Caffeine Remaining Calculator (Half-Life)
Caffeine does not vanish at a fixed hour — it decays exponentially. After one half-life (often about 5 hours in healthy adults) half of the dose is still there. This tool shows the milligrams left, not medical advice.
Caffeine remaining47.5 mg
Eliminated47.5 mg
Percent still in system50%
Half-lives elapsed1
How this calculation works
Remaining = dose × 0.5^(hours ÷ half-life). After two half-lives a quarter remains; after three, an eighth.
A typical 8 oz brewed coffee is about 80–100 mg; espresso is similar per shot; energy drinks vary widely. Enter the milligrams from the label when you have it.
Pregnancy, liver disease, oral contraceptives and smoking change half-life. The default 5 hours is a population average, not your personal clearance.
Worked example
95 mg at 8:00 with a 5-hour half-life leaves about 47.5 mg at 13:00 and about 24 mg at 18:00.
Frequently asked questions
Why is the default half-life 5 hours?
Healthy adult plasma half-life is commonly cited around 3–7 hours, with 5 hours a practical midpoint. It is longer in pregnancy and with some medicines, and shorter in smokers.
Will this tell me when I can sleep?
Sensitivity varies. Some people feel 50 mg at bedtime; others do not. Use the remaining milligrams as a rough guide, not a sleep prescription.
Does food change the half-life?
Food can slow absorption, which delays the peak, but the half-life of caffeine already in blood is mainly a liver-metabolism story.
Are inputs stored?
No. The math runs in your browser.
This tool is provided for general information only. Verify important figures independently. · Last reviewed: August 25, 2026