You can deduct any business expense that is ordinary and necessary: ordinary means common and accepted in your field, necessary means helpful and appropriate. Several above-the-line deductions (HSA, retirement contributions, student loan interest) also cut your income without itemizing. This guide covers the test, the categories, and the documentation that survives scrutiny.

What is the actual test for deducting a business expense?

The whole deduction system for business owners runs on two words from IRC 162: ordinary and necessary. The IRS defines them plainly. An ordinary expense is one that is common and accepted in your field of business. A necessary expense is one that is helpful and appropriate for your business, and it does not have to be indispensable. (IRS Publication 334)

The flip side is IRC 262: no deduction for personal, living, or family expenses. Every deduction question you will ever ask lives on the line between those two sections. The expense has to clear 162 and avoid 262, and you have to prove it with records.

Our house rule, borrowed from the FAQ we send every new client: we would rather tell you no on something small than let you carry a deduction you cannot defend. A deduction you cannot document is not a deduction. It is a liability with a three-year fuse.

Which deductions do business owners actually use?

Most owner deductions fall into a short list of categories. Here is the map, with the rule that governs each.

Operating expenses and compensation

Rent for the office, the software stack, the accountant's invoice, the business insurance policy: if the expense is common in your field and helpful to the business, it is deductible. Wages you pay are deductible when you pay or incur them, and they have to be reasonable for the work performed. Contractor payments you report on Form 1099-NEC are part of the same category. Note the 2026 threshold change: for payments made after 2025, information reporting on Form 1099-MISC and 1099-NEC applies at $2,000, not the old $600. The income itself was always taxable from the first dollar; only the reporting trigger changed. (IRS Publication 334)

Vehicle expenses

You have two choices for business driving: track actual expenses (gas, maintenance, insurance, depreciation, apportioned by business-use percentage) or multiply business miles by the standard mileage rate. For 2026 the IRS set the rate at 72.5 cents per mile for January through June, then raised it midyear to 76 cents per mile for July through December, the first midyear increase since 2022. (IRS, Standard Mileage Rates)

Commuting from home to your regular workplace is never deductible, no matter how far the drive. Trips between business locations during the day are. One exception: if your home qualifies as your principal place of business, trips from home to other work sites count as deductible business travel, not commuting. And if you use the standard mileage rate, you still need a mileage log: date, destination, business purpose. The rate is simple. The substantiation is not optional.

Retirement and health contributions

These are the heavy hitters for owners because they are above-the-line: they reduce your adjusted gross income whether or not you itemize. The 2026 limits:

• Solo 401(k): $24,500 in employee deferrals, $72,000 total with employer contributions ($8,000 catch-up at 50+, $11,250 super catch-up at 60-63).

• SEP IRA: the lesser of 25% of compensation or $72,000. The employer funds it; there are no employee deferrals. If you are self-employed with no W-2 wages, the math works out to about 20% of net self-employment income after half the self-employment tax. The full 25% applies to W-2 pay, such as an S-corp owner's salary.

• HSA: $4,400 self-only, $8,750 family, plus a $1,000 catch-up at 55+. You need qualifying high-deductible coverage; since 2026, bronze and catastrophic marketplace plans count.

(IRS, SEP Contribution Limits; IRS, Rev. Proc. 2025-19; IRS, Notice 2026-05 guidance)

What can you deduct without itemizing?

Above-the-line deductions are adjustments to income listed in IRC 62. They are the deductions you get on top of the standard deduction, and for many owners they are worth more than itemizing. The ones our clients use most:

• HSA contributions (deductible; withdrawals for qualified medical expenses are tax-free).

• Retirement plan contributions for the self-employed (Solo 401(k), SEP, SIMPLE).

• Self-employed health insurance premiums, deducted on Schedule 1.

• Student loan interest, up to $2,500 per year, with an income phaseout the IRS adjusts annually. (IRS Publication 970)

• Educator expenses, up to $300 for eligible K-12 educators ($600 if both spouses qualify, capped at $300 each), claimed on Schedule 1. Starting in 2026, educator expenses above $300 can also be itemized. (IRS FAQ on educator expenses)

• Half of self-employment tax, which the tax code lets you deduct as an adjustment to income.

