Caffeine Calculator: Half-Life, Sleep Cutoff & Safe Daily Limit

The tool tracks every drink you've added today, decays each one with a first-order half-life, and sums what's left at any time you ask about. The headline is caffeine on board now. The tile grid covers the four questions people actually want answered: predicted level at bedtime, the latest safe time for one more cup, when caffeine drops below the 25 mg "negligible" line, and your personal daily safe limit.

Inputs feed one model. Change body weight and the daily limit shifts. Change the half-life and the curve in the chart relaxes or steepens. The output is an estimate, not a clinical reading; for medication interactions or arrhythmia risk, ask your doctor.

Each dose decays by half every t_½ hours:

remaining(t) = D × 0.5^{(t − t_i) / t_½}

Multi-dose total is the sum across drinks (linear pharmacokinetics, valid at dietary doses below CYP1A2 saturation):

total(t) = Σ D_i × 0.5^{(t − t_i) / t_½}

The verification line at the bottom of the dark panel restates this with your actual numbers, so you can match it against a spreadsheet or hand calculation.

Caffeine half-life varies by a factor of three to four across healthy adults, and almost an order of magnitude once pregnancy enters the picture. The tool ships with six state presets that drive a sensible default; flip on the custom half-life if you want to dial it in to your real-world response.

• Healthy adult: ~5 h. The number every cookie-cutter calculator assumes by default.
• Heavy smoker: ~3 h. Smoking induces CYP1A2, so caffeine clears faster (and the buzz fades faster too).
• Hormonal contraceptive or SSRI: ~7 h. Estrogens and several SSRIs slow CYP1A2; one cup hits like 1.5 cups would otherwise.
• Pregnancy 3rd trimester: ~11.5 h. Late pregnancy nearly doubles the half-life, which is why the ACOG limit drops to 200 mg/day.

The classic question is "when should I stop drinking coffee for sleep?" The tool answers it by back-solving. Given everything you've already had, project the level at bedtime, find the headroom under your threshold, then compute how late you could add one more drink and still land under it. Closed form:

t^* = bedtime − t_½ × log₂(D_next / headroom)

The threshold dropdown lets you pick how strict to be. Sleep researchers like Walker and Drake target zero. The conventional 50 mg line is where EEG sleep architecture begins to change measurably. If your sleep already feels off, drop to 25 mg or strict zero and watch the cutoff time march earlier.

Caffeine is the most-studied legal ergogenic in sport. The ISSN position stand and EFSA's safety opinion converge on a 3-6 mg/kg single dose taken 45-60 min before exercise as the well-evidenced range for endurance and high-intensity work. Toggle the pre-workout block to see a personalized dose and recommended take-time.

Two warnings the calculator can't print. First, the dose still counts toward your daily limit and your sleep cutoff. Late-afternoon training plus a 200 mg pre-workout is the most common reason coffee drinkers can't fall asleep at night. Second, 6 mg/kg is a ceiling, not a target; taller doses raise jitter and arrhythmia risk without extra performance.

Linear pharmacokinetics break down at very high doses (CYP1A2 saturates above ~600 mg in a single sitting), and the tool doesn't model that. It also can't predict interactions with prescription drugs, arrhythmia history, anxiety disorders, or caffeine sensitivity at the genetic CYP1A2*1F level. Outputs are educational estimates, not medical advice.

Pregnancy guidance is the firmest part. The 200 mg/day cap is not negotiable in clinical guidance, regardless of body weight, and high consumption is linked to miscarriage and low birth weight in observational data. If you're pregnant or breastfeeding, talk to your obstetrician before changing intake based on this calculator.