It might be more than you think but it’s not enough to kill you
The amount of caffeine in coffee depends on a variety of factors, including the type of coffee you're brewing, the ratio of water to coffee grounds, and the method of preparation. That's why every source seems to have a different answer to the question, "How much caffeine is in a cup of coffee?" Since there are so many factors to consider, knowing exactly how much caffeine in a cup of coffee really depends on what kind of coffee you're drinking. But there are some basic guidelines that you can consider if you want to better understand how much caffeine is in coffee—especially if you're looking to cut back on your caffeine consumption.
According to the U.S. Department of Agriculture, an 8-ounce cup of hot, brewed coffee has 95 milligrams of caffeine. If you go to the Starbucks website though, you'll find that a cup of its signature Pike Place Roast has 155 milligrams of caffeine per 8-ounce serving—a difference that could be chalked up to the roast or the quality of the beans. That's why the Mayo Clinic gives a range, rather than an exact number, and estimates that the average cup of coffee will have between 95 and 200 milligrams of caffeine per 8-ounce serving.
But if you're drinking from a cup that's larger than eight ounces—as many people do—the numbers can quickly add up. For example, that 8-ounce coffee is called a "short" size in Starbucks signature parlance; if you're ordering a "tall," which is generally considered to be the Starbucks equivalent of a small coffee, you'll be consuming 235 milligrams of caffeine. For reference, a 12-ounce Red Bull energy drink has 107 milligrams of caffeine, according to the USDA.
According to the Mayo Clinic, an adult human can safely consume up to 400 milligrams of caffeine per day, which is about four 8-ounce servings. The best way to keep track of your caffeine consumption, then, is to think of a serving of coffee as eight ounces, rather than a full mug. If you're seriously trying to limit the amount of caffeine you ingest everyday, consider getting an 8-ounce mug of your own, so you don't accidentally pour yourself 20-ounces of coffee and think that you're drinking a single serving of caffeine.
But you don't really have to worry about overdosing on caffeine (which is a thing that happens to people, seriously) if you're just drinking black coffee. Even if you consume 500 or 600 milligrams of caffeine, you're not going to die; you might just have some trouble falling asleep or feel a little jittery.
That's why Dr. Peter Martin, director of the Institute for Coffee Studies at Vanderbilt University, told The Atlantic, "What I tell patients is, if you like coffee, go ahead and drink as much as you want and can." If you do have trouble sleeping, or seeing a negative impact on your health, then cut out a cup at the end of the day, he recommends. In much the same way that every cup of coffee has a different caffeine content, every human processes caffeine differently. So there's no reason to get too hung up on the specific amount of caffeine in a cup of coffee, unless it's having a negative impact on your health.