Caffeine Calculator
See caffeine still active using half-life decay, when it drops below 100 mg, and your total.
Written by Golam Rabbani, Founder & Lead Engineer
How to use this caffeine calculator
- Set your caffeine half-life (5 hours is the adult average) and your daily limit in milligrams.
- Add each drink from the preset list, or pick "Custom amount" to enter a precise mg dose.
- For each drink, enter how many hours ago you consumed it.
- Click Add drink to log additional servings.
- Hit Calculate to see how much caffeine is still active, when it will fall below the 100 mg sleep threshold, and whether you exceeded your daily limit.
About this caffeine calculator
The Caffeine Calculator models how much caffeine is currently in your bloodstream using a first-order pharmacokinetic decay: remaining = dose × 0.5^(hours / half-life). The default half-life of 5 hours matches the population mean for healthy adults; pregnancy, oral contraceptives, and certain liver conditions can lengthen it to 8–10 hours. Bundled drink presets follow USDA FoodData Central and published industry averages — for example, a 240 ml brewed coffee at 95 mg, a 250 ml energy drink at 80 mg, or a 60 ml energy shot at 200 mg. The 400 mg daily limit reflects the U.S. FDA position for most healthy adults. As a worked example, a 200 mg pre-workout taken 3 hours ago plus a 95 mg coffee 1 hour ago leaves about 200·0.5^(3/5) + 95·0.5^(1/5) ≈ 132 + 83 = 215 mg active, which would still take roughly 5.6 hours to fall below 100 mg. This tool is for general education and is not medical advice — consult a qualified healthcare professional before changing caffeine intake during pregnancy or with heart, anxiety, or sleep disorders.
FAQ
- How accurate are the caffeine amounts in the preset drinks?
- They reflect published averages from USDA FoodData Central and major beverage brands. Real values vary ±30% with brewing method, bean variety, and serving size — use Custom amount when you need to be precise.
- Why does the 5-hour half-life sometimes feel too short?
- Slow metabolizers (about 10% of adults) and people on certain medications can have half-lives of 8–10 hours. Raise the half-life input if caffeine consistently keeps you awake longer than this tool predicts.
- What is the 100 mg sleep threshold?
- Studies show caffeine doses above ~100 mg taken within 6 hours of bedtime measurably reduce total sleep time and slow-wave sleep. The calculator estimates when your active amount drops below that line.
- Is 400 mg/day safe for everyone?
- No. The FDA limit is for healthy non-pregnant adults. Pregnancy and children should consume far less (≈ 200 mg/day during pregnancy, none for children under 12), and people with arrhythmias or anxiety may need to cut intake even lower.
- Does the calculator track adenosine receptor sensitivity or tolerance?
- No. It only models bloodstream concentration. Habitual users feel less subjective stimulation at the same blood level because of receptor up-regulation; the milligrams in your system are still real.