Hard to say exactly.  In addition to what jrbancel said about how the calculations differ, you can also refer to BLS to see which industries Americans are employed in:
<a href="http:&#x2F;&#x2F;www.bls.gov&#x2F;emp&#x2F;ep_table_201.htm" rel="nofollow">http:&#x2F;&#x2F;www.bls.gov&#x2F;emp&#x2F;ep_table_201.htm</a><p>I would guess that construction, manufacturing, retail, and hospitality account for a significant portion of those jobs with less than 30 hours per week of work.