About

What lastcup.app is

lastcup.app is a free caffeine half-life calculator. You tell it what you drank and when, and it tells you roughly how much caffeine is still in your system at bedtime — and what time your "last cup" should be if you want a clean night's sleep. No signup, no app to install. Everything runs right in your browser.

Who it's for

It's for anyone who loves coffee but doesn't love lying awake at 1 a.m. Late-afternoon coffee drinkers, shift workers, students, parents running on espresso, and anyone curious about why that 4 p.m. latte might be quietly wrecking their deep sleep. You don't need to give up caffeine to use it — you just need to know where your cutoff is.

The science, in plain terms

Caffeine has a half-life of about five hours for an average adult. "Half-life" means that every five or so hours, roughly half of what's in your body is cleared away. So 200 mg at 2 p.m. leaves about 100 mg by 7 p.m., about 50 mg by midnight, and still around 25 mg by 5 a.m. That lingering caffeine blocks adenosine — the molecule that makes you feel sleepy — which is why a coffee that felt harmless in the afternoon can still keep you out of your deepest, most restorative sleep stages.

Your personal half-life varies. Genetics (the liver enzyme CYP1A2), pregnancy, and smoking all shift it — some people clear caffeine in about three hours, others take eight or more. The calculator lets you adjust for fast, average, or slow metabolism, pregnancy, and smoking, so the estimate fits you rather than an average stranger.

What it does

• Estimates caffeine remaining at your chosen bedtime.
• Tells you the latest time to have your last cup ("last sip by...").
• Charts the 24-hour decay curve in several views.
• Lets you stack multiple drinks across the day and adjust for your own metabolism.

An honest note

This is an educational estimate built on well-established pharmacokinetics, not a medical device. Individual responses vary, and the numbers are guidance, not a diagnosis. For health decisions, talk to a clinician.

Try the calculator →

Educational only. Not medical advice. Consult a clinician.