Caffeine Calculator

What a half-life actually means. Every five hours, roughly half your caffeine is gone. A 200 mg coffee leaves 100 mg after 5 hours, 50 mg after 10, and 25 mg after 15. It doesn't drain away steadily — it halves, then halves again, which is why a late-afternoon coffee lingers far longer than people expect.

The timing trap. A 200 mg coffee at 8 am leaves about 29 mg at 10 pm — negligible. The same coffee at 3 pm leaves about 76 mg at 10 pm — roughly a strong cup of tea, going into your bloodstream as you try to sleep. Same drink, same dose. The only difference is when. That's the single most actionable fact on this page.

Everyone's clock is different. The same coffee at 8 am leaves 8 mg at bedtime in a fast metaboliser (3-hour half-life) but 50 mg in a slow one (7-hour). Genetics (the CYP1A2 gene), pregnancy, oral contraceptives, liver function and smoking all shift it. If coffee after lunch reliably ruins your night, you are probably a slow metaboliser — and that's information worth having.

What caffeine actually does

Caffeine doesn't give you energy — it blocks tiredness. As you're awake, a molecule called adenosine builds up in your brain and binds to receptors that make you feel sleepy. Caffeine is shaped enough like adenosine to sit in those receptors without activating them, so the sleepiness signal never lands. The tiredness is still accumulating; you just can't feel it. When the caffeine clears, all that adenosine is waiting — which is exactly what the afternoon crash is.

How much is safe

The FDA cites 400 mg a day as an amount not generally associated with negative effects in healthy adults — roughly four 8 oz cups of brewed coffee. That's a population-level figure, not a target and not a promise. Sensitivity varies enormously: some people get palpitations from a single espresso, others sleep fine after a late one. Pregnant people are generally advised to stay under 200 mg. For children and adolescents, Health Canada's 2.5 mg per kg is the common reference — about 150 mg for a 60 kg teenager.

Half-life: the thing everyone gets wrong

Caffeine's half-life averages about five hours, meaning half of it is gone in that time — then half of the remainder in the next five, and so on. It's the compounding that catches people out. A 200 mg coffee at 3 pm still leaves around 76 mg in your bloodstream at 10 pm. That's a strong cup of tea, arriving as you're trying to fall asleep. The same coffee at 8 am leaves about 29 mg — effectively nothing.

This is why the single highest-leverage change most people can make isn't drinking less caffeine. It's drinking it earlier.

Why your friend can drink coffee at 9 pm

Half-life is highly individual, and largely genetic. A variant of the CYP1A2 liver enzyme gene makes some people fast metabolisers who clear caffeine in around three hours, while slow metabolisers take seven or more. Pregnancy dramatically slows clearance — in the third trimester the half-life can more than double. Oral contraceptives roughly double it. Smoking speeds it up. Some medications interfere in both directions.

You can infer your own type without a test: if a coffee after lunch reliably ruins your night, you're likely a slow metaboliser, and the calculator's "slow" setting will model you better than the average.

Caffeine and sleep

Even when caffeine doesn't stop you falling asleep, it degrades the quality of the sleep you get — reducing deep slow-wave sleep, the stage that does most of the physical restoration. People often insist caffeine doesn't affect them because they fall asleep fine, then wake unrefreshed, and reach for coffee to fix it. A stable loop, and a self-inflicted one. A common recommendation is to stop caffeine six to eight hours before bed; the calculator works out a personalised cut-off from your actual drinks, bedtime and metabolism.

Performance and exercise

Caffeine is one of the few genuinely well-evidenced performance supplements, improving endurance, power output and perceived exertion. The researched range is roughly 3–6 mg per kg of body weight, taken 45–60 minutes before exercise — for a 75 kg athlete, 225–450 mg. Note the top of that range exceeds the 400 mg daily guidance, and more is not better: past about 6 mg/kg, benefits plateau while jitters, elevated heart rate and disrupted sleep do not.

Does it dehydrate you?

Mostly a myth. Caffeine is a mild diuretic, but at ordinary doses the fluid in a coffee or tea more than offsets the extra urine, so caffeinated drinks still count toward your daily fluid intake. That said, they're not a substitute for water, and at very high doses, during hard exercise, or in heat, the diuretic effect matters more.

Too much: what it feels like, and when to get help

Excess caffeine causes jitteriness, anxiety, a racing or irregular heartbeat, insomnia, nausea, headaches, tremor and gastrointestinal upset. Most people's ceiling makes itself known well before anything dangerous happens. But concentrated products deserve real caution: powdered and pure caffeine supplements have caused deaths, because a teaspoon of powder can hold as much caffeine as dozens of coffees and kitchen scales aren't accurate enough to dose it safely. The FDA has taken action against bulk caffeine powder for exactly this reason. If someone has severe chest pain, an irregular heartbeat, confusion, seizures or persistent vomiting after taking caffeine — particularly from pills or powder — treat it as an emergency and seek medical help immediately (in the US, Poison Control: 1-800-222-1222).

Tolerance and withdrawal

Regular use builds tolerance: your brain grows more adenosine receptors, so the same dose does less, and your baseline without caffeine feels worse than it used to. Stopping abruptly brings on headaches, fatigue, irritability and difficulty concentrating, typically peaking at 24–48 hours and passing within a week or so. Tapering — cutting a quarter every few days, or switching some cups to decaf — makes it much easier than going cold turkey.

Cutting down without misery

Move your last cup earlier before you cut the amount; you'll often find you sleep better and need less. Replace one coffee with tea (roughly half the caffeine), then with decaf. Watch the hidden sources — energy drinks, pre-workout, some sodas, dark chocolate and certain medications all count toward the same total. And know your true starting number, which is what this page is for: most people are surprised by it.