Caffeine Intake & Half-Life Calculator
Add up the caffeine in your coffee, tea and other drinks, compare the total with the FDA’s 400 mg a day for healthy adults, and see how much is still in your body at bedtime and when it drops below a level you choose.
How to Use
- Add each drink with the time you had it. Choose a type to fill in its caffeine, or use Other and enter the milligrams from the label.
- Set how many of each you had at that time.
- Enter your bedtime and the level you would like to be under by then (50 mg to start; it is your choice).
- Leave the half-life at 4 hours, the EFSA average, or try 2 or 8 hours to see the range between people.
- Read the total against 400 mg, the amount left at bedtime and the time it falls below your level, and check Show Work for each step.
Worked Example
Two coffees, 7:30 and 12:30, bed at 10:30 PM. Two 95 mg cups are 190 mg, 47.5% of the FDA’s 400 mg. With a 4-hour half-life, the first cup has had 15 hours to clear and the second 10 hours: 95 × 0.515/4 + 95 × 0.510/4 = 7.1 + 16.8 = 23.9 mg left at bedtime. Just after the second cup the level is 134.9 mg, and it takes 4 × log₂(134.9 ÷ 50) = 5.7 hours to fall below 50 mg, at 18:14.
One coffee at 3 PM. 95 × 0.57.5/4 = 25.9 mg at 10:30 PM with a 4-hour half-life; 7.1 mg with a 2-hour half-life; 49.6 mg with an 8-hour one.
The common mistake: treating the half-life as the time it is all gone. After one half-life, half is still there. A 160 mg energy drink at 15:00 is not gone by 19:00: with a 4-hour half-life, 80 mg remains then, and 43.6 mg is still there at 10:30 PM. It takes about 6.7 hours just to get below 50 mg.
Show Work
Formulas
Where the Numbers Come From
Caffeine occurs naturally in more than 60 plants, including coffee beans, tea leaves, kola nuts and cacao pods, and is added to soft drinks, energy drinks and some medicines. The liver breaks almost all of it down, mainly through the enzyme CYP1A2, and the speed of that varies a great deal between people, which is why the half-life is an input here.
The 400 mg a day figure is the FDA’s, for most healthy adults; a 2017 systematic review by the Institute for the Advancement of Food and Nutrition Sciences supported it. In Europe, the European Food Safety Authority’s 2015 opinion on the safety of caffeine reached the same daily figure and described the half-life as about 4 hours on average, ranging from about 2 to 8 hours. The per-cup values come from USDA FoodData Central (SR Legacy), and the FDA adds a reminder that decaf is not caffeine-free: typically 2 to 15 mg in an 8 fl oz cup.
About This Tool
This calculator adds up caffeine from timed drinks, compares the day’s total with the FDA’s 400 mg, and draws how much is left in your body hour by hour using the half-life you choose, with your bedtime and your chosen level marked. It finds the time the level drops below that line for good, even when several drinks overlap. It treats each drink as absorbed at once, so the real peak is a little later and lower, and it cannot know how sensitive you are; the level to be under is yours to pick.
Everything runs in your browser; nothing you enter is sent anywhere.
For general information only — not medical advice. Talk to a doctor or other qualified professional about your own health.
Related tools: Sleep Calculator, Alcohol Units & Standard Drinks Calculator, and Blood Pressure Category Checker.
Frequently Asked Questions
How much caffeine is too much?
For most healthy adults the FDA cites 400 mg a day, about two to three 12 fl oz cups of coffee, as an amount not generally associated with negative effects, while noting wide variation in how sensitive people are. If you are pregnant, trying to become pregnant or breastfeeding, the FDA recommends asking your health care provider. It also notes that toxic effects such as seizures can be seen with rapid consumption of around 1,200 mg.
What is the half-life of caffeine?
The time for the body to clear half of it. The European Food Safety Authority gives a plasma half-life of about 4 hours in healthy adults, with a range of about 2 to 8 hours, and a longer half-life in pregnancy. With 4 hours, a 95 mg coffee leaves 47.5 mg after 4 hours and 23.75 mg after 8 hours.
How much caffeine is in coffee and tea?
USDA FoodData Central lists brewed coffee at 40 mg per 100 g, about 95 mg in an 8 fl oz cup; a 1 fl oz espresso at about 63 mg; black tea at about 47 mg and green tea at about 28 mg per 8 fl oz; and decaf coffee at about 2 mg. Real cups vary a lot: the FDA gives 113 to 247 mg for 12 fl oz of brewed coffee and 41 to 246 mg for 12 fl oz of energy drink, so use the label when there is one.
How late can I have coffee and still sleep?
It depends on how fast you clear caffeine and how sensitive you are, which no calculator knows. With a 4-hour half-life, a 95 mg coffee at 3 PM still leaves about 26 mg at 10:30 PM; with an 8-hour half-life, about 50 mg. MedlinePlus says the effects can last four to six hours, and the FDA lists sleep disruption among the signs of too much caffeine.
How does the calculator work out the curve?
Each drink is treated as absorbed when you have it (MedlinePlus says caffeine peaks in the blood within an hour) and then halves every half-life: remaining = mg × 0.5^(hours since ÷ half-life). The amounts from all drinks are added. Two 95 mg coffees at 7:30 and 12:30 peak at 135 mg just after the second and fall below 50 mg at 18:14.
How do I use the Caffeine Intake & Half-Life Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Checking a daily habit
Five 8 fl oz cups of brewed coffee are about 475 mg, 119% of the FDA’s 400 mg a day; four cups are 380 mg.
An afternoon espresso
A coffee at 7:30 and a double espresso at 16:00 add up to 221 mg and still leave 48 mg at 10:30 PM.
Energy drinks
A can labelled 160 mg at 15:00 drops below 50 mg at 21:43 with a 4-hour half-life, leaving 44 mg at 10:30 PM.
Slow or fast clearance
The same two morning coffees leave 3.5 mg at bedtime with a 2-hour half-life but 66 mg with an 8-hour one.
Tea through the day
Three 8 fl oz cups of black tea at 8:00, 11:00 and 15:00 are 141 mg in total and leave about 23 mg at 10:30 PM.
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