How the calculators work

Step by step

1. 1. Find your business profit

   Your mileage deduction is your business miles times the IRS standard mileage rate: 72.5¢ per mile from January 1 to June 30, 2026, and 76¢ per mile from July 1 to December 31, 2026. We add your other expenses and subtract the total from your earnings.

   Sources: IRS: Standard mileage rates

2. 2. Self-employment tax

   92.35% of your profit is taxed at 15.3%: 12.4% for Social Security (only on the first $184,500 of W-2 wages and self-employment earnings combined) plus 2.9% for Medicare. If that 92.35% figure is under $400, there is no self-employment tax.

   Sources: IRS: Self-employment tax, IRS Topic 554, SSA: Contribution and benefit base

3. 3. Additional Medicare Tax

   An extra 0.9% applies to wages and self-employment earnings above $200,000 ($250,000 if married filing jointly, $125,000 if married filing separately). W-2 wages use up that threshold first.

   Sources: IRS Topic 560: Additional Medicare Tax

4. 4. Federal income tax

   We start from your W-2 wages plus your profit, minus half of your self-employment tax. Then we subtract the 2026 standard deduction ($16,100 single, $32,200 married filing jointly, $24,150 head of household), the tips deduction (up to $25,000; not available if married filing separately; reduced when modified AGI is over $150,000, or $300,000 if married filing jointly), and the qualified business income deduction (generally 20% of business income, at least $400 if you have $1,000 or more, and reduced at high incomes). The 2026 tax brackets are applied to what is left.

   Sources: IRS Rev. Proc. 2025-32 (PDF), IRS: No tax on tips, IRS: Qualified business income deduction

5. 5. Only the extra tax from your gig income

   We figure your income tax twice, with and without your gig income, and show the difference. If you have a W-2 job, its own tax (usually withheld from your paycheck) is not part of the result.

6. 6. Quarterly payments

   The total is divided into four equal payments, due April 15, 2026, June 15, 2026, September 15, 2026 and January 15, 2027.

   Sources: IRS Form 1040-ES (2026, PDF)

Figures and sources (2026)

Federal income tax brackets (2026)

Sources: IRS Rev. Proc. 2025-32 (PDF)

What is not included

The calculators ask only a few questions, so they make assumptions. Your actual return can differ from the estimate because:

• Federal tax only. State and local income taxes are not included.
• No tax credits, such as the Earned Income Tax Credit or the Child Tax Credit. Credits would lower your actual tax.
• The standard deduction is always used. If you itemize, your result will differ.
• The only income counted is your gig income and W-2 wages. Interest, dividends, capital gains, retirement income, a spouse's self-employment income and other income are not included.
• Car costs use the standard mileage rate only, not actual expenses. Other expenses are taken as the total you enter.
• If you enter only your total miles, they are split evenly between the 72.5¢ and 76¢ periods.
• All tips you enter are assumed to qualify for the tips deduction.
• Your work is treated as one business with no employees. This matters only for the qualified business income deduction at high incomes.
• A business is treated as a specified service business only where a page asks and you tick the box. Pages that don't ask assume it is not one.
• W-2 wages are assumed to count in full for both Social Security and Medicare.
• Quarterly payments are the total split into four equal parts. The calculators don't work out the underpayment penalty, the safe harbor based on last year's tax, or tax already withheld from a W-2 job.
• Results are not rounded the way a tax return is. Many returns use the IRS Tax Table, which works in small income ranges instead of exact amounts, so your return may differ by a few dollars.
• Tax year 2026 only.

How the numbers are checked

The calculation code is separate from the web pages and has automated tests. The test cases cover ordinary situations and edge cases: income below the $400 self-employment tax floor, W-2 wages above the Social Security wage base, a business loss, the tips deduction phase-out, and the qualified business income deduction phase-in, with and without a specified service business. Their expected results were worked out independently of the calculator code, following the IRS forms and worksheets, and the tests check the calculator against them to the cent.

The tests also check that the tax brackets reproduce the amounts printed in the IRS rate tables. Every change to the calculation code has to pass all of these tests.

When figures are updated

The IRS and the Social Security Administration usually publish the next year's inflation-adjusted amounts in October or November. The figures here are reviewed when they do, and whenever an amount changes during the year. For example, the standard mileage rate for 2026 changed on July 1, 2026, from 72.5¢ to 76¢ per mile, and the calculators use both rates.