How Much Caffeine Is Too Much?

The U.S. FDA cites 400 milligrams a day as an amount not generally associated with negative effects for healthy adults — roughly two to three 12-ounce cups of coffee. It is a general guide, not a target, and sensitivity varies widely between people. The FDA recommends that anyone who is pregnant, trying to become pregnant or breastfeeding talks to their health care provider about whether to limit caffeine.

The spread is large: one coffee shop’s “medium” may hold double the caffeine of another’s. When a label gives an exact figure, use it. The caffeine calculator uses the midpoints of these ranges and lets you add exact amounts.

Caffeine’s half-life — the time for your body to clear half — is commonly cited as about 3–7 hours (around 5 on average). That means a coffee at 4 p.m. still leaves a meaningful amount at 10 p.m. The remaining amount is total × 0.5^(hours ÷ half-life). For two coffees and 40 mg from other sources (400 mg), with the last drink at noon and bed at 10 p.m., about about 100 mg remains at bedtime. Move the last coffee to 5 p.m. and the figure is much higher.

• Jitteriness, a racing heart or anxiety
• Trouble falling asleep or poor-quality sleep
• Stomach upset or headaches
• Needing more and more to feel normal

If you want to cut back, reduce gradually over a week or two to avoid withdrawal headaches.

1. Stop caffeine 8–10 hours before bed if you sleep lightly.
2. Count energy drinks, pre-workout powders and some pain relievers.
3. Pair caffeine with water and food; do not use it to replace sleep — check your sleep with the sleep debt calculator.
4. Check with a doctor if you take medication or have a heart or anxiety condition.