How to Use This Calculator
Select your beverages and quantities. The calculator totals your caffeine intake, compares it to the 400mg daily safe limit, and estimates how long caffeine stays active in your system using the half-life decay model.
Formula & Background
Total caffeine = Σ(servings × mg per serving). Typical values: drip coffee ~95mg/cup, espresso ~63mg/shot, black tea ~47mg/cup, energy drink ~160mg/can. Decay model: remaining caffeine = initial × 0.5^(hours/5), based on caffeine's ~5-hour half-life.
Calculation Example
2 cups of drip coffee at 8am (190mg) + 1 energy drink at noon (160mg) = 350mg total. By 5pm: ~247mg still active. By 10pm: ~174mg — enough to delay sleep onset for most people by 1–2 hours.
Expert Tips
- 400mg/day is the consensus safe limit for healthy adults (FDA, WHO, EFSA). During pregnancy: cap at 200mg.
- Avoid caffeine after 2pm if you're sensitive to sleep disruption — it takes ~10 hours to clear 95%.
- Caffeine tolerance builds quickly. A 2-week break fully resets sensitivity.
- Decaf is not caffeine-free: a 12oz decaf coffee still contains 2–15mg.