This is a vocabulary error you likely see and hear frequently: the wrong choice of word to describe relative measurements. I've seen and heard it recently in local news coverage -- and late last week, noticed it in the supermarket, too.
Words like "less" and "fewer" might seem synonymous at first, but they aren't. Fortunately, there's an simple trick to help you choose the right word.
If you're talking about something you can't literally count, use these words:
- less (e.g. less money, less energy, less sand)
- amount (e.g. a smaller amount of money, a smaller amount of energy, a smaller amount of sand)
Thus, "50% less calories" is incorrect, because while you can't literally count energy without getting more specific about unit measurement, you can literally count calories.
If you're talking about something you can literally count, use these words:
- fewer (e.g. fewer dollars, fewer calories, fewer grains of sand)
- number (e.g. a smaller number of dollars, a smaller number of calories, a smaller number of grains of sand)
Saying "50% fewer energy" would be just as incorrect, since "energy" isn't literally countable.
To summarize: if you can't literally count the item in question, use "less" or "amount;" if you can literally count it, use "fewer" or "number."
This orange juice, the manufacturer would like us to understand, delivers 50% fewer calories than its full-sugar alternative.