Caffeine

3 cups · 285 mg ·

Last cup by 10:45

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Today

You

Body weight kg

Half-life h

Bedtime

Sleep line mg/kg

How it’s calculated

Each drink is absorbed over roughly an hour (peak ≈ 70 min after drinking) and then halves every 5 hours. Caffeine is almost completely absorbed, peaks 15–120 min after drinking, and has an average half-life of about 5 h, with a range of 1.5–9.5 h [1].
Your half-life is personal: smoking shortens it, while oral contraceptives and pregnancy lengthen it [1, 5]. Change it under “You” if you know you are faster or slower.
Sleep line. A meta-analysis of 24 studies recommends drinking a coffee (107 mg) at least 8.8 h before bed and a 217.5 mg pre-workout at least 13.2 h before bed [2]. In this model both leave about 33–37 mg in the body at bedtime, so the default line is 0.5 mg per kg (35 mg at 70 kg).
Body weight. Caffeine spreads through body water (about 0.7 L per kg) [1], so the same milligrams mean a higher blood level in a lighter body. That is why the line scales with your weight. Scaling the study cut-offs by weight is this page’s assumption, not a study result.
For scale: 400 mg taken even 6 h before bed cut measured sleep by more than an hour [3]. EFSA considers up to 200 mg at once (≈ 3 mg/kg) and 400 mg a day (≈ 5.7 mg/kg) safe for healthy adults, and notes that about 100 mg (≈ 1.4 mg/kg) close to bedtime can affect sleep. In pregnancy the EFSA figure is 200 mg a day [4].
Drink values are typical amounts [6, 4]. Real cups vary a lot with beans, brewing and size, so use the label or enter your own number.

Sources

An estimate for healthy adults, not medical advice. Sensitivity to caffeine differs from person to person.