Caffeine Calculator

How to use this calculator

1. Count drinks honestlyA mug is not a cup. The figures here assume a 250 ml mug, which is what most people actually drink from — a 150 ml cup contains proportionally less.
2. Adjust the guideline if it applies to you400 mg is the general adult figure. UK pregnancy guidance is 200 mg. If you have been advised a different limit, enter it.
3. Use the sleep panel for the evening questionEnter the size of your last dose and how long before bed you had it. The half-life model shows how much is still circulating.
4. Treat the numbers as rangesCaffeine content varies by a factor of two or more between brews of the same drink. These are central estimates, not measurements of your cup.

How the calculation works

The daily total is straightforward multiplication and addition. The interesting part is the elimination model.

Caffeine is cleared by first-order kinetics, meaning a constant proportion is removed per unit time rather than a constant amount. That produces exponential decay with a half-life — the time taken for the amount in your bloodstream to halve.

In healthy adults that half-life averages about five hours. So a 200 mg coffee at 2pm leaves 100 mg at 7pm, 50 mg at midnight and 25 mg at 5am. The last figure is the one that surprises people: a lunchtime coffee is still measurably present when you wake up.

The individual variation is enormous and genuinely matters. Genetics — particularly variants of the CYP1A2 enzyme that metabolises caffeine — split the population into fast and slow metabolisers, with half-lives ranging from around two hours to ten. Smoking roughly halves it. Pregnancy can more than double it, reaching fifteen hours or more in the third trimester, which is a large part of why the guideline drops.

Hormonal contraceptives, some antibiotics and liver conditions all lengthen it too. If caffeine affects you noticeably more than it affects other people, that is a real physiological difference rather than a matter of tolerance.

Typical caffeine content

Espresso-based drinks are the ones people misjudge most. A single-shot latte contains less caffeine than a mug of filter coffee, because the filter method extracts more in total despite the espresso being far more concentrated per millilitre. Concentration and total dose are different things, and it is the total that matters.

Worked example

Two mugs of filter coffee, one instant and two black teas: 190 + 60 + 94 = 344 mg. That is 86 per cent of the 400 mg guideline, so within it but not by much — one more coffee would take you over.

Now the sleep question. Suppose the last of those coffees was a 95 mg mug at 4pm and you go to bed at 10pm, six hours later. Remaining = 95 × 0.5^(6 ÷ 5) = 95 × 0.435 = 41 mg.

Forty-one milligrams is roughly the caffeine in a mug of black tea, still circulating as you try to sleep. Research on caffeine and sleep consistently finds that doses well below what people consider stimulating still reduce total sleep time and slow-wave sleep, often without the person noticing any subjective effect.

Work the model backwards and the practical rule falls out: to have under 25 mg left at bedtime from a 95 mg coffee, you need about 9.6 hours. That is why the standard advice is no caffeine after early afternoon, and why people who insist coffee does not affect their sleep are frequently sleeping worse than they think.

What the guideline is and is not

The 400 mg figure is a general population guideline for healthy adults, based on evidence about cardiovascular effects, anxiety, and sleep disruption. It is not a threshold where something happens at 401 mg — it is a level below which no consistent adverse effect has been demonstrated across the population.

Pregnancy guidance in the UK is 200 mg, and the reason is specific: caffeine crosses the placenta, the foetus cannot metabolise it, and maternal clearance slows dramatically as pregnancy progresses. That guidance is worth following precisely rather than approximately.

Adolescents are advised considerably lower again, and energy drinks are the practical concern there rather than coffee — a large can can exceed 160 mg in a single serving, and they are marketed and consumed differently from a mug of tea.

Caffeine also produces genuine physical dependence. Regular consumption downregulates adenosine receptors, so stopping abruptly causes headache, fatigue and irritability for several days. That withdrawal is real pharmacology rather than habit, and it is why cutting down gradually works better than stopping outright.

One thing worth separating: tolerance to the alerting effect develops quickly, but tolerance to the sleep disruption does not develop nearly as completely. Someone who drinks coffee all day and sleeps badly may have full tolerance to feeling stimulated and none to the effect on their sleep architecture.

Assumptions and limitations

• Caffeine content per serving is a typical figure — real variation is a factor of two or more
• Assumes a 250 ml mug; smaller cups contain proportionally less
• The five-hour half-life is a population average, ranging from about two to ten hours between individuals
• Does not account for pregnancy, smoking, medications or liver conditions, all of which change clearance substantially
• Models a single last dose rather than accumulation across multiple drinks
• General guidance only — not medical advice, and not tailored to any health condition

Questions people ask about this in United Kingdom