Caffeine Calculator
Enter your weight, metabolism type, and the beverages you have had today. The calculator works out your personalised daily limit using health authority guidelines, shows how much caffeine is still in your system and estimates how much will remain when you go to bed. Switch between metric and imperial at any time.
How is your safe caffeine limit calculated?
The European Food Safety Authority (EFSA) and the U.S. Food and Drug Administration (FDA) both set 400 mg per day as the general ceiling for healthy adults, equivalent to about four 240 ml cups of brewed coffee. Because caffeine distributes through body water, its effects scale with body weight. EFSA specifically endorses 3 mg per kg as a single-dose limit that is safe for all healthy adults. Sensitivity varies widely: slow metabolisers carry caffeine in their system far longer than fast metabolisers, which is why this calculator lets you choose your metabolism type. Pregnancy slows caffeine clearance dramatically, raising the half-life from around five hours to eleven or more, which is why the WHO and NHS cap intake at 200 mg per day during pregnancy.
Caffeine half-life and sleep
Caffeine blocks adenosine receptors in the brain, the receptors that signal sleepiness. The time it takes your body to eliminate half of a dose is called the half-life, typically three to five hours in adults, but as long as eleven hours during pregnancy and as short as two and a half hours in smokers (nicotine speeds up the CYP1A2 enzyme that breaks caffeine down). If you drink a 200 mg coffee four hours before bed with a five-hour half-life, roughly 130 mg is still circulating when you try to sleep. Research shows that 200 mg at bedtime reduces slow-wave (deep) sleep by about 20 percent and shifts the circadian rhythm forward. Practical rule of thumb: aim to stop caffeinated drinks at least 8 to 10 hours before you want to sleep if you are a normal or slow metaboliser.
Who should limit caffeine intake?
Pregnant and breastfeeding people should stay below 200 mg per day, as caffeine crosses the placenta and the foetus cannot metabolise it. Teenagers are advised to keep intake below 2.5 mg per kg per day (Health Canada). People with anxiety disorders, heart arrhythmias, high blood pressure, or GERD may be advised by their doctor to reduce or eliminate caffeine. Caffeine also interacts with several medications: it can reduce the absorption of some antibiotics, potentiate stimulant drugs, and interact with certain antidepressants and bronchodilators. If you take regular medication, check with your pharmacist.
Signs of too much caffeine
Acute caffeine overdose is rare at normal dietary doses but can occur with concentrated supplements or energy drinks. Warning signs of excessive intake include rapid or irregular heartbeat, severe headache, trembling, extreme anxiety, nausea, and in very high doses (roughly 10 g or more) potentially life-threatening toxicity. At ordinary over-the-limit levels (say, 500-600 mg), the most common effects are jitteriness, insomnia, stomach upset, and an elevated heart rate. Tolerance develops quickly: regular coffee drinkers may feel few acute effects from doses that would disturb a non-habitual user. Cutting back abruptly after regular use can cause withdrawal headaches and fatigue for one to four days.
Caffeine content of common drinks and foods
| Source | Serving size | Caffeine (mg) |
|---|---|---|
| Brewed coffee | 240 ml (8 oz) | 80-120 (avg 95) |
| Espresso | 30 ml (1 shot) | 60-70 (avg 63) |
| Instant coffee | 240 ml (8 oz) | 27-100 (avg 60) |
| Black tea | 240 ml (8 oz) | 25-60 (avg 47) |
| Green tea | 240 ml (8 oz) | 20-45 (avg 28) |
| Matcha | 240 ml (8 oz) | 50-90 (avg 70) |
| Energy drink 250 ml | 250 ml (8.4 oz) | 50-100 (avg 80) |
| Cola (regular) | 355 ml (12 oz) | 30-40 (avg 34) |
| Dark chocolate | 30 g (1 oz) | 12-30 (avg 23) |
| Pre-workout supplement | 1 scoop | 100-300 (avg 150) |
Approximate caffeine per standard serving. Actual content varies by brand and preparation.
Frequently asked questions
How much caffeine is safe per day?
For healthy adults, both the U.S. FDA and the European Food Safety Authority consider up to 400 mg of caffeine per day safe. This is roughly equivalent to four 240 ml cups of brewed coffee. The limit is lower for specific groups: 200 mg during pregnancy, around 85-100 mg for teenagers, and zero for young children. Because caffeine spreads through body water, heavier people generally tolerate a higher absolute amount, which is why weight-based calculations (3-6 mg per kg) are also used.
How long does caffeine stay in your system?
Caffeine has a half-life of about 3-5 hours in a typical adult, meaning half of a dose is metabolised in that time. After five half-lives (15-25 hours), less than 3 percent remains. The actual figure varies significantly: slow CYP1A2 metabolisers may take 8-10 hours per half-life, while smokers metabolise caffeine roughly twice as fast as non-smokers. During pregnancy the half-life can extend to 11.5 hours.
How many hours before bed should I stop drinking coffee?
A general rule is 8-10 hours before your planned bedtime if you are an average metaboliser. With a 5-hour half-life, a 200 mg coffee taken 10 hours before sleep has decayed to about 12 mg by the time you lie down. If you are a slow metaboliser (8-hour half-life), the same 200 mg drink leaves about 50 mg active at bedtime 10 hours later. Use the timing fields in the calculator above to get a personalised figure.
Does caffeine affect sleep quality even if I fall asleep easily?
Yes. Even moderate caffeine at bedtime can reduce the proportion of slow-wave (deep) sleep and suppress melatonin. A study in Science Translational Medicine found that 200 mg of caffeine taken before sleep delayed the internal circadian clock by nearly 40 minutes, similar to exposure to a room of bright light. If you notice feeling unrefreshed in the morning despite sleeping a full night, late-day caffeine may be a factor worth experimenting with.
Why does caffeine affect some people more than others?
The main driver is a genetic variation in the CYP1A2 gene, which encodes the liver enzyme responsible for breaking caffeine down. People with the fast-metaboliser variant clear caffeine quickly and may feel less stimulated and sleep better after afternoon coffee. Slow metabolisers retain caffeine longer, meaning the same dose produces a stronger and more prolonged effect. Pregnancy, oral contraceptives, some antibiotics, and liver disease also slow CYP1A2 activity.
Is caffeine addictive?
Caffeine produces physical dependence rather than addiction in the clinical sense: regular users who stop abruptly experience withdrawal symptoms (headache, fatigue, irritability) that peak at 1-2 days and resolve within a week. This is different from the compulsive drug-seeking that defines addiction. The WHO does not classify caffeine dependence as a substance use disorder, though it does recognise caffeine withdrawal as a condition. Gradually reducing intake over several days typically avoids or minimises withdrawal symptoms.