I speculate that X % does mean (as Graeme mentioned before) X g pure caffeine/100 g coffee bean. Percentages do not have units because the same unit on the top and the bottom of the fraction gets cancelled. However, this % unit could be more specific: X % pure caffeine/coffee bean.
Lil and K cupper both mentioned volume in various units--(fluid) ounces, mL, cup. The volume of water, I would believe, has less to do with the final amount of caffeine extracted, and more to do with the final concentration of caffeine in the brew.
For example, I can extract 14 g of coffee beans containing 1.37% caffeine with either 9 oz or 18 oz or water. Both would give me roughly 191.8 mg caffeine:
(14 g coffee bean x 1.37 g caffeine)/100 g coffee bean x 1000 mg/1 g = 191.8 mg
As you can see from above, the amount of water never enters the equation on how to calculate how much caffeine I have in the final brew (assuming most of the caffeine is extracted in the first 9 oz of water). But water goes into the equation to figure out concentration. The one with more water will just be twice as dilute:
191.8 mg/9 oz = 21.3 mg/oz, 191.8g/18 oz = 10.7 mg/oz
In summary, it's not the amount of water used to extract from the coffee beans that determines how much caffeine is in the brew, but only how many grams of coffee beans are used in the first place. Thus, grams is the only unit of measure that would be helpful, not ounces, mL, cup or anything to do with volume.
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