This question is actually quite tricky, and the answer just depends. There are many factors that determine how much caffeine a coffee drink contains. One misconception is that the greater volume of coffee inside your cup indicates that the drink is more caffeinated than others. Another misconception is that one shot of strong- tasting espresso is far more caffeinated than a cup of brewed coffee. In terms of a coffee drink’s caffeine concentration, the amount of coffee used to make a drink is less important than the way the coffee is brewed.
There are many different species of coffee, and coffee beans from different plants vary in their caffeine content. The two main species include: Arabica and Robusta. Caffeine is actually a defense mechanism produced by coffee plants as a repellant against insects. Robusta produces more caffeine because it is grown at lower elevation, which has a higher presence of insects. Arabica grows at higher elevation, which experiences fewer insects. Robusta coffee beans typically contain around twice as much caffeine as Arabica beans. Through this 2016 study, researchers discovered that Arabica beans contain 34.1–38.5 g of caffeine per kilogram (kg) of dry coffee whereas Robusta beans contain 68.6–81.6 g of caffeine per kg of dry coffee. Depending on the variety of coffee plant will influence caffeine content.
Darkness of Roast
Lighter roasted coffee also has slightly higher caffeine levels than dark roasts. Towards the end of a roast, the hotter the coffee beans get the more its caffeine vaporizes. Darker roasted beans will reach an internal temperature of 460-470 F while light roasted coffee reaches an internal temperature of 356 to 401. The longer coffee roasts, the lighter in density and the lesser in caffeine content they become.
Caffeine Extraction Variables
Even more than coffee variety and darkness of roast, different brew methods result in significantly different levels of caffeine extraction. Caffeine is extremely water soluble and washes out of the coffee into the cup quite easily. The extraction of caffeine depends on 4 main variables: coffee to water ratio, time, grind size, heat/ pressure.
Despite having fewer grams of coffee, a double shot of espresso tastes much more coffee concentrated than a V60 pour over since espressos have a higher coffee to water ratio. However, since pour overs are brewed using more coffee, pour overs and drip coffee contain more caffeine than espresso. The flavor intensity is less since the coffee to water ratio of drip coffee is lower. Below is a breakdown of caffeine levels between espresso and drip coffee.
Single shot espresso = 64 mg caffeine
Double shot espresso 125 mg caffeine
8 oz drip = 95 mg caffeine on average
12 oz drip = 200-300 mg caffeine on average
In summary, a double shot of espresso contains less caffeine than a 12 oz cup of drip coffee. However, by caffeine concentration, espresso is the highest. For example, if you were to fill a 12 oz cup with pure espresso, you would have a coffee drink that contains 768 mg of caffeine. In other words, a coffee drink with a double shot of espresso is less caffeinated than a 12 oz drip coffee, while a quad shot espresso drink and 12 oz drip coffee contain about the same caffeine level.
Considering total brew times, coffee to water ratio, grind, and temperature/ pressure, below is a ranking of brew methods from most to least caffeinated, BY VOLUME.
By caffeine concentration, espresso is definitely the highest. Clearly if water is added to any coffee drink, the caffeine concentration per ounce will get diluted. There are many other factors that can contribute to differing amounts of caffeine, such as coffee to water ratio or the recipe used or any of the variables listed above, so it is impossible to say with any degree of specificity which coffee drink has the highest level of caffeine.