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Caffeine Half-Life Calculator

Computes the caffeine remaining after a given time by first-order elimination with an adjustable half-life (about 5 hours in healthy adults), the time until the amount falls below a threshold, and a 24-hour decay schedule.

When to use

You want to know how much of a coffee, tea or energy-drink dose is still active after some hours, or when it will drop below a chosen level before bedtime.

Do not use when: You need guidance for pregnancy, liver disease, children or drug interactions, where the half-life can be two to three times longer; the result is informational only, not medical advice. For generic exponential decay use half-life.

Formula

remaining_mg = dose_mg × 0.5^(hours_elapsed / half_life_hours); percent_remaining = 100 × 0.5^(hours_elapsed / half_life_hours); hours_until_threshold = half_life_hours × log2(dose_mg / threshold_mg), 0 when dose_mg ≤ threshold_mg; hours_until_threshold_from_now = max(0, hours_until_threshold − hours_elapsed)

Assumes first-order (exponential) elimination and complete absorption at time zero; absorption actually takes 30–60 minutes and the half-life ranges from about 1.5 to 9.5 h between adults. Informational only, not medical advice.

Inputs

Parameter	Type	Unit	Required	Description
`dose_mg`	number	mg	yes	Caffeine taken at time zero. Typical: brewed coffee ≈ 95 mg per 240 mL cup, espresso ≈ 63 mg per 30 mL shot, black tea ≈ 47 mg per cup, cola ≈ 34 mg per 355 mL, energy drink ≈ 80 mg per 250 mL. Range: > 0, ≤ 2000
`hours_elapsed`	number	h	yes	Time since the dose was taken. Range: ≥ 0, ≤ 72
`half_life_hours`	number	h	default 5	Elimination half-life: about 5 h in healthy adults (FDA), shorter in smokers (≈ 3 h), longer with oral contraceptives, in pregnancy or with liver disease. Range: ≥ 1.5, ≤ 10
`threshold_mg`	number	mg	default 25	Amount below which the remaining caffeine is considered negligible (25 mg ≈ a quarter cup of coffee). Range: > 0, ≤ 2000

Outputs

Output	Type	Unit	Description
`remaining_mg`	number	mg	Caffeine still in the body after hours_elapsed.
`percent_remaining`	number	%	remaining_mg as a percentage of the dose.
`half_lives_elapsed`	number		hours_elapsed / half_life_hours.
`hours_until_threshold`	number	h	Time from the dose until the amount falls to threshold_mg (0 when the dose is already at or below it).
`hours_until_threshold_from_now`	number	h	hours_until_threshold minus hours_elapsed, floored at 0.
`schedule`	list		Objects with hours and remaining_mg at 0, 2, 4, 6, 8, 10, 12, 16, 20 and 24 hours after the dose.

Example

200 mg after 10 h, half-life 5 h: {"dose_mg":200,"hours_elapsed":10,"half_life_hours":5,"threshold_mg":25} → {"remaining_mg":50,"percent_remaining":25,"half_lives_elapsed":2,"hours_until_threshold":15,"hours_until_threshold_from_now":5}

One 95 mg coffee after 6 h: {"dose_mg":95,"hours_elapsed":6} → {"remaining_mg":41.35,"percent_remaining":43.53,"hours_until_threshold":9.63,"hours_until_threshold_from_now":3.63}

GET https://tttkmbb.com/api/v1/calculate/caffeine-half-life?dose_mg=200&hours_elapsed=10&half_life_hours=5&threshold_mg=25

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/caffeine-half-life (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/caffeine-half-life · Markdown: https://tttkmbb.com/health/caffeine-half-life.md · JSON definition: https://tttkmbb.com/health/caffeine-half-life.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="caffeine-half-life"
OpenAPI operationId: calculate_caffeine_remaining
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

FDA – Spilling the Beans: How Much Caffeine is Too Much? (government)
Wikipedia – Caffeine (pharmacokinetics) (reference)

FAQ

How much caffeine is in my drink?

Roughly: brewed coffee 95 mg per 240 mL cup, espresso 63 mg per shot, black tea 47 mg, green tea 28 mg, cola 34 mg per 355 mL can, energy drink 80 mg per 250 mL. Check the label when it states a value.

When should I stop drinking coffee before bed?

With a 5 h half-life, half of a dose is still present after 5 h and a quarter after 10 h; many sleep guidelines suggest no caffeine within 6–8 hours of bedtime. Use the threshold outputs to find the time for your own dose.

Related calculators

Half-Life Calculator — Generic exponential decay for any substance or half-life.
Sleep Cycle Calculator — Plan a bedtime once the caffeine has worn off.