The calculator
Bedtime in, last-coffee time out — with an optional log of everything you drank today and a live decay curve. Set your own half-life and bedtime target, or just use the defaults (5.5 h half-life, 50 mg at bed). All of it runs 100% in your browser — no sign-up, nothing uploaded.
Enter your bedtime and cup size.
Your caffeine decay curve
Stacked exponential decay of every logged dose against your bedtime target. All computed on your device — nothing leaves the page.
The math it uses
where H is half-life in hours, T is when the drink is finished, and target is the maximum caffeine you accept at bedtime. Multiple doses sum: each dose decays independently from its own finish time.
How to use your cutoff
- The cutoff is per drink. A 160 mg energy drink cuts off hours earlier than a 64 mg espresso shot — pick your actual cup above.
- Use the dose log before your afternoon order. If you're already over your bedtime target, the calculator tells you — that's the afternoon decision made for you.
- Know you're slow or fast? Set your own half-life. Afternoon-anything wrecks some sleepers because they clear caffeine near the 12-hour end of the range; fast metabolizers sit near 3–4 hours.
- It's an estimate, not a prescription. The model simplifies real caffeine absorption, and nothing here is medical advice.
Caffeine half-life, explained
A caffeine half-life is the time your body takes to clear half of the caffeine in your blood. In healthy adults it averages roughly 5–6 hours (one clinical study reported a mean of 5.7 hours). That's the number the calculator defaults to.
But the same cup isn't the same for everyone. The Sleep Foundation cites a 2–12 hour range: your genes, age, whether you're pregnant, medications you take, and your liver health all move the number. Some people clear caffeine near the fast end; others are up at the slow end wondering why a noon coffee still has them wired at midnight.
One more wrinkle people miss: one half-life doesn't mean "worn off." After one half-life, half the dose is still active; after two, a quarter is still there. It takes about five half-lives (~27–30 hours at the default 5.5 h) before caffeine is essentially gone (about 3% remaining).
And that matters for sleep. A study published in the Journal of Clinical Sleep Medicine (Drake et al.) found that a fixed dose of caffeine taken 6 hours before bed still measurably reduced total sleep — the researchers recommend avoiding substantial caffeine for a minimum of 6 hours prior to bedtime.
Peak blood levels arrive 1–2 hours after ingestion, so the "finish your drink" time you enter in the calculator is the right anchor: the decay curve starts there.
How long does 200 mg of caffeine take to wear off?
Here's a 200 mg dose (a strong coffee or a standard energy drink) dying out at a 5.5-hour half-life:
Computed at the 5.5 h default; the curve compresses or stretches with your own half-life — drag it in the calculator and watch.
The practical version: a 2 PM 200 mg coffee leaves ~69 mg in your system at 10:30 PM — more than a full espresso shot in bed with you. Move it to 4 PM and ~88 mg is still there, nearly half the original cup. (Model estimates at 5.5 h.) The decay chart at the top runs the same calculation on your own doses.
What is the 2-hour coffee rule?
The 2-hour coffee rule is a popular guideline: stop all caffeine at least 2 hours before you go to bed. It's one of the most-asked caffeine questions online — Google's own "People also ask" box features it alongside these calculator queries.
The honest verdict: it's better than nothing, but weak against the evidence. At a 5.5-hour half-life, a 200 mg coffee 2 hours before bed leaves ~155 mg in your system at lights-out — over three-quarters of the original cup still circulating.
The evidence-backed rule is harsher: 6 hours or more. The Drake et al. sleep study found measurable sleep reduction even at the 6-hour mark, so six hours is a minimum, not an ideal. And if you're a slow metabolizer toward the 12-hour end of the range, even early-afternoon coffee can linger.
So treat the 2-hour rule as the floor and the calculator as your personal version: pick your bedtime, your cup, and the amount you're willing to have left at bedtime — and it gives you your cutoff, not a slogan.
Frequently asked questions
Real answers, all sourced — mirrored in the page's schema for search engines.
What is a caffeine cutoff time?▼
Your caffeine cutoff is the latest time you can have caffeine so that what remains by bedtime stays under a limit you choose. With a ~5.5-hour half-life, coffee roughly halves every 5.5 hours — so a 200 mg coffee at 6 PM is still ~100 mg at 11:30 PM. Enter your bedtime in the calculator to get your own.
How does this calculator work?▼
It uses the standard exponential decay of a chosen half-life: remaining = dose × ½^((now − drink time) ÷ half-life). Your cutoff is solved backwards from bedtime: drink late enough that decay leaves no more than your target at bed. Logged doses each decay independently and sum together on the curve.
Is 6 hours before bed really enough?▼
Six hours is the evidence-backed minimum: Drake et al.'s study in the Journal of Clinical Sleep Medicine found that caffeine 6 hours before bed "has important disruptive effects on sleep," even for people who fall asleep fine. Slow metabolizers — up to a 12-hour half-life — typically need a wider margin, which is why the calculator lets you set your own target and half-life.
How long does 200 mg of caffeine take to wear off?▼
At a 5.5-hour half-life: ~100 mg after 5.5 hours, ~50 mg after 11, ~25 mg after 16.5, and effectively gone (~3% left) after about 27–30 hours. See the full decay table; plug your real bedtime into the calculator for your personal numbers.
How much caffeine is in coffee, tea, and energy drinks?▼
Cups vary widely — always check the label — but as typicals:
How much caffeine is safe per day?▼
The U.S. FDA advises that up to 400 mg per day is generally acceptable for healthy adults — roughly four 8-oz cups of brewed coffee. Guidance is stricter during pregnancy: Health Canada lists 300 mg/day for pregnant and breastfeeding people (Health Canada).
Does this tool save or upload my data?▼
No. Every calculation runs in your browser on your device — your bedtime and doses are never uploaded, stored on a server, or shared. Close the tab and it's gone.
Is this medical advice?▼
No. It produces mathematical estimates from a half-life you choose, and real half-life spans 2–12 hours by person. If you have a sleep disorder or a medical condition, talk to a clinician.