Taxes on Side Income and Multiple Income Streams

A side business, freelance work or a second job can add real money, and the tax bill on it often comes as a shock. Extra income is taxed at your highest rate, not your average one, and self-employment income carries a second tax that employees never see on their pay stub. This chapter explains how income from several sources stacks up, what self-employment tax is, which deductions and retirement accounts belong to a side business, and when changing the business's legal structure is worth the cost. Figures are 2026 federal amounts.

Income stacks: the side job is taxed at your top rate

The IRS does not tax each job separately. All your income is added together on one return, and the brackets from chapter 1 are applied to the total. A side income is effectively taxed on top of your salary, at your marginal rate.

On a salary of $70,000 this filer's effective federal rate is 9.4%. With side income bringing the total to $90,000, income tax rises from $6,570 to $10,970. Every side dollar here was taxed at the 22.0% marginal rate, far above the effective rate on the salary. Planning a side income around the effective rate is the most common reason people underestimate what they will owe.

The example counts income tax only. If the side income is self-employment income, a second tax comes on top.

Self-employment tax

An employee and their employer each pay Social Security and Medicare tax on wages. A self-employed person is both, so they pay both halves: 15.3% in total, made of 12.4% for Social Security and 2.9% for Medicare. It applies once net earnings from self-employment reach $400 for the year.

Three details soften it:

• The tax applies to 92.35% of your profit, not all of it. The result is that the effective rate on profit is about 14.1%.
• Half of the tax is deductible as an adjustment, which lowers your income tax.
• The Social Security part stops at the yearly wage base, $184,500 in 2026, and that limit is shared with any wages. Someone whose salary already reaches the base pays only the 2.9% Medicare part on side profit.

Put the two taxes together and a side dollar in the 22% bracket can lose a third or more of its value to federal tax alone, before state tax. Setting aside a fixed share of every payment you receive, often between a quarter and a third depending on your bracket and state, avoids a painful April. The side hustle tax calculator works out the share for your income.

Deductions that belong to the business

Self-employment income is taxed on profit: what you earn minus ordinary and necessary business expenses. Keeping good records is what turns real costs into real deductions.

• Direct costs: supplies, software, equipment, advertising, professional fees, a share of your phone and internet used for the business.
• Home office: a part of your home used regularly and exclusively for the business can be deducted, either with the simplified method (a set rate per square foot, up to a size limit) or by allocating actual costs. A spare-room desk that is also the family computer area does not qualify. IRS Publication 587 has the tests.
• Vehicle: business miles at the IRS standard mileage rate for the year, or the business share of actual costs. Commuting to a regular workplace does not count.
• Health insurance: premiums for yourself and your family, if you are not eligible for a plan through an employer or a spouse's employer.
• Qualified business income deduction: many owners of sole proprietorships, partnerships and S corporations can deduct up to 20% of their qualified business income. Above $201,750 of taxable income ($403,500 for a couple filing jointly), limits apply that depend on the type of business, wages paid and property owned. The 2025 tax law made this deduction permanent.

One line matters here: the activity must be a business, carried on to make a profit. If it is really a hobby, the income is still taxable but the expenses are not deductible.

Retirement accounts for the self-employed

Self-employment opens retirement accounts with much higher limits than an IRA.

A solo 401(k) lets you contribute twice: as the employee, up to $24,500 in 2026, and as the employer, up to 20% of net self-employment earnings after the deduction for half of self-employment tax. The employee limit is shared across every 401(k), 403(b) and similar plan you take part in. Someone who already contributes the full amount at a day job has only the employer part left in a solo 401(k).

A SEP IRA allows only the employer contribution, at the same 20% of net earnings, but is simpler to open and run.

Both lower income tax at your marginal rate. Neither lowers self-employment tax. The solo 401(k) contribution calculator shows how much each allows for your profit and day-job contributions.

Sole proprietor, LLC or S corporation

By default a one-person business is a sole proprietorship, reported on Schedule C. Forming a single-member LLC adds legal separation but, by default, changes nothing about federal tax.

An S corporation election can change the tax. The owner pays themselves a reasonable salary, which carries payroll taxes, and takes the rest of the profit as distributions, which do not. The saving is the self-employment tax avoided on those distributions. The costs are real: payroll processing, a separate corporate return, state fees in some states, and the risk that the IRS treats too low a salary as unreasonable. The salary must match what the work would pay an employee. A lower salary also lowers the room for retirement contributions and future Social Security benefits.

For these reasons the election tends to pay off only when profit is steady and comfortably above what a reasonable salary for the work would be, plus the extra costs. A tax professional who knows your state's rules is worth paying for this decision.

Records that protect you

Payment platforms and clients report what they paid you on Forms 1099-NEC and 1099-K, and the IRS matches them against your return. Keep business money in its own bank account, save receipts, log business mileage as you go, and report all income even if no form arrives. Large losses year after year, round-number expenses and a home office claimed alongside a full-time job elsewhere all draw attention; that is a reason to document them carefully, not to skip deductions you are entitled to.