Two students can have wildly different-looking GPAs and be equally strong applicants — or the same-looking GPA and not be comparable at all. The reason is that "GPA" isn't one number; it's at least two, and they measure different things.
What unweighted GPA measures
Unweighted GPA uses a standard 4.0 scale, full stop. An A is worth 4.0 whether it came from AP Physics or a study hall–adjacent elective — the letter grade is all that counts, not how hard the class was. This is the "pure" measure of how well you performed, stripped of any credit for course difficulty.
What weighted GPA measures
Weighted GPA adds points for course rigor, usually on a 5.0 scale (though the exact scale varies by school). A typical structure gives an extra 0.5 points for an Honors-level A and a full extra point for an AP, IB, or dual-enrollment A — so a 4.0 in AP Chemistry might weight out to a 5.0, while a 4.0 in a standard-level class stays at 4.0. Weighted GPA is a measure of course rigor combined with performance, not performance alone.
Why colleges look at both — and often calculate a third number
Here's the part that surprises a lot of applicants: many colleges don't just take the GPA your high school reports. High schools use wildly different weighting formulas — some give 1.0 extra point for AP, some give 0.5, some weight IB differently than AP, some don't weight at all — which makes a raw weighted GPA useless for comparing applicants across different schools. So selective colleges frequently recalculate your GPA themselves, usually using a standardized formula (often unweighted, or weighted only for a consistent set of "core" academic courses) so every applicant is measured on the same scale regardless of which high school's formula produced the number on the transcript -- see our piece on exactly how that recalculation works, with real examples, for the specifics.
This is also why your high school sends a school profile along with your transcript — a document that explains your school's specific grading scale, weighting system, and course offerings, so admissions readers can interpret your GPA in context rather than at face value.
The mistake this causes
The most common error: assuming a higher weighted GPA is automatically a stronger application than a lower one. A student with a 4.6 weighted GPA built on a light course load isn't necessarily stronger than a student with a 3.9 unweighted GPA who took the most rigorous schedule their school offers. Admissions officers are trained to read transcripts in context — what courses were available, which ones you chose, and how you performed in them — not just to rank-order the GPA number itself.
The reverse mistake also happens: a student assumes a "low" unweighted GPA (say, a 3.6) looks bad, without realizing that number reflects five AP classes and will read very differently than the same 3.6 from an all-standard-level schedule.
What this means for you
Track both numbers, but don't optimize for either one in isolation. The actual thing colleges are evaluating is whether you challenged yourself as much as your school reasonably allowed and performed well doing it — a strong unweighted GPA on a rigorous schedule is the clearest signal you can send, and it's read the same way no matter which weighting formula your specific high school happens to use.