Is taking 18 units online for one semester doable?

I’m registered for 6 classes this fall online at University of Utah. I’ll only be online this semester and then I’ll move on campus in the spring. All my classes are general ed classes, so these were the easiest available. Is this doable? My online schedule:

Macroeconomics (easy since I took micro)
Music Theory (ge)
Art Now History (ge)
US National Government (required)
Earthquakes and Volcanoes
Intro Study Language (ge)

I also know music theory since I’ve played the piano all my life, so that class should be easy for me.

That depends on whether they are self-paced or if there are multiple due dates over the course of the week, and whether you work full-time/have other responsibilities. I’ve seen students take 21 credits online without a problem, but they went to colleges with extremely self-paced courses (for example, all the work was due on Sunday night; some were simply just before the end of the semester). Other students take 6 credits and struggle because their professors require multiple discussion posts throughout the week and they work full-time. It all depends on what your responsibilities are and how the courses are structured.