Decaf coffee is not caffeine-free: an 8-fluid-ounce cup typically has about 2 to 15 milligrams of caffeine, according to the U.S. Food and Drug Administration. That compares with roughly 95 milligrams in a typical 8-ounce cup of regular brewed coffee, because decaffeination usually removes about 97% of the caffeine in the green beans (National Coffee Association).
The exact amount varies by bean, decaffeination method, brewing strength, and cup size, so a large café decaf can land at the top of that range or above it. Decaf tea also keeps a small amount of caffeine. The FDA advises that people who react strongly to caffeine may want to avoid decaf drinks altogether, and notes that restaurants are not required to disclose caffeine content.
Source: FDA, “Spilling the Beans: How Much Caffeine is Too Much?”.
Image: Sack of green decaffeinated Brazilian coffee by Jkafader, CC BY-SA 3.0, via Wikimedia Commons.