The short answer
The FDA says up to 400 mg of caffeine per day is safe for most healthy adults — roughly four 8 oz cups of brewed coffee. Below that, caffeine hasn't been linked to dangerous effects in healthy people. Above it, side effects climb, and single doses around 1,200 mg or more can cause serious toxicity.
The catch: real-world cups are often bigger than 8 oz, so people hit 400 mg on fewer "coffees" than they think. This is educational guidance, not medical advice — check with a clinician about your own situation.
What 400 mg looks like in real cups
400 mg is fewer drinks than the "four cups" rule suggests once size is counted:
• Four 8 oz home coffees (~95 mg each) ≈ 380 mg — right at the line.
• Two 16 oz café coffees can each be 200–300 mg — two of them may already exceed 400 mg.
• Cold brew is the fast track: a couple of large servings (150–240 mg each) can top the limit alone.
Add up your actual day — by drink and size — with the caffeine intake calculator, and see how much caffeine is in each drink.
Signs you've had too much
Common signs of exceeding your personal tolerance include:
• Jitteriness, shakiness, or a racing heart
• Anxiety or restlessness
• Trouble sleeping (see when to stop coffee before bed)
• Upset stomach, headache, or irritability
Tolerance varies widely — some people feel these at 200 mg, others not until much higher. If moderate coffee makes you anxious or wrecks your sleep, that's your signal to cut back regardless of the 400 mg number.
Who should have less than 400 mg
Several groups have lower recommended limits:
• Pregnant or breastfeeding: ACOG advises keeping caffeine under ~200 mg per day, since it clears the body much more slowly in pregnancy.
• Teens and children: experts suggest well below adult limits — often 100 mg or less for adolescents.
• People with heart conditions, anxiety, or on certain medications: may need to limit further; check with a doctor.
• Caffeine-sensitive or slow metabolizers: listen to your body over the number.
For everyone, spreading intake through the day and stopping mid-afternoon keeps caffeine working for you rather than against your sleep.