Form 1040 Line 21
is for miscellaneous taxable income, such as jury duty pay, hobby income, or
income from the rental of personal property.
There are some other instances where line 21 may be used, but income
from performing work or services would generally not be reported on line 21.
The type of
income you described should be reported as self-employment income on Schedule
C, ultimately being included on line 12 of Form 1040. This would be correct even if you do not have
any expenses to offset the income.
