A semester GPA covers just one term, unlike a cumulative GPA which blends every term together. List only this semester's courses, convert each letter grade to points on the standard 4.0 scale (A = 4.0, A− = 3.7, B+ = 3.3, B = 3.0, B− = 2.7, C+ = 2.3, C = 2.0, C− = 1.7, D+ = 1.3, D = 1.0, D− = 0.7, F = 0.0), multiply by each course's credit hours, and divide the total points by the term's total credits.
Use it mid-semester with your expected grades to see what you are tracking toward, then update the grades as results come in. Switching one expected grade at a time shows exactly which course has the most leverage on your term GPA — almost always the highest-credit one.
The calculation assumes the standard unweighted 4.0 scale and that pass/fail courses are excluded, which matches how most colleges report a term GPA. If your school counts A+ as 4.3 or ignores plus and minus grades, your official figure may differ slightly from this estimate.