For side income, the deductions that matter most are the ordinary expenses of running the activity, business mileage, a home office if you truly qualify, the deduction for half of your self-employment tax, and in some cases the qualified business income deduction and retirement contributions. A write-off does not erase tax. It reduces the profit you are taxed on, so each dollar of real expense saves only a portion of that dollar. This article covers US federal rules in general terms.
Many people chase small or shaky deductions and overlook the routine, well-supported ones. The sections below go roughly in order of how often each applies.
How a write-off works for side income
If you have a side business, you report revenue and expenses on Schedule C. Net profit is revenue minus allowed expenses, and you pay both income tax and self-employment tax on that profit. That is why a legitimate expense helps more than a deduction on a plain W-2 return: it reduces your income tax base and the earnings subject to self-employment tax.
It also means spending money to save tax rarely pays. An expense saves your marginal tax rate plus self-employment tax on that amount, which is always less than the expense. Buy things because the activity needs them.
The “ordinary and necessary” test
The IRS generally allows business expenses that are ordinary (common in your type of work) and necessary (helpful and appropriate for it). They must relate to the income-producing activity and be reasonable in amount. Personal costs do not qualify, and costs with mixed personal and business use must be divided sensibly. Publication 334, the IRS tax guide for small business, explains the standard in detail.
The activity also has to be a business. Hobby income is still reportable, but expenses do not reduce it the same way. The IRS looks at factors such as a profit motive, businesslike records, time and effort, and whether you depend on the income. If you are unsure, a tax professional can help.
The deductions that most often matter
Direct business costs
These are the easiest to support:
- Supplies, materials and inventory.
- Software and subscriptions used for the work.
- Platform, marketplace and payment processing fees.
- Advertising and website costs.
- Professional services such as bookkeeping tied to the business.
- Contract labor you pay others, which can bring filing duties of its own.
- Business insurance, licenses and permits.
Business mileage
If you drive for the business, you can generally deduct either actual vehicle costs or a standard mileage rate that the IRS sets and updates. Publication 463 covers both methods, and the current rate is listed on the IRS site, so check it for the year in question. Commuting from home to a regular workplace does not count. Trips to meet clients, buy supplies or deliver goods generally can. A log written at the time, with date, destination, miles and purpose, is far more convincing than one rebuilt later.
Home office
The IRS says you must use part of your home exclusively and regularly for the business, for example as your principal place of business. The exclusive-use rule is strict. A desk in a bedroom that you also use for personal tasks typically does not qualify. Self-employed people can claim this deduction, and employees generally cannot under current federal law. There is a regular method based on actual expenses, and a simplified method based on a set rate per square foot up to a size cap. Publication 587 explains both, and the deduction is limited by the income of the business.
Phone and internet
You can deduct the business-use share of a phone or internet bill if you can reasonably support the split. Keep a note of how you worked out the percentage.
Equipment and larger purchases
Computers, cameras, tools and similar items can generally be deducted, either over several years through depreciation or sometimes sooner under special expensing rules. Those rules have limits and have changed over time, so check current IRS guidance or ask a professional before a large purchase, and keep the receipt and the date you started using the item.
Meals and travel
Business travel costs such as transportation and lodging are generally deductible when the trip is primarily for business. Business meals are often limited, and personal meals do not qualify. Keep a note of who you met and why.
Deductions on the personal side of the return
Some valuable deductions appear on Form 1040 instead of Schedule C:
- Half of self-employment tax. It is figured on Schedule SE and lowers your adjusted gross income.
- Self-employed health insurance. You may be able to deduct premiums you pay yourself, subject to conditions such as not being eligible for employer coverage.
- Retirement contributions. Self-employed plans such as a SEP IRA or solo 401(k) may allow larger contributions than an ordinary IRA. Limits and deadlines vary by plan and year, so check the current figures.
- Qualified business income deduction. Many owners of pass-through businesses may deduct a percentage of qualified business income, subject to income limits and other rules that Congress has modified over time. It is calculated on the return and is not a separate expense.
Records that back up the deductions
An unsupported deduction can be disallowed in a review, with tax, interest and possibly penalties added. Basic habits help:
- Keep a separate account or card for the activity.
- Save receipts or invoices, including digital ones.
- Keep a mileage log.
- Write a short note on the purpose of unusual expenses.
- Record estimated tax payments.
How to organize tax records throughout the year lays out a practical setup.
Traps to avoid
- Overclaiming. Large deductions compared with income can draw attention, particularly repeated losses.
- Mixing personal and business use without a reasonable split.
- Hobby status. If the activity is not a business, deductions are generally not available against its income.
- Forgetting to pay as you go. Deductions lower the bill, but you still owe tax during the year. See how quarterly estimated taxes work.
- Relying on old rules. Depreciation, expensing and the qualified business income deduction have all been changed by legislation, so confirm the rules for the year you are filing.
For the bigger picture of how these fit on a return, read how to file taxes with side income and a regular job.
Gray areas to ask about
Some costs sit in a gray zone: a share of rent or utilities for a home office, a vehicle used for both work and personal trips, a computer used by the family, or clothing you wear for work but could wear elsewhere. The answer usually turns on how much is truly business use and whether you can show it. When the amount is large, the IRS publications named above and a tax professional are better guides than a general article. A written note made at the time, explaining how you split the cost, goes a long way.
Timing and a year-end check
Small businesses on the cash method generally deduct expenses in the year they pay them, which is the usual setup for side businesses. A purchase in late December therefore counts for that year, while one in January counts for the next. Before the year closes, total your income and expenses to date and decide whether any planned purchase is genuinely needed.
To keep the records that support what you report, print the side-income record log.
Where this comes from
This article is based on the IRS pages on business use of the home and on deducting business expenses, which point to Publications 334, 463 and 587, and the IRS page on self-employment tax. It does not give figures such as the mileage rate or deduction caps, because they change. Check the current figure at irs.gov (IRS) or in the form instructions, and ask a tax professional about your own situation.