Moderate caffeine is safe for most adults, but too much causes jitters, a fast heartbeat and poor sleep. Caffeine has a half-life of about 5 hours in adults – longer in pregnancy and with some medicines – so an afternoon coffee can still affect sleep at night.
How to use the Caffeine Calculator
- Brewed coffee, 8 oz (95 mg)
- Espresso shot (63 mg)
- Black tea, 8 oz (47 mg)
- Green tea, 8 oz (28 mg)
- Cola, 12 oz (34 mg)
- Energy drink, 8.4 oz (80 mg)
- Other caffeine
- Weight
- Group
- Caffeine half-life
- Hours between last caffeine and bedtime
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter how many of each drink you have, your weight, and how many hours before bed you have your last caffeine.
Inputs explained
| Field | What to enter |
|---|---|
| Brewed coffee, 8 oz (95 mg) | A number |
| Espresso shot (63 mg) | A number |
| Black tea, 8 oz (47 mg) | A number |
| Green tea, 8 oz (28 mg) | A number |
| Cola, 12 oz (34 mg) | A number |
| Energy drink, 8.4 oz (80 mg) | A number |
| Other caffeine | In mg |
| Weight | Units: kg, lb, st, g |
| Group | Choose one: Healthy adult, Pregnant or breastfeeding, Child or teen (3 mg/kg) |
| Caffeine half-life | In hours |
| Hours between last caffeine and bedtime | A number |
What each result means
- Total caffeine = [Brewed coffee, 8 oz (95 mg)]×95 + [Espresso shot (63 mg)]×63 + [Black tea, 8 oz (47 mg)]×47 + [Green tea, 8 oz (28 mg)]×28 + [Cola, 12 oz (34 mg)]×34 + [Energy drink, 8.4 oz (80 mg)]×80 + [Other caffeine]
- Share of your limit = [Total caffeine]÷[Your daily limit]×100
- Caffeine per kg = [Total caffeine]÷[Weight]
- Caffeine left at bedtime (if all taken at once) = [Total caffeine]×0.5^([Hours between last caffeine and bedtime]÷[Caffeine half-life])
Example values
How the main result changes when Brewed coffee, 8 oz changes and everything else stays at the example values above:
| Brewed coffee, 8 oz (95 mg) | Total caffeine (mg) |
|---|---|
| 1 | 142 |
| 1.5 | 190 |
| 2 | 237 |
| 2.5 | 285 |
| 3 | 332 |
| 4 | 427 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Total caffeine = Σ servings × mg per serving
Safe limits: 400 mg/day adults, 200 mg/day in pregnancy, 3 mg/kg/day for children (EFSA)
Caffeine remaining = total × 0.5^(hours ÷ half-life)
Worked example
Inputs — Brewed coffee, 8 oz (95 mg): 2; Espresso shot (63 mg): 0; Black tea, 8 oz (47 mg): 1; Green tea, 8 oz (28 mg): 0; Cola, 12 oz (34 mg): 0; Energy drink, 8.4 oz (80 mg): 0; Other caffeine: 0 mg; Weight: 70 kg; Group: Healthy adult; Caffeine half-life: 5 hours; Hours between last caffeine and bedtime: 8.
Results — Total caffeine = 237 mg; Your daily limit = 400 mg; Share of your limit = 59 %; Caffeine per kg = 3.4 mg/kg; Caffeine left at bedtime (if all taken at once) = 78 mg.
Key terms
- Half-life
- Time for the body to clear half of a substance.
Frequently asked questions
How much caffeine is safe?
Up to 400 mg a day for healthy adults and 200 mg a day in pregnancy.
How long does caffeine last?
Its half-life is about 5 hours, so a quarter remains after 10 hours.
How much caffeine is too much at once?
Single doses above 200 mg are more likely to cause side effects; very high doses can be dangerous.