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Caffeine Calculator (Intake + Half-Life)

Audit daily caffeine vs the 400 mg safe limit and model how much is still in your system after N hours using the 5-hour half-life.

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Two modes: audit your daily caffeine intake against the FDA 400 mg safe limit (200 mg if pregnant), and model how much caffeine remains in your bloodstream N hours after consumption using the 5-hour half-life.

Caffeine pharmacokinetics: oral absorption is ~99% within 45 minutes; peak blood concentration at 30-60 min; elimination follows first-order kinetics with a half-life of 5 hours (range 3-7, longer in pregnancy and liver disease). Remaining caffeine after t hours = initial × 0.5^(t / 5). FDA safe upper limit is 400 mg/day for adults, 200 mg/day for pregnant women (ACOG). The "sleep-safe" threshold is ~50 mg in the blood at bedtime.

Disclaimer: Half-life varies 3-7 hours by genetics (CYP1A2), smoking status, oral contraceptives, and liver function. Caffeine-sensitive individuals should cut off intake 8-12 hours before bed. Not medical advice.

Calculator information

How to use this calculator

  1. Open the Daily Caffeine Total tab and choose a beverage from the database (americano 150 mg, espresso 64 mg, black tea 47 mg, Red Bull 80 mg, cola 34 mg) and the number of servings.
  2. Add more beverages or foods (dark chocolate, pre-workout supplements) to accumulate today's total caffeine in mg.
  3. Compare your total against the FDA safe limit of 400 mg/day for healthy adults, or 200 mg/day for pregnant women.
  4. Half-life & Sleep Impact tab: enter your last consumption time and dose; the calculator projects caffeine levels in your body at bedtime (assuming a 5-6 hour half-life).
  5. Tolerance & Withdrawal tab: enter your routine intake; the calculator outlines the 12-24 hour withdrawal symptom timeline (headache, fatigue, mood dip) if you stop abruptly.
  6. mg-to-Equivalent Conversion tab: enter a target mg value and see how many cups of coffee, tea, or energy drinks are equivalent.

Caffeine Pharmacokinetics (First-Order Elimination)

Level_t = Initial_dose x (0.5)^(t / half_life)
  • Level_t = caffeine level in the body t hours after consumption (mg)
  • Initial_dose = amount of caffeine consumed (mg)
  • t = time since consumption (hours)
  • half_life = 5-6 hours in healthy adults (varies 1.5-9.5 hours depending on CYP1A2 genetics, pregnancy, smoking)

Caffeine reaches peak plasma concentration 30-60 minutes after consumption. With an average half-life of 5 hours: 200 mg of coffee at 3:00 PM still leaves 100 mg in your system at 8:00 PM and 50 mg at 1:00 AM, enough to disrupt sleep cycles. Pregnancy slows elimination (half-life 10-15 hours).

Worked example: Office worker drinks 3 cups of coffee throughout the day, last cup at 4:00 PM

Given:
  • 7:00 AM: cup of drip coffee = 100 mg
  • 11:00 AM: americano = 150 mg
  • 4:00 PM: espresso = 64 mg
  • Total caffeine today: 314 mg
  • Target bedtime: 10:00 PM
Steps:
  1. Total intake 314 mg, below the FDA limit of 400 mg
  2. Remaining caffeine at 10:00 PM from the 4:00 PM dose = 64 x (0.5)^(6/5) = 64 x 0.435 = 27.8 mg
  3. Remaining from 11:00 AM (11 hours elapsed) = 150 x (0.5)^(11/5) = 150 x 0.217 = 32.5 mg
  4. Remaining from 7:00 AM (15 hours elapsed) = 100 x (0.5)^(15/5) = 100 x 0.125 = 12.5 mg
  5. Total remaining at 10:00 PM = 27.8 + 32.5 + 12.5 = 72.8 mg (enough to disrupt sleep)

Result: 72.8 mg of caffeine remains in the body at bedtime - potentially extending sleep latency by 30-60 minutes.

Frequently asked questions

What is the safe daily caffeine limit?
The FDA and EFSA set 400 mg/day as safe for healthy adults (about 4 cups of coffee). Pregnant women should limit intake to 200 mg/day per ACOG. Children and adolescents should avoid caffeine or stay under 100 mg/day per AAP guidance. People with anxiety disorders, arrhythmias, or hypertension may need stricter limits.
Why does evening coffee disrupt sleep but afternoon coffee doesn't?
Because caffeine's half-life is 5-6 hours. A 200 mg coffee at 2:00 PM leaves only ~50 mg by midnight (3 half-lives). But 200 mg at 6:00 PM leaves ~100 mg at midnight, which is enough to interfere with adenosine receptors. Rule of thumb: stop caffeine 6-8 hours before bed for quality sleep.
What are the symptoms of caffeine overdose?
Consuming >500-600 mg at once can trigger jitters, palpitations, nausea, anxiety, and insomnia. Doses >1,000 mg (rarely from beverages, usually from powdered supplements) are dangerous: severe tachycardia, seizures, even death. Pre-workout supplements sometimes contain 300-400 mg per scoop, so watch the dose carefully.
Is decaf coffee really caffeine-free?
Not entirely. Decaf coffee still contains 2-15 mg of caffeine per cup (vs. 80-150 mg in regular coffee). Industry standard requires at least 97% caffeine removal via Swiss Water or CO2 processes. For people who are highly caffeine-sensitive or in late pregnancy, consider herbal teas (chamomile, rooibos) that are truly caffeine-free.
How long do caffeine withdrawal symptoms last?
For regular drinkers consuming >200 mg/day, symptoms appear 12-24 hours after the last dose: headache (most common), fatigue, low mood, difficulty concentrating. Symptoms peak on days 1-2 and resolve within 5-9 days. A tapering strategy (reducing by 25% per week) is more comfortable than quitting cold turkey.

Last updated: May 11, 2026

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