You don’t need a tool.
Take your core classes - not PE, Drivers Ed, Health.
Each A gets 4. B gets 3. C gets 2. Doesn’t matter if + or - just 4, 3, 2
Add them up. Divide by # of classes. That’s your unweighted.
Straight As, as an example, is a 4.0 exactly half As, half Bs is 3.5.
Weighted tells users little because there are many weighting systems. So unweighted gives you an apples to apples comparison with other applicants. .