How much caffeine is safe to drink in a day based on your exact body weight in pounds or kilograms?

Two ceilings matter. The U.S. Food and Drug Administration describes 400 mg per day as the amount not generally associated with dangerous negative effects in healthy adults. Toxicology literature adds a weight-scaled figure of roughly 6 mg per kilogram of body weight, which is more forgiving for a large adult and considerably stricter for a small one. This calculator computes both at once from your weight, the caffeine content of the drink you selected, and the number of servings, then flags whichever limit you are closest to breaching.

What are the exact formulas used to model caffeine dose, safe limits and clearance over time?

Weight in kg        = weight in lbs / 2.2046
Total intake (mg)   = caffeine per serving x servings
Weight-based limit  = weight in kg x 6 mg
FDA absolute limit  = 400 mg (healthy adults)
Remaining after t h = total x 0.5 ^ (t / 5.7)
Hours down to 50 mg = ln(50 / total) / ln(0.5) x 5.7
Body weight         = clamp(value, 10, unit maximum)

The 5.7-hour half-life is the healthy-adult population average. Fast metabolisers clear closer to four hours; slow metabolisers, pregnant people and users of oral contraceptives can run past nine.

What is the full caffeine reference chart for popular US coffee chains, sodas and energy drinks?

Values are manufacturer and chain published averages. Brewed coffee in particular varies by roast, grind and contact time, so treat every drip-coffee figure as a range of roughly plus or minus 20 %.

What happens if you exceed the FDA 400 mg daily limit with energy drinks stacked on top of coffee?

The failure mode is almost always stacking rather than a single oversized drink. A Grande brewed coffee at 310 mg plus one 16 oz energy drink at 160 mg reaches 470 mg before lunch, and a pre-workout scoop can add another 200. At that level most adults report tremor, a resting heart rate 10 to 20 beats above baseline, reduced deep sleep the same night, and a rebound fatigue crash the following afternoon that invites another dose. The dose meter in this tool exists specifically to make that stacking visible before it happens.

When should you drink your last coffee of the day if you want to fall asleep at eleven at night?

Work backwards from the half-life. To be under roughly 50 mg of circulating caffeine at bedtime after a 200 mg dose you need about eleven hours, which puts your last cup around noon for an 11 pm bedtime. After a 400 mg day that window stretches past seventeen hours. The clearance card on the right prints the exact figure for your own intake, including the residue remaining at 6, 12 and 24 hours.

Is this caffeine safety calculator a substitute for medical advice from a doctor or dietitian?

No. It models a healthy, non-pregnant adult using published population averages for caffeine content, half-life and tolerated dose. Pregnancy caps intake near 200 mg, adolescents near 100 mg, and cardiac, anxiety or reflux conditions can lower the safe ceiling much further. Use the output as a planning aid and defer to a clinician for any personal medical decision.