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General adult guidance is up to about 400mg of caffeine a day; in pregnancy the NHS advises no more than 200mg. Caffeine's half-life is around 5 hours, so an afternoon coffee is still half-present at bedtime. Contents vary widely by brew strength and brand. Not medical advice.
What The Caffeine Calculator Works Out
- Daily caffeine: your drinks added up at typical contents: a mug of filter or shop-bought coffee around 140 mg (chain sizes often carry more), an espresso shot about 80 mg, tea roughly 50 mg, a 250 ml energy drink 80 mg (large cans double it) and a cola about 35 mg. Two coffees and two teas, an entirely ordinary British day, is 380 mg, brushing the line before a single energy drink or afternoon top-up.
- Against the guidelines: the general adult guideline is 400 mg a day and the NHS pregnancy limit is 200 mg. Above the guideline, jitters, anxiety, palpitations and reflux climb; regular high intake breeds tolerance plus withdrawal headaches on skipping a day, which taper better than they cold-turkey.
- Half still in your system at bedtime: the half-life is the useful science. Around five hours in a typical adult, so half of a 2pm coffee is still circulating at 7pm and a quarter at midnight, degrading sleep depth even in people who fall asleep fine.
- Caffeine per coffee and tea (advanced): override the defaults to match your brew. A strong home cafetiere or a large chain latte can run 200 mg or more, while a weak builder's tea may be 30 mg.
- Other sources (advanced): the stealth sources: dark chocolate (about 25 mg per 50 g), pre-workout powders (sometimes 200 mg or more a scoop) and painkillers with added caffeine (65 mg a tablet). Decaf carries a token few milligrams.
- Half-life (advanced): stretched by pregnancy, the contraceptive pill and some medications, shortened for smokers, and genetically fast or slow by CYP1A2 variant, meaning the same latte wires one person and washes over another.
- Left at bedtime and caffeine curfew (advanced): how much of your last drink is still circulating at bedtime, and the latest time to have it so that under 25 mg (about a decaf) remains. The sleep-hygiene rule of a caffeine curfew eight-plus hours before bed has more evidence behind it than most such rules; the decay table shows the hour-by-hour drop.
- Per kg of bodyweight (advanced): the European Food Safety Authority puts the safe daily intake at about 5.7 mg per kg (400 mg for a 70 kg adult) and a single dose at 3 mg per kg, so lighter people reach the line sooner.
- Headroom to your limit (advanced): how much more you could have under the 400 mg adult or 200 mg pregnancy limit, in coffees and teas. Energy drinks concentrate risk in teenagers, where UK guidance steers them to little or none.
- Caveats: caffeine is safe and even mildly beneficial for most adults at moderate doses; anyone with heart rhythm issues, anxiety disorders or pregnancy on board has a lower right number. Not medical advice.
Source and further reading: NHS guidance (caffeine in pregnancy).