There are a lot of factors that will affect the amount of caffeine in a cup of coffee, including the size of the drink and the strength of the brew. How much caffeine is in your coffee also depends on the type of roast you're drinking. There's a real difference between light roast and dark roast coffee when it comes to caffeine content as well as taste, and the way your coffee is roasted can also have an effect on how much caffeine it has. But in order to understand the difference between dark roast and light roast, you need to understand how and why coffee beans are roasted in the first place.
Coffee beans don't come ready to grind and brew, straight off the shrub. (Did you know that coffee beans grow on shrubs? They were discovered in Ethiopia by a farmer whose goats were eating the coffee berries and then dancing around uncontrollably because caffeine.) Coffee beans are picked and stored green. In order to prepare them for your French press, coffeemakers must roast the beans to bring, "out the aroma and flavor that is locked inside the green coffee beans," according to the experts at the National Coffee Association.
In the simplest terms, the roasting process—in which green beans are heated up in an oven or a even frying pan until they're brown—is what makes coffee beans ready to drink. "A perfect roast is ultimately about the balance between flavors in the green coffee and those that develop during roasting," writes Nick Cho in Serious Eats, and, unsurprisingly, there are many ways to find that balance.
This is where the terms light roast and dark roast originate. As the names might suggest, darker roasts are beans that have been roasted for a longer time, and light roasts are, well, more lightly roasted. Light roasted beans are lighter in color, and the flavor of light roasts is generally more acidic. If you really want to taste a coffee bean, you want to be drinking a lighter roast.
Darker roasts, on the other hand, are kept in the oven until dark brown, and the outsides of the beans are oily. That's why the flavor of dark roasts depends more on the roast than the type of bean; you taste the notes from the roasting process, not the bean itself. According to the experts at Zagat, dark roasts, "will also lose brightness, and gain bitterness, along with a fuller body."
That more bitter taste isn't correlated with a higher caffeine content, though. Contrary to what you might think, darker roasts have less caffeine than light roasts—though, at the end of the day, the difference is negligible. "Caffeine is actually extremely stable during the roasting process," writes Julie R. Thomson for Huffington Post. "The effect of roasting on caffeine is so minimal it can really only be observed in a controlled laboratory setting."
So stop worrying about which one will give you a buzz quicker and focus instead on the taste because, at the end of the day, you'll be getting a caffeine fix from any cup of coffee, whether it's light or dark roast.