Can ChatGPT calculate your quarterly estimated taxes? What goes wrong

This post is general information, not tax advice. Rates, limits and dates are for tax year 2026 from the IRS and SSA pages listed under Sources, checked October 24, 2026. Your own estimate depends on facts this post can't see: use the worksheet in Form 1040-ES and check it with a CPA or enrolled agent.

"How much should I send the IRS this quarter?" is one of the most natural things to ask a chatbot, and the answer comes back fast and sure. The trouble is that the self-employment tax formula has a few steps that are easy to skip, and a model that skips one still sounds right. Below is what the IRS actually asks for, the five mistakes to look for, a worked example with the numbers, and a split of who should do which part.

What the IRS asks for

The IRS's estimated taxes page says that if you're in business for yourself you generally need to make estimated tax payments, and that individuals generally must pay if they expect to owe $1,000 or more when they file. Self-employment tax is figured on Schedule SE when net earnings from self-employment are $400 or more, at 15.3%: 12.4% for Social Security and 2.9% for Medicare (IRS Topic 554). The year is split into four payment periods, and Form 1040-ES has the worksheet and the dates. A date that falls on a weekend or legal holiday moves to the next business day.

Five places chatbots get the math wrong

1. Applying 15.3% to all profit. Self-employment tax is figured on 92.35% of net earnings, not 100%. Skipping that step overstates the tax on every dollar.
2. Ignoring the Social Security wage base. The 12.4% Social Security part only applies up to $184,500 of combined wages and self-employment earnings for 2026 (SSA). Medicare's 2.9% has no cap. A W-2 job on the side uses up part of that $184,500.
3. Forgetting the Additional Medicare Tax. IRS Topic 560 adds 0.9% on earnings above $200,000 for a single filer ($250,000 married filing jointly). It's figured separately, on Form 8959.
4. Using calendar quarters. The second payment period covers only April and May, and the fourth covers September through December. A chatbot that splits the year into four three-month quarters gives the wrong amount per period and sometimes the wrong due date.
5. Getting safe harbor wrong. You generally avoid the underpayment penalty by paying at least 90% of this year's tax or 100% of last year's, or 110% if last year's AGI was over $150,000 ($75,000 if married filing separately), per Form 1040-ES and Publication 505. A single "pay 25% a quarter" answer misses this.

Two smaller slips are common too. Half of self-employment tax is deductible when figuring adjusted gross income (Topic 554), which lowers income tax, not self-employment tax. And state estimated taxes have their own rules and dates, which a federal answer won't cover.

Worked example: $60,000 and $250,000 of profit

A fair test you can run on any chatbot

I'm a sole proprietor in the US with no W-2 wages. My net profit from self-employment for tax year 2026 is $[amount].

1. Calculate my 2026 self-employment tax step by step. Show net earnings (profit × 92.35%), the Social Security part with the 2026 wage base, and the Medicare part.
2. Tell me if the Additional Medicare Tax applies and on what amount.
3. List the 2026 estimated tax payment periods and due dates from Form 1040-ES.
4. Explain the safe harbor rules in one paragraph.
5. Cite the IRS page for each rule.

Then compare its answer with: net earnings = profit × 0.9235; Social Security = 12.4% of the smaller of net earnings and $184,500; Medicare = 2.9% of net earnings.

If the chatbot's figure differs from the formula at the bottom, the formula wins. OpenAI's own help center says ChatGPT can give incorrect answers with high confidence, and Anthropic's documentation says techniques to reduce made-up answers lower the risk but don't remove it. The self-employment tax calculator does the same arithmetic in code.

Who should do what

How SoloWOS splits the work

SoloWOS has a Taxes tab in Expenses that estimates what to set aside, and the code does all the math. For a US workspace it works per Form 1040-ES payment period: profit for the period and the year from invoices paid and expenses logged, self-employment tax on the year to date minus what earlier periods already counted (so the wage base is applied once, assuming no W-2 wages), and federal income tax as the set-aside percentage you choose times the period's profit, without brackets. The page shows the total, the due date with the weekend and holiday rule applied, a 12-month calendar, the official sources with the date they were checked, and "How we got these numbers".

Every result carries the same notice: "Estimate to help you set money aside. It isn't tax advice and doesn't file anything: check it with your contador / CPA." SoloWOS doesn't file or pay; the page links to IRS Direct Pay. Five days before a payment is due, you get a notification in the app with the amount and the breakdown.

Explain with AI is optional. It sends Anthropic only the estimate SoloWOS calculated (the period, due date, totals on each line, your set-aside percentage and the assumptions), never client names, invoices or expenses, and returns 3 to 6 sentences on what each number means and what to bring to your accountant. The AI is told not to suggest strategies, deductions or tax positions, and SoloWOS drops any sentence that tells you to deduct something. It uses one AI draft: 3 a month on Basic, 200 on Solo. Under Anthropic's commercial terms the data isn't used to train their models.

For the full walkthrough of quarterly payments, read quarterly estimated taxes for freelancers, and for a rule of thumb on the percentage, how much to set aside for taxes. The estimate starts from the expenses you log with receipts. Other posts on what AI should and shouldn't do with your money are in AI for freelancers, including AI bookkeeping for 1099 contractors.