Decaffeinated is not caffeine-free
Decaf is coffee with most of its caffeine removed, not all of it. The FDA puts a typical 8 fl oz cup of decaf at 2 to 15 mg, and says decaffeinated coffees and teas “still contain some caffeine.” USDA’s lab figure for decaf brewed with tap water is 1 mg per 100 g, about 2 mg a cup, the bottom of the FDA’s range.
Coffee-shop decaf lands higher. Starbucks lists 15 mg for a Short Decaf Pike Place Roast, rising to 30 mg for a Venti. That Venti is roughly a third of an 8 oz cup of regular drip coffee (95 mg by USDA’s figures), and a little less than a can of Coca-Cola (34 mg).
When a few milligrams matter
For most people, decaf’s caffeine is noise. A healthy adult working to the 400 mg daily figure the FDA cites could drink twelve Venti decafs and stay under it.
It matters in three situations. The first is pregnancy, where the ceiling ACOG uses is 200 mg a day: switching from regular to decaf is one of the easiest ways to stay under it, but four or five large decafs are not zero. The second is anyone told to avoid caffeine entirely; the FDA suggests that people who react strongly to caffeine may want to avoid decaf altogether. The third is evening coffee. With caffeine’s mean half-life of about five hours (NCBI), a decaf after dinner adds little, but it is still more than a cup of herbal tea, which USDA records at zero.
Decaf espresso drinks
Decaf espresso follows the same pattern. USDA measures restaurant decaf espresso at 1 mg per 100 g, against 212 mg for regular espresso. A decaf latte is close to caffeine-free, as long as every shot in it is decaf.