The words this site uses, and what they mean here
Half-life, serving, per 100 ml, threshold, cutoff. Several of these have a loose everyday sense and a precise one; this page fixes the precise one, because the calculators use it.
By Merey Tleugazin Revised 2026-09-18
Several words on this site have a loose everyday sense and a precise one, and the calculators use the precise one. A "cup" is the clearest case: in conversation it means whatever you drank out of, and here it means 240 ml of brewed coffee carrying 96 mg of caffeine, which is the unit every "1.8 cups" figure on the site is measured against. This page fixes the precise sense of each term so that a number you read somewhere on the site can be checked rather than guessed at.
Three groups of term, and why they are easy to confuse
Amounts. Milligrams, servings and concentration per 100 ml describe how much caffeine is in a thing. The trap is that per-serving and per-100 ml rank drinks differently, and both rankings are correct. An espresso has more caffeine per millilitre than any brewed coffee and less caffeine per serving than most of them, because it is 30 ml. Which one you want depends on whether you are asking how strong a drink is or how much you are about to take.
Time. Half-life, first-order elimination, residual caffeine and cutoff time describe what happens after you drink. The trap here is that the number printed on a can is the start of a curve, not the answer to any question about your evening. A 160 mg energy drink at three in the afternoon is still above the sleep threshold at eleven at night; a 200 mg coffee at nine in the morning is not.
Limits. The daily limit, the sleep threshold and the pregnancy figure all sound like the same kind of statement and are not. Two of them come from agencies and are about intake across a day; the sleep threshold is a working line this site uses to classify a residual amount at a moment in time, and it is ours rather than anyone's official position. Where a definition below comes from a named body, it says so.
A note on units and serving sizes
Volumes on this site are millilitres, with the familiar US fluid-ounce equivalent given alongside where a drink is sold that way. A "tall" is 354 ml, a "grande" 473 ml, a "venti" 591 ml for hot drinks; a standard energy-drink can is 250 ml and the large format is 473 or 500 ml depending on the market. Caffeine is always milligrams, never a proportion and never a percentage, because a percentage of a drink you have not specified the size of is not a quantity.
Where a term below has more than one common spelling — decaf and decaffeinated, K-Cup and capsule, half-life and t½ — the alternatives are listed with it, because people arrive here searching for all of them.
The terms
- Caffeine
- A stimulant alkaloid found naturally in coffee, tea, cacao, guarana and yerba mate, and added to energy drinks, soft drinks and supplements. Everything on this site is measured in milligrams of caffeine, never in cups.
- Milligram also mg
- The unit every figure here is expressed in. One brewed 240 ml cup of drip coffee is about 96 mg, which is the reference dose the site is calibrated against.
- Serving
- The quantity a figure refers to — one can, one shot, one scoop, one 240 ml cup. A caffeine number without a serving is not a number; where a product is sold in several sizes, each size is a separate row in the dataset.
- Per 100 ml
- Caffeine concentration: milligrams divided by millilitres, scaled to 100 ml. It is the right comparison between two liquids and a meaningless one between a liquid and a tablet, so the field is empty on 46 of the 87 rows rather than filled with a division nobody should make.
- Half-life also t½
- The time it takes for the amount of caffeine in the body to fall by half. This site uses 5.7 hours, the commonly cited population average; an individual value can sit anywhere from under three hours to over nine.
- First-order elimination
- A model in which the rate a substance leaves the body is proportional to how much is present, which produces an exponential decay with a constant half-life. It is the simplest model that fits aggregate caffeine data, and it is the one every calculator here runs.
- Absorption phase
- The period after drinking during which caffeine is still entering the bloodstream, typically peaking within 30 to 60 minutes. This site does not model it: doses are treated as available immediately, which overstates the first half hour and is accurate afterwards.
- Sleep threshold
- The residual amount below which caffeine is unlikely to be what is keeping you awake. This site uses 100 mg. It is a working line rather than a clinical boundary, and it is the only thing the two colours on this site classify against.
- Cutoff time also last cup
- The latest clock time a given dose can be taken and still fall under the sleep threshold by bedtime. It is derived from the dose, the half-life and your stated bedtime, not looked up.
- Daily limit
- The 400 mg per day figure cited by the FDA and EFSA for healthy adults. It is a population figure: it is not a target, it does not apply during pregnancy, and it says nothing about how a day of 400 mg is distributed.
