Track how much caffeine is still in your system, how much is left at bedtime, and when it drops to a sleep-friendly level — half-life model, browser-only.
- Runs locally
- Category Calculator
- Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.
Tap a drink to add it at the chosen time, or enter custom mg.
Your intake today
- 08:00 · 95 mg
- 13:30 · 95 mg
Not medical advice. This is a rough first-order pharmacokinetic estimate; individual caffeine metabolism varies widely with genetics, medication, pregnancy, liver function, and age. For health decisions, consult a clinician.
What this tool does
A caffeine metabolism calculator that estimates how much is still circulating in your body at any moment, using a first-order half-life model. Log each drink — pick a preset (brewed coffee ~95mg, single espresso 63mg, black tea 47mg, cola 34mg, an energy drink, matcha) or type custom milligrams — and set the time you had it. The tool sums the decay of every dose, so two coffees five hours apart stack realistically instead of being treated in isolation. Set your bedtime and it shows the residual caffeine still in your system when your head hits the pillow, plus how long until you drop under a sleep-friendly 50mg. The half-life is adjustable from 4 to 6 hours because metabolism varies: slower with oral contraceptives or pregnancy, faster in heavy smokers. A 400mg/day adult ceiling warning flags an over-caffeinated day. Everything runs in your browser, the share link reproduces your exact intake, and the math is clearly labelled as an estimate, not medical advice.
Tool details
- Input
- Numbers
- The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
- Output
- Live result
- The result area focuses on usable output, with copy, download, or preview actions when supported.
- Privacy
- Browser-side processing
- The main tool logic does not call an external API, so inputs normally stay in the current tab.
- Save / share
- Shareable URL state
- Key settings are encoded in the URL so another person can reopen the same setup.
- Performance budget
- Initial JS <= 11 KB
- No WASM budget is declared, keeping the tool quick to open on mobile.
- Best fit
- Calculator · Student
- Category and role tags drive related tools, internal links, and quick fit checks.
How to use
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1. Input
Paste or drop your content into the tool panel.
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2. Process
Click the button. All processing is local in your browser.
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3. Copy / Download
Copy the result or download to disk in one click.
How Caffeine Calculator fits into your work
Use it for fast estimates, comparisons, and planning numbers before you make the final call.
Calculation jobs
- Getting a realistic range before a purchase, plan, workout, or schedule decision.
- Comparing scenarios by changing one input at a time.
- Turning rough assumptions into a number you can discuss.
Calculation checks
- Double-check units, dates, rates, and rounding assumptions.
- Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
- Save the inputs that produced an important result so you can reproduce it later.
Good next steps
These links move the current task into a more complete workflow.
- 1 Sleep Cycle Calculator Sleep cycle calculator — 90-minute REM cycles. Find best bedtime if you must wake at X, or best wake time if you sleep at Y. Open
- 2 Water Intake Calculator Water intake calculator — daily water target by body weight, activity, climate. With drink-log to track today's progress. Open
- 3 Half-Life Calculator Solve any radioactive-decay unknown — initial amount, remaining amount, elapsed time, or half-life — with exact exponential math, all in your browser Open
Real-world use cases
Decide if a 4pm coffee will cost you sleep
You are tempted by a late-afternoon coffee but have an early morning. Log a 95mg cup at 16:00, set your bedtime to 23:00, and read the bedtime residual: around 47mg still onboard at lights-out. Since that is near the 50mg sleep-friendly line, you either skip it or switch to decaf — and you can test "what if I have it at 2pm instead" in one click to see the residual drop.
Build a taper plan to cut back caffeine
You want to wean off three coffees a day without the withdrawal headache. Log your current intake, see the total (285mg) and the bedtime residual, then experiment: drop the afternoon cup, halve the morning one to a single espresso. The tool shows each change's effect on both your daily total and what is left at night, so you taper with numbers instead of guesswork.
Plan caffeine for an all-nighter before an exam
You are cramming and plan to study until 3am. Instead of one giant energy drink that spikes then crashes, log smaller doses (an espresso at 22:00, another at 00:30) and watch the residual stay in a useful band rather than overshooting the 400mg ceiling. You also see how late the caffeine lingers, so you know the 9am exam-day grogginess is the trade-off you are signing up for.
Time a pre-workout dose without ruining your night
You train at 18:00 and like a 150mg pre-workout hit for the performance boost. Log it and check the bedtime residual: if you sleep at 22:30, roughly 96mg is still circulating — too much. The tool tells you to either move the workout earlier, cut the dose, or accept a later bedtime, turning a vague worry into a concrete number.
Compare two coffees' real caffeine load
A friend insists espresso is "stronger" than drip coffee. Add a single espresso (63mg) and an 8oz brewed coffee (95mg) at the same time and the tool makes the point per-serving: the larger drip cup actually delivers more total caffeine, even though espresso is more concentrated per ml. It is a quick, shareable way to settle the argument with real numbers.
Common pitfalls
Assuming caffeine is "gone" a couple of hours after the cup. With a 5-hour half-life, half is still in you at hour 5 and a quarter at hour 10 — that 3pm coffee is why you are still wired at bedtime.
Reading the 400mg/day ceiling as a personal safe target. It is a population guideline; caffeine-sensitive people, and anyone pregnant (often advised under 200mg), feel effects well below it.
Trusting the milligram numbers as exact. Brewed-coffee caffeine swings from ~70 to ~140mg per cup with the bean, roast, and grind, so log your real drinks and treat presets as ballpark starting points.
Privacy
Every calculation — the half-life decay, the dose superposition, the clearing-time search — is plain JavaScript that runs in your browser tab. Nothing you log about what or when you drank is sent anywhere, and there is no external API or analytics on your intake. The one thing to know: the shareable URL encodes your dose list, half-life, and bedtime in the query string (e.g. ?d=95@0800,95@1330&bed=23:00), so a "share link" leaves that data in the destination's access log. That is harmless for a coffee log; if you would rather not share it, just copy the result instead of the URL.
FAQ
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