Caffeine Calculator

How Caffeine Affects Your Body

Caffeine is absorbed quickly and peaks in blood about 45 minutes after consumption. How long it lasts depends on your metabolism.

Caffeine Half-Life

Half-life is the time for half the caffeine to leave your bloodstream. Average is ~5 hours. Slow metabolizers (CYP1A2 variants) may have 7+ hours; fast metabolizers around 3 hours. After one half-life, 50% remains; after two, 25%; after three, 12.5%.

Safe Caffeine Limits

The FDA recommends up to 400mg per day for healthy adults. A body-weight guideline is 3–6mg per kg. Above 6mg/kg or 400mg/day is considered high; very high doses can cause anxiety, palpitations, and sleep disruption.

Caffeine and Sleep

Caffeine blocks adenosine receptors that promote sleep. Even small amounts (50mg+) can reduce sleep quality. Most experts suggest stopping caffeine 6–8 hours before bed. Use this calculator to find when your level drops below 50mg.

Tolerance

Regular caffeine use builds tolerance — you need more for the same effect. Reducing intake for a few days can reset sensitivity. Avoid escalating doses; stick to moderate daily limits.

Caffeine Content by Source

An 8oz cup of brewed coffee contains about 95mg of caffeine, but this varies widely — from 75mg to 200mg depending on the bean, roast, and brewing method. A single espresso shot has about 63mg. Green tea averages 28mg per cup, black tea about 47mg. Energy drinks range from 80mg to 300mg per can. Pre-workout supplements typically contain 150–300mg. Knowing the exact caffeine content of your preferred source is essential for accurate timing and dosage calculations.

Caffeine and Exercise Performance

Caffeine is one of the most well-researched ergogenic aids. A dose of 3–6mg per kg of body weight, consumed 30–60 minutes before exercise, can improve endurance, power output, and focus. It works by blocking adenosine receptors and increasing adrenaline. However, habitual users may experience reduced performance benefits due to tolerance. Cycling caffeine intake (reducing for a few days before important events) can restore its ergogenic effects.

Caffeine Sensitivity and Genetics

Your response to caffeine is largely determined by the CYP1A2 gene, which controls how fast your liver metabolizes caffeine. Fast metabolizers (CYP1A2 AA genotype) clear caffeine quickly and may tolerate afternoon coffee without sleep issues. Slow metabolizers (AC or CC genotype) process caffeine much slower, meaning even a morning cup can affect sleep. If caffeine makes you anxious or disrupts your sleep, you may be a slow metabolizer — consider cutting off earlier or reducing your dose.

Caffeine Withdrawal Symptoms

Regular caffeine users who suddenly stop can experience withdrawal symptoms within 12–24 hours: headaches, fatigue, irritability, difficulty concentrating, and even flu-like symptoms. Withdrawal peaks at 1–2 days and typically resolves within a week. To avoid withdrawal, reduce intake gradually — cut by 25% every few days rather than going cold turkey. This is especially important if you consume more than 200mg daily.