How the Math Works
Getting to taxable income. Total income comes from Parts 2 through 7. Adjustments in Part 8 come off to give adjusted gross income, and half of any self-employment tax is subtracted automatically. From there the worksheet takes the larger of your standard deduction or your itemized deductions, then subtracts the Schedule 1-A deductions for tips, overtime, car loan interest and age 65, and finally the qualified business income deduction.
Two sets of rates. Ordinary income fills the brackets from the bottom up. Qualified dividends and gains on assets held more than a year sit on top of that ordinary income and are taxed at 0, 15 or 20 percent depending on where your total taxable income falls. That is why the bracket chart shows your ordinary income against the ordinary bands and treats the preferential income separately.
Self-employment tax. Net business income is multiplied by 92.35 percent, then taxed at 12.4 percent for Social Security up to the annual wage base and 2.9 percent for Medicare with no ceiling. W-2 wages use up the Social Security wage base first. Half of the resulting tax is an adjustment to income.
Social Security benefits. None, half or 85 percent of your benefits are taxable depending on provisional income, which is your other income plus tax exempt interest plus half your benefits. The thresholds have never been indexed for inflation, which is why more people pay tax on benefits every year.
The 90 percent safe harbor. If at least 90 percent of your total tax is paid by the original due date, and the rest is paid when you file by the extended deadline, the IRS waives the late payment penalty. Interest still runs on whatever is unpaid, so the safe harbor is a floor, not a target.
Failure to file penalty. This is the expensive one, and it is the whole reason to file an extension. Without an extension the penalty is 5 percent of the unpaid tax per month or part of a month, up to 25 percent, reduced by the late payment penalty in months where both apply. Filing Form 4868 by April 15 removes it entirely through October 15.
Estimated tax penalty. This one is separate from the extension. If your withholding and quarterly estimates during the year came in under the smaller of 90 percent of this year's tax or 100 percent of last year's tax (110 percent if last year's AGI was over $150,000), the IRS can charge an underpayment penalty on Form 2210. A payment made in April does not cure it, because the penalty is based on when payments were made during the year.
What This Worksheet Does Not Do
It covers federal tax only. Texas has no state income tax, so for most clients of the firm that is the whole picture, but income taxed by another state is outside its scope.
The qualified business income deduction is a reasonable ceiling rather than a precise figure. Below the income threshold it is 20 percent of your business income. Above it, the deduction depends on the wages your business pays and disappears entirely for many professional practices.
The alternative minimum tax is not calculated. Since 2018 it reaches very few people who do not exercise incentive stock options or hold private activity bonds. If either applies, this worksheet will understate your tax.
Married filing separately, part year residency, the earned income credit, reconciling a health insurance subsidy, the net operating loss rules, and the passive activity loss limitations are all outside its scope. So is the underpayment penalty itself: the worksheet tells you what you need to pay in to be safe and what you are short, but it does not price the penalty if you were late earlier in the year.
If your situation is at all unusual, a business sale, a large stock position, a move between states, or a first year of self-employment, the answer here is the beginning of the conversation rather than the end of it.