All Arabica coffee beans have the same amount of caffeine in them, and you cannot roast the caffeine out. If you tried, you would be left with nothing but ash. So here’s the scoop: it comes down to a weight vs. volume thing.
Visual of 40g light roast vs 40g dark roast
As the coffee goes through the roasting process, you roast the moisture out and the beans expand. Beans are roasted longer to get a dark roast, so the darker the roast, the more moisture is taken out, therefore taking more weight out of the coffee. This means it takes more dark roast to weigh out 40g of coffee than it does a light roast. So, using this method you may have a tiny bit more caffeine in your brew because you are using more dark roast to brew with.
Visual of 40g ground light roast vs 40g ground dark roast
Light roast coffee is more dense than dark roast, so if you use the same volume of dark roast as you do light roast when brewing your coffee (for instance if you typically use a coffee scoop), you are using less coffee with a dark roast, therefore less caffeine, if that makes sense.
Get to know Patrick Dinkins, Cameron’s Roastmaster and Certified Arabica Q Grader.