Caffeine starts working in 15–45 minutes and its effect lasts about 3–5 hours; its half-life is around 5 hours, so an afternoon coffee can still affect your sleep at night. Sensitivity varies a lot from person to person.
How to use the Coffee Calculator – How Much Caffeine Do I Need?
- Body weight
- Goal
- Caffeine sensitivity
- Drink
- Caffeine per serving (custom)
- Servings already today
- Pregnant or breastfeeding?
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter your weight, choose your goal, sensitivity and drink, and how many servings you have had today.
Tips
- Weigh coffee beans rather than using scoops – roast level and grind change how much a scoop holds.
- Grind just before brewing and match the grind size to the method (fine for espresso, coarse for French press and cold brew).
- Use filtered water just off the boil (90–96 °C / 195–205 °F) for hot coffee.
- Adjust the ratio to taste in small steps – one change at a time.
Common mistakes to avoid
- Using boiling water on green or white tea, which turns it bitter.
- Forgetting that caffeine content varies widely between drinks and brands.
- Comparing ratios written different ways (1:16 coffee:water vs 60 g per litre).
Inputs explained
| Field | What to enter |
|---|---|
| Body weight | Units: kg, lb |
| Goal | Choose one: Stay alert, Strong focus, Exercise performance |
| Caffeine sensitivity | Choose one: Very tolerant, Tolerant, Average, Sensitive … |
| Drink | Choose one: Brewed (drip) coffee, 8 oz / 237 ml, Espresso, single shot (1 oz), Espresso, double shot, Americano, 12 oz (2 shots) … |
| Caffeine per serving (custom) | In mg |
| Servings already today | A number |
| Pregnant or breastfeeding? | Choose one: No, Yes |
What each result means
- Servings of your drink for that dose = [Suggested caffeine dose now]÷[Caffeine per serving]
- Caffeine so far today = [Servings already today]×[Caffeine per serving]
Example values
How the main result changes when Body weight changes and everything else stays at the example values above:
| Body weight (kg) | Suggested caffeine dose now (mg) |
|---|---|
| 35 | 53 |
| 52.5 | 79 |
| 70 | 105 |
| 87.5 | 131 |
| 105 | 158 |
| 140 | 200 |
Formula
Dose (mg) = body weight (kg) × target (1.5 mg/kg for alertness, 3 mg/kg for strong focus, 4.5 mg/kg for exercise) × sensitivity factor
Single dose kept ≤ 200 mg (EFSA); daily total ≤ 400 mg (FDA, EFSA); 200 mg in pregnancy
Servings = dose ÷ caffeine per serving
Worked example
Inputs — Body weight: 70 kg; Goal: Stay alert; Caffeine sensitivity: Average; Drink: Brewed (drip) coffee, 8 oz / 237 ml; Servings already today: 1; Pregnant or breastfeeding?: No.
Results — Caffeine per serving = 95 mg; Suggested caffeine dose now = 105 mg; Servings of your drink for that dose = 1.1; Caffeine so far today = 95 mg; Servings left before the daily limit = 3.2; Daily limit = Within the daily limit.
Frequently asked questions
How much caffeine is too much?
The FDA and EFSA say up to 400 mg a day is safe for most healthy adults; 200 mg in pregnancy.
How much caffeine is in a cup of coffee?
About 95 mg in 8 oz of brewed coffee, 63 mg in a single espresso.
How long does caffeine last?
Its half-life is about 5 hours, so half is still in your body 5 hours later.