I have to correct myself on this because I've told multiple people this and have believed it...but...science is always right.
So, it's always been said dark roasts have higher caffeine content than light roasts. I've always said light roasts have more caffeine than dark roasts...Well, I was kinda right.
So, using my coffee for example because shameless plug, take our light roast it comes in an 8oz bag... take our dark roast it comes in an 8oz bag.... both weigh 8oz...
Arabica beans have less caffeine than Robusta beans...I use Arabica because they taste better. But, I made sure to purchase beans that have the most caffeine content for Arabica
Now take the beans...both roast blends use the same beans...the dark roast is roasted longer which means it expands and loses water...this means it's bigger and weights less than the light roast which has less size but more water.
Now I measure my coffee when filling the bags by weight... lemme add one more thing, caffeine doesn't burn off till about 600 degrees Fahrenheit...My roaster can barely get to 350; So caffeine never really gets burnt off. Now, back to what I was getting at; I have to add more dark roast beans to a bag to reach 8oz, which, both beans used in my dark roast are the same in the light roast. So, more dark roast beans...mean more caffeine...which means our dark roast has more caffeine...I was wrong; Kinda...
So, if the coffee you buy are Arabica beans and measured by weight; go with a dark roast for the extra kick.
Fun fact: Robusta coffee beans have double the amount of caffeine than Arabica.
Cheers