Free Online Caffeine Calculator

How to use

1. 1

   Add your drinks. Choose a drink from the list, or Custom amount to type the milligrams from a label, set the time you had it, and press Add. Repeat for each drink today.

2. 2

   Set your bedtime. Enter the time you plan to go to sleep. The default is 11 p.m.

3. 3

   Adjust the assumptions if you need to. Open Adjust assumptions to change the half-life from 5 hours or the bedtime limit from 30 mg.

4. 4

   Read the results. See the caffeine left at bedtime, when it drops below your limit, the latest time for one more drink of your choice, and today’s total against 400 mg.

How long caffeine stays in your system

Caffeine’s half-life is the time your body takes to clear half of it. For a typical healthy adult that is about five hours, the figure the CDC Yellow Book uses; the European Food Safety Authority puts the average nearer four, with a range of two to eight. The halving compounds: about half of a drink is still in your system five hours after you have it, a quarter after ten hours, and an eighth after fifteen. That is why a 95 mg cup of coffee at 2 p.m. still leaves about 28 mg at 11 p.m., while the same cup at 8 a.m. is down to about 12 mg by then.

Your own half-life can sit well away from five hours. Smoking speeds clearance: in one study, smokers averaged 3.5 hours against 6 for non-smokers. Estrogen-containing birth control pills slow it: the half-life went from about 5.4 to 7.9 hours in another. Pregnancy slows it more as the months pass, most of all in the third trimester, when the half-life can reach about 15 hours. Liver disease can stretch it much further, and some medicines slow it too. Genes play a part as well, because CYP1A2, the liver enzyme that clears about 95 percent of caffeine, works faster in some people than in others. The half-life setting runs from 1.5 to 15 hours to cover most of this; if you are pregnant, have a liver condition or take a regular medicine, a clinician or pharmacist can tell you more than an estimate can.

What caffeine does to sleep

Caffeine keeps you alert by blocking adenosine, the signal that builds sleep pressure through the day, and it goes on blocking it for as long as enough is circulating. A 2023 meta-analysis of 24 studies by Gardiner and colleagues in Sleep Medicine Reviews found that caffeine cut total sleep time by 45 minutes on average, lowered sleep efficiency by 7 percent, added 9 minutes to the time it took to fall asleep and 12 minutes of waking in the night, and trimmed about 11 minutes of deep sleep. To avoid losing sleep, the authors concluded, a 107 mg coffee should come at least 8.8 hours before bed, and a 217.5 mg pre-workout drink at least 13.2 hours before.

Studies of timing point the same way. In a 2013 study by Drake and colleagues, 400 mg of caffeine taken six hours before bed still disrupted sleep, and the American Academy of Sleep Medicine reported that it cut measured sleep by more than an hour. EFSA notes that a single 100 mg dose can affect sleep in some adults when it is taken close to bedtime. Falling asleep easily does not mean caffeine had no effect, either: in these studies most of the cost was in how long and how deeply people slept, not in how long they took to drop off.

Caffeine in common drinks

Caffeine per serving varies widely. The bean, the brew, the cup and the café all change it, so the figures below are typical amounts from USDA FoodData Central and the other sources listed, not a measurement of your cup. The widest spreads are in drinks sold by the can or the scoop: the FDA puts energy drinks anywhere from 54 to 328 mg per 16 fl oz, and among 100 top-selling pre-workout powders, the ones with caffeine averaged 254 mg per serving, give or take about 80. When a label gives a number, use Custom amount.

Decaf still has a little: the FDA gives 2 to 15 mg for an 8 fl oz cup, and the table uses USDA’s figure of 2. Chocolate counts too, though not for much; 30 g of 70 to 85 percent dark chocolate has about 24 mg. Cold brew strength differs so much between cafés that the table uses one large chain’s published figure for its standard 16 fl oz cup.

How this calculator works

Each drink is treated as one dose that is absorbed and then cleared at steady rates, the standard one-compartment model used in pharmacology. Absorption runs at a rate of 5 per hour, which, at the default half-life, puts each drink’s peak about 45 minutes after the time you enter. From there the amount halves every half-life: 5 hours unless you change it, anywhere from 1.5 to 15. The curve is the sum of every drink on the list, and the day runs from 4 a.m. to 4 a.m., so a 12:30 a.m. bedtime counts as the end of the same day.

The bedtime limit, 30 mg unless you change it (5 to 100), is how much you are willing to have left when you go to sleep. It comes from the Gardiner review’s cutoffs: under this model, a 107 mg coffee 8.8 hours before bed leaves about 32 mg, and a 217.5 mg pre-workout 13.2 hours before leaves about 36 mg, so 30 mg is a slightly cautious reading. The time you drop below the limit is found to the minute, starting after your last drink has peaked and looking up to 48 hours ahead. The latest time for one more drink only counts times that leave it room to peak before bed, is rounded down to the nearest 5 minutes, and says so plainly when you are already over the limit or when no time today would keep you under it. Today’s total is shown against the 400 mg a day the FDA cites for healthy adults.

All of this is an estimate for healthy adults, not medical advice. Real absorption depends on food, how quickly you drink, your body and your medicines, and no calculator knows how sensitive you are to caffeine. If you are pregnant or breastfeeding, have a heart, liver or anxiety condition, or take regular medication, ask a clinician how much caffeine suits you.

Link parameters

Everything travels in the address bar, so a link can open the calculator with drinks, a bedtime and settings already filled in.

Frequently Asked Questions