Weighted and unweighted, both at once
High schools usually publish two GPAs and they answer different questions. The unweighted GPA is the plain 4.0 scale, where an A is a 4.0 whether you earned it in AP Chemistry or in a study period. The weighted GPA adds a bonus for harder courses, typically half a point for honors and a full point for AP or IB, which is how a transcript ends up showing a 4.3.
This calculator gives you both from one set of entries, because colleges ask for one or the other and sometimes both, and because the gap between them is itself informative.
The formula
For the weighted version, each course's grade points gain its bump first: an A in an AP class is 4.0 + 1.0 = 5.0 grade points, and an A- in an honors class is 3.7 + 0.5 = 4.2.
Credits matter, even in high school
Most high school courses run a full year and carry one credit, which is why leaving the credits field blank works fine for the majority of transcripts. But not every course is the same size. A one-semester elective is usually half a credit. Some double-period lab sciences and career and technical courses carry two. When those are in the mix, averaging by course count rather than by credit gives the wrong answer, and it is wrong in the direction that flatters small courses at the expense of big ones.
So the credits field is here, optional and defaulted to one. If your transcript lists credits next to each course, use them. If it does not, leave the field alone and the arithmetic will be right anyway.
Worked example
Six classes: an A in Honors English, a B+ in AP Calculus, an A- in AP Chemistry, a B in regular History, an A in regular Spanish, and an A in a half-credit PE course.
Unweighted quality points: 4.0 + 3.3 + 3.7 + 3.0 + 4.0 + (4.0 × 0.5) = 20.00 across 5.5 credits, so the unweighted GPA is 3.64.
Weighted, the honors class becomes 4.5 and the two AP classes become 4.3 and 4.7, giving 22.50 quality points and a weighted GPA of 4.09.
The whole weighting is worth 0.45 points of GPA. Three harder classes out of six, and the payoff is under half a point.
What one more AP class is actually worth
This is the question the calculator exists to answer honestly, and the answer is smaller than most people assume. A bump does not apply to your GPA, it applies to one course and then gets spread across every credit you are carrying. Adding one AP course to the six-class example above raises the weighted GPA by about 0.15. Adding one to a full four-year transcript of 24 credits raises it by about 0.04.
That is a real gain and it is worth having. It is also not the transformation the workload might lead you to expect, and knowing the actual size of it is the only way to weigh one against the other sensibly. The results table gives you the exact figure for your own transcript rather than a general rule.
There is a better argument for taking the hard class anyway, and it is not the GPA. Admissions officers read the transcript itself, not just the number at the bottom of it. A demanding schedule shows up there directly, in a way that a decimal place never quite captures.
Weighting is a convention, not a standard
The 0.5 for honors and 1.0 for AP used here is the most common US arrangement by a wide margin, but it is a local decision rather than a national rule. Some districts add a full point for honors. Some cap the weighted value of any single course. Some run an entirely different 5.0 scale, and a few weight only certain subjects. If your school publishes its own convention, that is the one that governs your transcript, and this calculator is giving you the standard version rather than yours.
The caveat that outranks all of the arithmetic
Many colleges recalculate GPA themselves. They commonly strip the weighting back out, drop non-academic courses such as PE and study hall, count only core academic subjects, and apply their own scale. Which means the weighted number your school prints and the number an admissions office actually uses are frequently not the same number at all.
This is not a reason to avoid hard classes, and it is not a reason to distrust your school's figure. It is a reason to hold any single GPA loosely, and to remember that it is a summary of a transcript rather than a replacement for one.
What about courses that do not count?
Pass or fail classes normally earn credit without grade points, so they sit outside the calculation and should be left out here. Withdrawals usually carry neither. Some schools exclude PE, health and study hall from the GPA entirely while others include them, which is one of the more common reasons a calculated GPA and a transcript disagree by a few hundredths. Your school's guidance counselor is the authority on all of these, and it is a normal thing to ask.