Caffeine Half-Life Calculator
Free caffeine half-life calculator based on peer-reviewed science. Calculate caffeine decay, find your optimal cutoff time for sleep, and get study timing recommendations. Includes 100+ drink database.
How It Works
C(t) = C₀ × (1/2)^(t/t½)Caffeine follows exponential decay: C(t) is the remaining caffeine at time t, C₀ is the initial dose, and t½ is the half-life (averaging 5 hours in healthy adults). After one half-life, 50% remains; after three half-lives (~15 hours), only 12.5% remains.
Quick Tips
FDA Limit
Healthy adults: max 400mg/day (about 4 cups of coffee). Pregnant women: max 200mg/day.
Sleep Tip
Stop caffeine at least 6 hours before bedtime. For 11 PM sleep, last coffee by 2 PM.
Half-Life
Average half-life is 5 hours, but ranges from 3h (fast metabolizers) to 9h+ (slow metabolizers).
Pregnancy
Caffeine half-life increases to 15-20 hours during pregnancy. Limit to 200mg/day.
Step-by-Step Instructions
- 1Choose your mode: Half-Life, Sleep, Study, Metabolism Quiz, or Drinks Database.
- 2Select a drink from the database or enter a custom caffeine amount.
- 3Set your metabolizer type (or take the quiz to find out).
- 4See your current caffeine level, sleep impact, and personalized recommendations.
Frequently Asked Questions
How long does caffeine stay in your system?▼
Caffeine has a half-life of about 5 hours in healthy adults. After 5 hours, 50% remains; after 10 hours, 25%; after 15 hours, 12.5%. It takes about 24 hours for caffeine to be nearly eliminated.
When should I stop drinking coffee before bed?▼
For most adults, stop caffeine at least 6 hours before bedtime. If you're a slow metabolizer, stop by noon for an 11 PM bedtime. Our Sleep mode calculates your exact cutoff time.
How much caffeine is safe per day?▼
The FDA recommends no more than 400mg per day for healthy adults (about 4 cups of brewed coffee). Pregnant women should limit to 200mg/day. Adolescents: 100mg/day maximum.
What factors affect caffeine half-life?▼
Genetics (CYP1A2 enzyme), smoking (speeds up metabolism), oral contraceptives and pregnancy (slow metabolism significantly), age, liver function, and certain medications.
Can caffeine disrupt sleep even if I fall asleep fine?▼
Yes. Caffeine reduces REM sleep and deep sleep quality even if you don't notice trouble falling asleep. As little as 25mg at bedtime can measurably reduce sleep quality.
Is decaf coffee completely caffeine-free?▼
No. A typical 8oz cup of decaf contains 2-5mg of caffeine. It's much less but not zero. Very sensitive individuals may still notice effects from multiple decaf cups.