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When you wake up and drink your morning cup of coffee, you don't dwell on what it is about the brown beverage that wakes you up. You just know that it's easier to go about your day after you've downed a few cups.

Of course, it's the caffeine that's hard at work, making your brain think you're less tired than you really are.

Whether you're looking out for your body or you just want a stronger effect, the question is, which coffee beverage has the highest caffeine content? The question is simple enough, though the answer is anything but.

Caffeine is a wild card

Energy drinks or other caffeinated beverages often have their caffeine content plainly listed or readily available with a quick search. Those numbers are pretty specific and accurate.

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Unlike mixing ingredients and following a strict recipe to make a soda, however, making coffee isn't a precise science. Something as simple as where the coffee was grown can have an effect on its caffeine content. So can the roast level, blend or variety of the coffee, the brew method and a handful of other factors.

That's why, when you see a caffeine content listing for coffee, it's generally an approximation, at best. But based on these approximations, you can guess -- to a decent level of certainty -- how much caffeine you're ingesting with each cup.

Espresso

You'll often hear that espresso has more caffeine than a cup of coffee. Generally, that's wrong.

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The average shot of espresso contains approximately 64 milligrams of caffeine. And Starbucks lists a single shot of espresso at 75 milligrams of caffeine.

In other words, that's anywhere from 64 to 75 milligrams of caffeine per fluid ounce (30 milliliters), so a single shot of espresso doesn't contain more caffeine than a single cup of coffee. It's when you start stacking shots that things start to add up.

Even then, espresso drinks -- like lattes -- are typically diluted with water, milk or cream. A venti latte macchiato, 20 fluid ounces (591 milliliters), from Starbucks contains three shots of espresso, or approximately 225 milligrams of caffeine. A drip coffee in the same size from Starbucks contains nearly double the amount of caffeine. Ounce for ounce, espresso drinks are significantly less caffeinated than black coffee.

Drip coffee

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Your standard drip coffee varies even more in caffeine content due to the variety and variation of brewing methods, sizes in which it is served and the varying strengths at which it can be brewed.

One 8-fluid-ounce (237-milliliter) cup of hot brewed coffee generally has about 95 milligrams of caffeine. Caffeine Informer, a database of caffeine content in food and drinks, estimates the same size coffee contains 163 milligrams of caffeine. This is two to five times the caffeine content of your average soda, which ranges from 34 milligrams (Coca-Cola) to 69 milligrams (Pepsi Max).

And that's just the beginning. The caffeine content value changes dramatically, based on who you ask or where you buy your coffee.

For example, a 14-fluid-ounce (414-milliliter) cup of coffee from Dunkin' Donuts is listed at 210 milligrams of caffeine and a 24-fluid-ounce extra large (710-milliliter) has 359 milligrams of caffeine.

Starbucks, on the other hand, changes its caffeine content based on the roast level. Its Pike Place house brew is listed at 260 milligrams for a 12-fluid-ounce (355-milliliter) cup. Up that to a 20 fluid ounce venti (591 milliliter) and you're looking at 415 milligrams of caffeine. And a venti blonde roast is listed at a whopping 475 milligrams, according to Caffeine Informer.

A single venti drip coffee from Starbucks alone contains more caffeine than the average American ingests in a day, according to a 2012 report by the FDA, which showed that number was around 300 milligrams of caffeine per person per day. And though the numbers tend to vary by agency, the level of daily caffeine intake that is considered safe is between 400 and 450 milligrams.

Then there are the outliers. The increasingly popular Death Wish Coffee claims to pack upward of 660 milligrams of coffee into every 12-fluid-ounce (355-milliliter) cup. And Black Insomnia Coffee claims over 702 milligrams of caffeine for the same size.

One cup of either of these coffees is nearly double the amount of the recommended safe caffeine intake. By no means are these coffees normal or average. Instead of the typical 100% arabica beans, Death Wish and Black Insomnia source robusta coffee beans, which are typically less flavorful but contain much higher concentrations of caffeine, and blend them with arabica coffee.

Cold brew

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Cold brew is a different beast entirely. While heat helps extract more caffeine, cold brew is typically brewed as a concentrate, with a higher than normal coffee-to-water ratio of between 1:4 and 1:8, compared with a more typical drip coffee ratio of 1:15 or 1:25. This alone leads to a higher caffeine concentration.

However, as a concentrate, cold-brew coffee is also typically cut with water or creamer, which levels things out a bit.

Still, even some cold brews that are brewed and sold as ready-to-drink (RTD) beverages contain higher levels of caffeine than your standard drip brew.

For instance, Stumptown Cold Brew Coffee is one of the first that comes to mind. Its 10.5-fluid-ounce (311-milliliter) stubby contains approximately 279 milligrams of caffeine. Its Nitro Cold brew contains 330 milligrams of caffeine in just 11 fluid ounces (325 milliliters).

Chameleon Cold Brew comes in both a concentrate, which is 32 fluid ounces (946 milliliters) with 2160 milligrams of caffeine, and an RTD, 10 fluid ounces (296 milliliters) with 270 milligrams of caffeine. For the concentrate, it's supposed to be cut with one part milk or water to one part coffee, which would effectively cut the caffeine content in half.

Starbucks Cold Brew, by comparison, is not nearly as strong. It has just 200 milligrams of caffeine in 16 fluid ounces (473 milliliters).

The verdict

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As you might expect, it's tough to come up with a clean answer. That said, if you take the average of all the hot brewed coffees above, you get approximately 27 milligrams of caffeine per fluid ounce (30 milliliters). Even though that estimate is high (thanks to Death Wish and Black Insomnia), this means drip coffee has nearly one-third the caffeine content per fluid ounce when compared to espresso.

The average of all the cold brews comes to about 26 milligrams of caffeine per fluid ounce (30 milliliters). This puts it directly in line with hot brewed coffee.

But to paint the full picture, you have to take into consideration the volumes at which all three are consumed. Espresso is served in two to four doses at a time, and cold brew is typically served in smaller containers -- 10 fluid ounces (296 milliliters), give or take -- and neither typically come with refills. Hot coffee, however, is served in anything from 8 to 34 fluid ounces (237 to 710 milliliters) and often comes with free or drastically cut prices for refills.

Even with an 8-ounce (237-milliliter) cup, one refill would more than double the caffeine intake of either of the other two beverages.