In order to obtain the caffeine levels for our coffee we send a sample of our coffee off to an independent lab for caffeine testing. They use the HPLC method (High-Performance Liquid Chromatography.
The sample passes through a column under high pressure, where caffeine is separated from other components.
The caffeine is detected, usually with a UV detector, and its concentration is quantified based on its retention time and peak area.
That gives us a result of the milligrams of caffeine per litre of brewed coffee. The weight of the coffee used in the brew is then factored in giving a weight per gram of coffee.
Then we can factor that weight into the different measures needed for different brew methods.
We've worked to 7grams of coffee for a single espresso, 18grams for a double espresso and 14grams for a filter coffee.
If you want to work out the exact caffeine levels in your Counter Coffee brew then you can use those weights and caffeine levels provided to calculate it.
It is worth mentioning that the caffeine levels in coffee are naturally occurring and so will fluctuate with time, and different brew methods, grind sizes and roast levels can all have an impact on caffeine levels.
Ultimately the specific number only has so much impact, rather awareness of the strength or weakness of the coffee you are consuming and how it makes you feel is the really important thing here, so that you can enjoy a delicious coffee with all the benefits whilst staying healthy and avoiding the negatives.