The self-employment tax applies to net earnings from self-employment whenever an individual has a trade or business that generates at least $400 in net earnings during the taxable year. There is no requirement that the activity be ongoing, regular, or repeated in future years. A one-year side gig — such as freelance work, gig economy services, or other short-term self-employment — is subject to self-employment tax if it meets the $400 threshold.

The "regular basis" definition in §1402(h) — which requires net earnings of at least $400 in two of three consecutive years — applies only to specific contexts (such as certain optional coverage provisions), not to the general self-employment tax obligation. The core rule remains: any individual with net earnings from self-employment of $400 or more in a tax year owes self-employment tax on that income, even if the activity occurs only once.

Income from gig economy work, freelance services, or other one-time self-employment is included in the self-employment tax calculation and must be reported on Schedule SE (Form 1040). The tax is levied, assessed, and collected as part of the income tax, and contributes to Social Security and Medicare coverage.

In short, a one-year side gig that generates $400 or more in net earnings triggers self-employment tax liability, regardless of whether you repeat the activity in future years.