, the standard-rate versus actual-expense choice, and the records §274(d) requires.

• The annualized income installment method

  Lower estimated payments when income arrives late in the year: how Schedule AI's four columns work, the cap that stops it raising early payments, and a worked schedule.

• SEP-IRA vs solo 401(k)

  The 2026 ceilings at five profit levels, why 25% becomes 20%, deadlines, and the federal and state tax a contribution saves.