26 USC 6157 - Payment of Federal unemployment tax on quarterly or other time period basis

(a) General rule 
Every person who for the calendar year is an employer (as defined in section 3306 (a)) shall
(1) if the person is such an employer for the preceding calendar year (determined by only taking into account wages paid and employment during such preceding calendar year), compute the tax imposed by section 3301 for each of the first 3 calendar quarters in the calendar year on wages paid for services which respect to which the person is such an employer for such preceding calendar year (as so determined), and
(2) if the person is not such an employer for the preceding calendar year with respect to any services (as so determined), compute the tax imposed by section 3301 on wages paid for services with respect to which the person is not such an employer for the preceding calendar year (as so determined)
(A) for the period beginning with the first day of the calendar year and ending with the last day of the calendar quarter (excluding the last calendar quarter) in which such person becomes such an employer with respect to such services, and
(B) for the third calendar quarter of such year, if the period specified in subparagraph (A) includes only the first two calendar quarters of the calendar year.

The tax for any calendar quarter or other period shall be computed as provided in subsection (b) and the tax as so computed shall, except as otherwise provided in subsection (c), be paid in such manner and at such time as may be provided in regulations prescribed by the Secretary.

(b) Computation of tax 
The tax for any calendar quarter or other period referred to in paragraph (1) or (2) of subsection (a) shall be computed by multiplying the amount of wages (as defined in section 3306 (b)) paid in such calendar quarter or other period by 0.6 percent. In the case of wages paid in any calendar quarter or other period during a calendar year to which paragraph (1) of section 3301 applies, the amount of such wages shall be multiplied by 0.8 percent in lieu of 0.6 percent.
(c) Special rule where accumulated amount does not exceed $100 
Nothing in this section shall require the payment of tax with respect to any calendar quarter or other period if the tax under section 3301 for such period, plus any unpaid amounts for prior periods in the calendar year, does not exceed $100.