How can a parent figure out their child's GPA when homeschooling?

GPA is short for grade point average, it is the average of all the grades TOTAL example being if you have a 90% in English 85% in Math and 70% in Science to get the average add all three then divide by 3 (or however many grades there are) 90+85+70=245 245/3=81.67
Most school systems are based on a 4.0 GPA scale, 4 being an A, so that average would be a so multiply the 81.67 by 4 then divide that total by 100. This GPA on a 4.0 scale would be a 3.26  

Answer

That may depend on the regulations of your state. It certainly depends on the style of the homeschooling parent. Most homeschooling parents test for mastery, so grades are to some extent meaningless. You move on when you understand the material and not before. Some parent never even consider giving their child a grade until it's necessary to get in to college or some other external reason. Others grade assignments and tests and keep careful grade books. Still other homeschoolers use a correspondence course, where work is sent off to be graded by others.