The formula
grade = Σ (score ÷ maximum) × relative weight × scale
Where it comes from
In a rubric each criterion is scored against its own maximum and weighted by whatever the rubric says. The common mistake is averaging the scores without looking at the weights, which gives a different number as soon as the criteria are not worth the same. The correct calculation turns each criterion into its percentage of achievement and weights it.
How to work it out by hand
- Divide each score by its criterion’s maximum: that is the achievement
- Work out the relative weight of each criterion over the total of the weights
- Multiply achievement by relative weight and by the scale
- Add up every criterion
What is worth knowing
The column that actually helps you improve is not achievement but what is lost, which combines both things: a weak criterion with little weight costs tenths, and a middling one with a lot of weight costs whole points. Sorting by that gives the order in which to work. For the person marking, comparing the weighted grade with the simple average is also a quick check on whether the rubric weights are doing what they were meant to.