- Residual caffeine
- How much of a dose is still in the body at a chosen moment. It is what the decay curve plots and what the calculator answers; the figure printed on a can is the starting point of that curve, not the answer.
- Decaf also decaffeinated
- Coffee or tea from which most, not all, of the caffeine has been removed. A decaf cup typically carries 2 to 15 mg, which is small enough to ignore for sleep and not small enough to call zero.
- Caffeine-free
- Containing no caffeine at all — herbal infusions, most clear sodas, roasted grain drinks. The distinction from decaf matters on this site and the dataset marks it with a separate field.
- Brewed coffee also drip, filter
- Coffee made by passing hot water through ground beans. The generic 240 ml cup is 96 mg here; a chain's large brewed coffee can be three times that, which is the single most common surprise in the dataset.
- Espresso
- A 25 to 35 ml shot pulled under pressure. A single is about 63 mg — less per serving than a mug of brewed coffee and far more per millilitre, which is why per-serving and per-100 ml rankings disagree.
- Cold brew
- Coffee steeped in cold water for 12 to 24 hours. The long contact time extracts more caffeine per millilitre than drip, so a cold brew is usually the strongest ordinary coffee on a café menu.
- Nitro cold brew
- Cold brew dispensed with nitrogen. The gas changes the texture and not the chemistry, so the caffeine is close to the cold brew it was made from; the small difference in the dataset is a difference in serving size.
- Instant coffee
- Dried coffee extract dissolved in water. A standard teaspoon gives roughly 60 to 80 mg, lower than brewed, and unusually consistent between cups because the dose is measured rather than extracted.
- Single-serve pod also K-Cup, capsule
- A sealed portion of ground coffee brewed by a machine. The caffeine depends on the grams of coffee sealed inside, not on the cup size selected, so brewing a pod long does not make it stronger.
- Matcha
- Powdered green tea leaf whisked into water, so the whole leaf is consumed rather than an infusion of it. A teaspoon carries roughly 70 mg, well above a cup of brewed green tea.
- Yerba mate
- An infusion of Ilex paraguariensis leaf, traditional in South America, carrying caffeine in the same range as tea to weak coffee depending on how it is prepared.
- Energy drink
- A soft drink formulated with added caffeine, usually 80 to 300 mg per container. On this site it is a category rather than a judgement: the figure is what matters, and some coffees exceed most of them.
- Pre-workout
- A powdered supplement taken before exercise, typically 150 to 400 mg of caffeine per scoop, sometimes more. It is the form where a serious overdose is realistic, because the dose is scooped rather than portioned.
- Guarana
- A plant extract used in energy drinks whose active stimulant is caffeine. Where a label lists guarana separately from caffeine, the total is higher than the caffeine line alone suggests.
- Theobromine
- The stimulant alkaloid that makes up most of the effect of chocolate. It is not caffeine and this site does not count it; the caffeine in a bar of dark chocolate is genuinely small.
- L-theanine
- An amino acid found in tea and sold as a supplement, often taken with caffeine. It is outside the scope of this site, which measures caffeine and does not model interactions.
- Tolerance
- The reduction in caffeine's subjective effect with regular use. It changes how a dose feels and not how fast it is eliminated, so the curves here are unchanged by it — which is exactly why a habitual drinker can sleep badly without feeling stimulated.
- Withdrawal
- Headache, fatigue and irritability following an abrupt stop after regular use, typically beginning 12 to 24 hours later and lasting a few days.
- Adenosine
- The signalling molecule that accumulates during waking hours and promotes sleep. Caffeine blocks its receptors rather than removing it, which is why alertness arrives without the underlying sleep pressure going away.
- Bedtime
- An input, not a fact about you. Every cutoff and every "left at bedtime" figure on this site is computed against the time you give it; the default is 23:00 and changing it changes every answer.
- Dose stacking
- Two or more drinks whose curves overlap. The calculator sums them, which is the whole reason a mid-afternoon second cup can matter more than the first one did.
- Dataset version
- The date stamped on the published dataset. Every figure on this site comes from one version, and the version is printed in the footer, so a number you quoted last month can be traced to the file it came from.
- Reference value
- A figure compiled from a public source rather than measured by us. Everything in the dataset is one; nothing here is a laboratory result, and the sources page says which source stands behind which class of value.
- Computed value
- A figure this site derived with its own model — residual caffeine, a cutoff time, a multiple of a standard cup. Computed values are reproducible from the published dataset and the published formula, and the tools show the assumptions they used.
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