If your total business income, from all Schedule Cs combined and from any partnership or S corporation income that is treated as self-employment income, is $400 or more, you must file a Schedule SE and pay SECA tax on your entire net business income, including the first $400.
Schedule SE (Form 1040), Self-Employment Tax (standard .pdf file)
Schedule SE (Form 1040), Self-Employment Tax (fill-in .pdf file)
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For more information, see our discussion of self-employment tax.