• IRA contributions, up to $7,500 for 2026 ($8,600 at 50+), subject to income phaseouts if you or your spouse are covered by a workplace plan.

The reason these matter disproportionately for business owners: every dollar of above-the-line deduction lowers AGI, and AGI is the number that drives phaseouts for everything else, including the SALT cap phaseout and IRA deductibility. Lowering AGI is the closest thing to a free lunch in the tax code.

What actually qualifies for the home office deduction?

A home office has to meet two tests at the same time: regular use and exclusive use. Regular means you use it consistently for business, not occasionally. Exclusive means no personal use of that space at all. A guest room that doubles as an office on some days usually fails the exclusive-use test, which is exactly how most home office deductions die.

If the space qualifies, you choose a method. The simplified method gives you $5 per square foot for up to 300 square feet, a maximum $1,500 deduction, with no depreciation and no records of actual expenses. The actual-expense method apportions real costs (a share of rent or mortgage interest, utilities, insurance, plus depreciation on the business portion of the home) and usually produces a bigger deduction in expensive housing markets, at the cost of full records and depreciation recapture when you sell. (IRS, Simplified Option for Home Office Deduction)

One hard boundary: W-2 employees cannot deduct a home office at all. The suspension of miscellaneous itemized deductions, which used to cover unreimbursed employee expenses, is now permanent, so there is no employee home office deduction to claim. S-corp owners are employees of their own corporation, which means they cannot take the deduction directly either; the correct route is an accountable-plan reimbursement from the corporation. We cover both methods and the S-corp mechanics in our dedicated home office guide.

How do you defend mixed-use expenses?

Mixed-use expenses are the ones that are partly business and partly personal: the phone, the car, the trip that combined a client meeting with a weekend. The rule is simple and the execution is not: you deduct the business portion, and the paper trail matters as much as the expense itself.

Three documentation habits separate defensible deductions from audit bait:

1. Separate the money first. Business expenses on the business card or account. A $20,000 business expense run through a personal card is deductible in theory and miserable to prove in practice. Separation is the cheapest audit insurance you can buy.

2. Log contemporaneously. For travel, meals, and vehicle use, IRC 274 requires strict substantiation: amount, time, place, and business purpose, recorded at or near the time. Reconstructed logs months later carry little weight.

3. Know what is categorically personal. Volunteer travel for a nonprofit sports team, for example, is personal, not a business write-off, unless there is a genuine business connection. The value of your volunteer time is never deductible; only out-of-pocket costs like mileage (at the 14-cent charitable rate) can be. (IRS Publication 526; IRS Publication 463)

Our full guide on mixed-use expenses walks through the documentation standard for each category.

When does side income become taxable?

Immediately. All income is taxable from the first dollar, whether or not anyone sends you a form. The forms are just reporting:

• Self-employment tax kicks in at $400 of net earnings for the year.

• Form 1099-NEC reporting applies at $2,000 for 2026 payments.

• Form 1099-K (payment apps and marketplaces) applies at $20,000 and 200 transactions under current federal law, after the One Big Beautiful Bill Act reverted the threshold. Several states still use lower thresholds.

• Not receiving a form never makes income tax-free. It just means the IRS has not been told yet.

What gets business owners in trouble?

The same four failures show up in almost every problem file:

1. Personal expenses dressed as business expenses. The daily coffee, the family vacation with one client lunch, the wardrobe. IRC 262 disallows personal expenses outright, and intent does not convert them.

2. No documentation. The expense was real but the receipt, log, or business purpose is missing. Under IRC 274's strict substantiation rules for travel and vehicles, undocumented means disallowed.

3. W-2 home office deductions. Permanently unavailable since the miscellaneous itemized deduction suspension. Claiming one as an employee is an automatic adjustment.

4. Ignoring the SALT cap on the individual side. Business owners who itemize face the $40,400 SALT cap for 2026 (phasing down above $505,000 of MAGI). Entity-level strategies like the PTET election exist precisely because the individual cap is so binding. Our SALT cap guide covers the workarounds.

Pending review by Brian Thomas, EA. Updated September 25, 2026.

This article is free educational content, not tax advice for your specific situation. If you want a plan built around your actual numbers, that is what our paid tax planning engagements do.