CalculatorCalc.

The European Food Safety Authority (EFSA) and the US FDA agree: for healthy adults, up to about 400 mg of caffeine a day carries no short-term health risk in most cases. That is roughly 3–5 cups of drip coffee. As a single dose, staying within 200 mg is the common guide.

For pregnant and breastfeeding women, the international recommendation is 200 mg a day or less; for children and adolescents, 3 mg per kg of body weight. Transfer through the placenta and breast milk, immature liver metabolism, and higher sleep sensitivity put those ceilings at half or below. With heart conditions or anxiety disorders, your doctor’s advice comes first.

The reason this calculator shows "mg remaining until the limit" is to keep you consciously below those official guidelines. Build your day so the total stays under 400 mg — roughly 200–300 in the morning and up to 100 in the afternoon — and your night’s sleep is much easier to protect.

In healthy adults, blood caffeine falls by half in about 5 hours on average. Take 100 mg at 8 a.m. and the math says 50 mg remains at 1 p.m., 25 mg at 6 p.m., and 12 mg at 11 p.m. That long half-life is exactly why the "one evening cup" creates sleepless nights.

Half-life varies widely between people, strongly shaped by your CYP1A2 liver-enzyme genotype and by factors like smoking, oral contraceptives, pregnancy and liver disease. That some people sleep fine on the same dose while others lie awake is not imagination — it is metabolic speed.

If you want to protect sleep reliably, the safe rule is: no new caffeine within 8–10 hours of bedtime. Going to bed at 11 p.m. makes 1–3 p.m. the practical last call. After that, consider switching to decaf or herbal tea.

• Average half-life is 5 hours (individual range 2–10 hours)
• Smokers metabolize faster; pregnant women and newborns far slower
• Grapefruit juice and certain medications slow metabolism

"A cup of coffee" varies hugely by method. As general guides: 150–200 ml of drip coffee runs 80–120 mg; one espresso shot (30 ml) 60–80 mg; a French press cup 100–130 mg; a cold brew 150–200 mg.

By concentration espresso wins by far (2–2.6 mg per ml), but by total intake, drip and cold brew tend to deliver more. "Espresso seems strong, I’ll hold back" — and yet that afternoon iced coffee often carries more caffeine than the shot would have.

Roast level matters less than commonly said. Dark roasts expand more, so the same scoop weighs less — making dark roast feel lower in caffeine — but compared by equal weight, total caffeine barely changes through roasting. The real levers are dose and brew time, not roast level.

One cup a day for a year is 365 cups. Swap a ¥150 convenience-store coffee for a ¥50 home brew (10 g from a ¥1,000/200 g bag) and the ¥100 daily difference compounds to ¥36,500 a year. Even specialty beans (¥2,000–3,000 per 200 g) come to ¥100–150 a cup — convenience-store money or less for something close to café quality.

This calculator works out per-cup, monthly and annual cost from your dose per cup and the bean’s unit price. Add the up-front cost of a dripper and a kettle and it still pays for itself within six months to a year. Even buying a scale and a grinder, home coffee wins by a wide margin over three years.

It also works in the other direction: not cutting cost, but raising quality on the same budget. A ¥4,500 convenience-store month buys two ¥2,250 bags of specialty beans instead. When the question is "what do I choose for the same spend," this calculator earns its keep.

Daily caffeine increases adenosine receptors until the effect dulls. This is tolerance: the same dose stops feeling like it wakes you. A full reset takes about 7–14 days off caffeine, with reports putting the return to peak sensitivity at around two weeks.

If quitting outright is unrealistic, stepped reduction works too: halve the dose, switch to decaf on weekends, replace afternoon cups with herbal tea. Withdrawal (headache, fatigue) typically peaks 12–48 hours after cutting down and settles within 3–5 days.

Decaf has come a long way — Swiss Water and CO₂ methods preserve most of the flavor. At 1–5% of normal caffeine per cup (2–7 mg), it serves perfectly well for evening unwinding or as the choice during pregnancy.