Caffeine Half-Life Calculator
Estimate how much caffeine may remain after one or more drinks, and see when it may fall below your chosen level.
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What The Caffeine Result Means
The estimate applies a half-life curve to each entered caffeine source. A half-life is the time it takes for the estimated amount in the body to halve. If the half-life is set to 5.5 hours, about half remains after 5.5 hours, about a quarter after 11 hours, and about one eighth after 16.5 hours. The number is not a blood test, sleep diagnosis or personal safety decision.
Caffeine clearance varies. Pregnancy, liver function, smoking status, some medicines, oral contraceptives, genetics, age, illness and regular intake can change how long caffeine stays active for a person. The result is best used as a planning estimate for sleep, daily intake notes and label checks. If symptoms such as chest pain, fainting, severe anxiety, palpitations or confusion are present, seek urgent medical help rather than relying on a web estimate.
Formula And Method
Formula for each source: remaining mg = dose mg x 0.5^(hours since dose / half-life hours).
Total now: add the remaining amount from each drink, food, supplement or medicine entered.
Time to target: half-life x log(current mg / target mg) / log(2), when current amount is above the target.
1 Add each caffeine source separately, because a fresh tea and an old coffee do not decay from the same starting time.
2 Choose a half-life assumption. A middle value is useful for planning, while a slower value is more cautious for late-day caffeine.
3 Read the remaining milligrams, planned sleep estimate and time to your chosen threshold together.
4 Keep label values with the result, because shop drinks, home brewing and supplements can vary widely.
When To Be Careful With The Estimate
The result cannot say whether caffeine is safe for you. The FDA notes that sensitivity depends on body weight, medicines, conditions and individual response. NHS guidance for pregnancy says caffeine can be taken but should be no more than 200 mg per day. EFSA lists 200 mg single doses and 400 mg per day as levels that do not raise safety concerns for the general healthy adult population, but that is not a personal rule for every user.
Speak with a pharmacist, GP, midwife or clinician if caffeine interacts with medicines, pregnancy, heart rhythm concerns, anxiety, sleep problems, high blood pressure or a child or teenager’s intake. Check medicine labels as well as drinks, because some pain relief, cold and flu products and alertness products contain caffeine.
Caffeine Remaining By Half-Lives
This table explains the shape of the result. It uses percentages rather than one drink size, so it applies to any starting dose. The actual clock time depends on the half-life you enter. With a 5.5 hour setting, three half-lives take 16.5 hours; with an 8 hour setting, three half-lives take 24 hours.
| Elapsed Half-Lives | Estimated Amount Left | Example From 200 mg |
|---|---|---|
| 0 | 100% | 200 mg |
| 1 | 50% | 100 mg |
| 2 | 25% | 50 mg |
| 3 | 12.5% | 25 mg |
| 4 | 6.25% | 12.5 mg |
| 5 | 3.125% | 6.25 mg |
Worked Caffeine Examples
Morning Filter Coffee
140 mg, 6 hours ago: with a 5.5 hour half-life, roughly 66 mg may remain. A slower half-life setting leaves more.
Tea After Lunch
75 mg, 2 hours ago: the estimate remains fairly high because less than one half-life has passed.
Late Energy Drink
160 mg, 1 hour ago: most of the entered caffeine remains, so the sleep estimate is likely to matter.
Common Caffeine Amounts To Check
Drink values vary by brand, serving size and preparation. Treat this table as a prompt for label checking, not a fixed database. NHS pregnancy guidance gives examples such as 75 mg in a mug of tea, 100 mg in a mug of instant coffee and 140 mg in a mug of filter coffee. Packaged drinks and supplements should be checked from their label.
| Source | Typical Amount To Enter | Check Before Copying |
|---|---|---|
| Mug of tea | About 75 mg | Strength and mug size vary |
| Mug of instant coffee | About 100 mg | Depends on spoon size and brand |
| Mug of filter coffee | About 140 mg | Can vary with brew method |
| Single espresso | Often 60 to 80 mg | Check cafe or capsule data |
| 250 ml energy drink | Often around 80 mg | Read the can label |
| 500 ml energy drink | Often around 160 mg | Serving may be more than one portion |
| Cola can | Often 30 to 45 mg | Brand and size vary |
| Pre-workout supplement | Can be high | Read scoop size and warning label |
| Dark chocolate | Varies widely | Cocoa percentage matters |
| Cold and flu medicine | Check pack | Some products contain caffeine |
FAQs
What half-life should I enter?
A 5 to 6 hour value is a reasonable middle assumption from NIOSH guidance for many adults. Some people clear caffeine faster or slower. If sleep is the concern, try the result with 5.5 hours and again with 8 hours to see a more cautious planning range.
Does the estimate show when caffeine has completely gone?
No practical half-life estimate reaches a true zero. It keeps halving. After five half-lives, about 3.125% of the entered amount remains. For most planning questions, a chosen threshold such as 50 mg is more useful than waiting for a mathematical zero.
Can I use this during pregnancy?
The arithmetic works, but pregnancy needs extra caution because caffeine clearance and public-health guidance differ. NHS guidance says caffeine should be no more than 200 mg per day in pregnancy. Ask a midwife, GP or pharmacist if caffeine, symptoms or medicines are a concern.
Why do I still feel alert when the number looks low?
The estimate tracks caffeine amount, not personal sensitivity, sleep debt, stress, anxiety, other stimulants or the effect of poor sleep. Some people notice symptoms at low amounts, while others notice less. Symptoms matter more than the number.
Should I count chocolate and medicines?
Yes, if you are trying to track total caffeine. Chocolate, cola, energy drinks, pre-workout products and some medicines can add caffeine. Use product labels where possible because exact amounts vary.
Is 400 mg per day safe for everyone?
No. EFSA and FDA material gives broad adult population reference points, not personal medical clearance. Sensitivity, body weight, pregnancy, medicines, heart rhythm issues, anxiety and sleep problems can change what is suitable.
When should I seek help rather than calculate?
Get urgent help for severe symptoms such as chest pain, fainting, severe confusion, seizure, breathing difficulty or serious palpitations. For non-urgent concerns, contact a pharmacist, GP, NHS 111, midwife or relevant clinician.
Sources
- National Institute for Occupational Safety and Health. (2018). Caffeine and long work hours. Centers for Disease Control and Prevention. https://archive.cdc.gov/www_cdc_gov/niosh/emres/longhourstraining/caffeine.html
- European Food Safety Authority. (2015). Scientific opinion on the safety of caffeine. EFSA Journal, 13(5), 4102. https://www.efsa.europa.eu/en/efsajournal/pub/4102
- NHS. (2023). Foods to avoid in pregnancy. NHS. https://www.nhs.uk/pregnancy/keeping-well/foods-to-avoid/
- U.S. Food and Drug Administration. (2024). Spilling the beans: How much caffeine is too much? U.S. Food and Drug Administration. https://www.fda.gov/consumers/consumer-updates/spilling-beans-how-much-caffeine-too-much