Vocabulary size has no official grading scale
Standardized vocabulary tests exist mainly for research and clinical assessment, and there is no published band table that turns a word-list result into a level. So this page never says your vocabulary is at a particular grade. It turns a check you ran on your own word list into a score out of 100, so that a later check on the same list can be compared with this one.
How the score is built
Each word you defined exactly counts as one, each half-known word counts for the partial credit you chose, and the sum is divided by the number of words you checked. There is no correct value for partial credit, so it stays an input, and the formula actually applied is printed with the result. Words still needed for your target are rounded up.
Limits
Pick easy words and the score goes up without anything changing. What matters is the movement between two checks on the same list, not the number itself. Understanding a whole passage is a different skill, so use the reading comprehension self-check for that.
Frequently Asked Questions
No. There is no official mapping from a word-list score to a grade or age, and the score swings widely with the words you happen to choose. Use it only to compare repeat checks on the same list.
There is no set value. Half credit at 50% is common when you roughly know the word, and 20 to 30% when you can only guess it from context. The important thing is to reuse the same value next time.
With a short list one or two words swing the score a lot. Fifty words steadies it, and around a hundred makes repeat comparisons much more stable.