Caffeine Calculator
An 8 oz cup of brewed coffee has about 95 mg of caffeine. Drunk at 08:00, about 48 mg is left at 13:00, with a 5 hour half-life.
How to use
- Pick a drink, the number of servings and the time you had it, then press Add. Repeat for each drink and remove any by mistake.
- Read the daily total and how it compares with the 400 mg guide.
- Set the "Check at" time to see how much caffeine is still in your body then, and read the 24 hour chart and table.
How it works
Total caffeine is the sum of each drink's caffeine times its servings. The daily guide used here is 400 mg, which the US FDA gives as an amount not generally associated with dangerous, negative effects for healthy adults. That is roughly four or five cups of coffee.
Caffeine left in the body is estimated with exponential decay: remaining = dose x 0.5^(hours since the drink / 5). A half-life of 5 hours is a commonly used average, so half of a dose is gone after 5 hours and a quarter after 10. The amounts for all drinks already taken by the chosen time are added together.
The model assumes the whole dose is in your body at the moment you drink it. In real life absorption takes some time, and the peak level comes roughly 30 to 60 minutes after drinking, so the curve is a simplification.
The chart covers one day from 00:00 to 24:00 and only counts drinks you enter for that day, so caffeine from the day before is not included.
Caffeine in common drinks
These are approximate amounts per serving, based on figures from the US FDA and USDA FoodData Central. The real amount varies with brand, bean, brewing method and cup size, so treat them as typical values and check the label when there is one.
| Drink | Serving | Caffeine, approx |
|---|---|---|
| Brewed coffee | 8 oz | 95 mg |
| Espresso | 1 oz shot | 64 mg |
| Instant coffee | 8 oz | 70 mg |
| Decaf brewed coffee | 8 oz | 5 mg |
| Black tea | 8 oz | 47 mg |
| Green tea | 8 oz | 28 mg |
| Cola | 12 oz | 34 mg |
| Energy drink | 8 oz | 80 mg |
| Dark chocolate | 1 oz | 22 mg |
Sources: FDA · USDA FoodData CentralLast reviewed 2026-10-13
FAQ
How much caffeine is too much?
The US FDA cites up to 400 mg a day as an amount not generally associated with dangerous, negative effects for healthy adults. Sensitivity differs from person to person, and pregnancy, some medicines and health conditions can lower the amount that is right for you. Ask a doctor if you are unsure.
Why a 5 hour half-life?
It is a typical average used for healthy adults. It varies widely. Pregnancy, liver disease, some medicines such as certain oral contraceptives, and smoking can make it much longer or shorter, so your own curve may differ a lot from the chart.
Are the caffeine amounts exact?
No. They are approximate typical values from FDA and USDA FoodData Central. A café coffee can have far more than a home cup, and energy drinks and teas vary by brand and steeping time.
Does caffeine left in my body affect sleep?
It can. Caffeine can still be in your system many hours later, which is why some people avoid it in the afternoon. This tool shows an estimate only and cannot say how it will affect you.
Is this medical advice?
No. It is a general estimate for information only and does not replace advice from a doctor or pharmacist.