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Side Business Taxes 101: Schedule C, Self-Employment Tax and Estimated Payments

Maya Brooks

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Red calculator with tax keys, a pencil and a yellow sticky note on a desk

Your first sale feels like a finish line. For tax purposes, it’s a starting line. The moment money comes in from your side business, the IRS treats it as income, and because nobody withholds tax from it the way your employer does from your paycheck, you’re the one who has to plan for it.

This guide walks through the federal basics in plain words: what counts, which forms show up, what self-employment tax is and when estimated payments come into play. It’s general information as of October 2026, not tax advice. State rules differ, and a tax professional can look at your actual numbers.

Every Dollar Counts, Form or No Form

The IRS is blunt about this: all income is taxable, whatever the amount, unless the law specifically says otherwise. Whether or not you receive a tax form doesn’t change that.

That matters because of Form 1099-K, the form payment apps and online marketplaces send to report what they paid you. After the One, Big, Beautiful Bill, the federal threshold for those platforms went back to its earlier level: they generally only have to file a 1099-K for you when you pass both more than $20,000 in payments and more than 200 transactions. Card payments processed directly have no threshold, and some states set lower limits of their own. Most new side businesses will never hit the federal platform threshold, and that income is still reportable.

Hobby or Business?

If you sell things now and then for fun, the IRS may see a hobby. If you’re working at it to make a profit, it’s likely a business. No single test decides it. The IRS looks at factors such as:

  • Whether you run it in a businesslike way, with accurate books and records
  • Whether the time and effort you put in show you intend to make a profit
  • Whether you’ve made a profit in some years, or your losses are normal for a startup
  • Whether you change your methods to improve profitability
  • Whether you and your advisers know what it takes to run it as a business

Either way, profit from selling goods or services is taxable. The difference is mainly in how income and expenses are handled, which is one reason to keep records from day one.

Schedule C: Where Your Business Shows Up

If you run the business on your own without forming a company, you’re a sole proprietor by default. Sole proprietors report business income and expenses on Schedule C, which is attached to your regular Form 1040. Income minus allowable business expenses gives you your net profit, and that profit is what most of the following is based on. The IRS’s Publication 334 is the long-form guide to what counts as a business expense.

Self-Employment Tax and the $400 Line

This is the part many first-timers don’t see coming. On top of income tax, self-employed people pay Social Security and Medicare tax on their business profit. Your employer pays half of that on your wages, but on side-business profit you cover both halves.

  • The rate is 15.3%: 12.4% for Social Security and 2.9% for Medicare.
  • If your net earnings from self-employment are $400 or more for the year, you file Schedule SE and pay it.
  • You can deduct the employer-equivalent portion of self-employment tax when figuring your adjusted gross income. That lowers income tax, not the self-employment tax itself.
  • The Social Security part applies only up to a yearly wage base that counts your day-job wages and self-employment earnings together. The limit changes every year, so check the current figure on irs.gov.

Estimated Taxes: Paying as You Go

Because nobody withholds tax from your side income, the IRS expects you to pay during the year. Individuals generally need to make estimated payments if they expect to owe $1,000 or more when they file. Paying too little, too late can mean a penalty, even if you end up getting a refund.

For the 2026 tax year, the payment dates on Form 1040-ES are:

  • April 15, 2026
  • June 15, 2026
  • September 15, 2026
  • January 15, 2027

If you have a day job, there’s a simpler option the IRS points out: ask your employer to withhold more by submitting a new Form W-4 with an extra amount per paycheck. The IRS Tax Withholding Estimator can help you figure out how much. For many people with modest side income, that’s easier than tracking quarterly deadlines.

A Simple System for Your First Year

You don’t need accounting software to start. You need a few habits:

  1. Keep business money apart from personal money, ideally in a separate account, so your records are clean.
  2. Log every payment you receive and every business expense, with date, amount, who it was to or from and what it was for. The IRS says records should show your income and expenses clearly and be backed by receipts, invoices or statements.
  3. Each time money comes in, move a fixed share into a savings account set aside for taxes. The right share depends on your tax bracket, your state and your day-job withholding, so check it with the withholding estimator or a tax professional rather than guessing.
  4. Put the estimated-tax dates in your calendar now, even if you’re not sure you’ll need them.
  5. Check your state revenue department’s site, since state income tax and sales tax rules are separate from everything above.

Taxes are a cost of doing business, not a sign you’ve done something wrong. Handled early, they’re a short monthly chore. Ignored, they become a stressful April.

Do This Today

Create a simple spreadsheet with four columns: date, source, amount and notes. Enter every payment your side business has received this year, add up the total, and put a reminder in your calendar for January 15, 2027, with a note to decide whether you need an estimated payment or a W-4 change. That’s under an hour, and it gives you a real number instead of a worry.

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Maya Brooks

Maya Brooks writes the Getting Started guides, focused on the first 90 days: picking one business model, setting it up cheaply and getting to a first sale without drowning in options.