How much caffeine is in a cup of coffee?

An 8 oz (240 ml) brewed coffee has about 95 mg. A 12 oz cup is often 140 mg. One espresso shot is about 63 mg. Cold brew and large cafe drinks can exceed 200 mg. Actual content varies by bean, roast, and brew.

Does a second coffee stack on the first?

Yes. Caffeine does not reset. If you drink coffee at 7 am and again at 10 am, most of the first cup is still in your system. This calculator adds every drink on top of what remains from earlier ones.

Caffeine Calculator

Track coffee, tea, and energy drinks. See half-life decay, caffeine left at bedtime, and your safe daily limit by weight and age.

Caffeine’s average half-life is 5 hours. A healthy adult’s daily cap is 400 mg (FDA). One 8 oz coffee is about 95 mg. Stop caffeine 8–10 hours before bed — 200 mg at 2 pm still leaves ~50 mg at 8 pm.

First-order decay using a 3 / 5 / 7 hour half-life. Estimates only — not medical advice.

What did you drink?

Add each coffee, tea, or energy drink and when you had it.

Weight

What time do you sleep?

Who is this for?

Does caffeine last a long time for you?

What Is a Caffeine Calculator?

A caffeine calculator turns what you drank into a timeline: total milligrams, how fast it leaves your blood, how much is left when you sleep, and whether you are under a safe daily cap. This page covers the same jobs as a half-life calculator, a bedtime / sleep tool, and a caffeine limit by weight and age — on one URL.

It models stacked drinks. A 10 am espresso does not start from zero if you already had coffee at 7 am. Pair timing with the water intake calculator if coffee is your main fluid, or the intermittent fasting calculator if you drink coffee in a fasting window.

What Is Caffeine Half-Life?

Half-life is the time for half the caffeine in your blood to clear. The population average is about 5 hours. The CYP1A2 liver enzyme does most of the work. Fast metabolizers (~40% of people) sit near 3 hours. Slow metabolizers (~15%) sit at 7–10 hours. Pregnancy can stretch half-life to 11–18 hours in the third trimester. Smoking often shortens it.

Decay is exponential. After one half-life, 50% remains. After two, 25%. After three, 12.5%:

Caffeine remaining from a 200 mg dose (about two small coffees)
Hours later	Fast (3 h)	Normal (5 h)	Slow (7 h)
1 hour	159 mg	174 mg	181 mg
3 hours	100 mg	132 mg	149 mg
5 hours	63 mg	100 mg	122 mg
8 hours	31 mg	66 mg	91 mg
10 hours	20 mg	50 mg	74 mg
12 hours	13 mg	38 mg	61 mg

Formula: remaining = dose × 0.5^{(hours ÷ half-life)}. Peak blood level is usually 30–60 minutes after you drink.

How Much Caffeine Is in Coffee, Tea, and Energy Drinks?

Labels lie less than cafe names. A “small coffee” can be 80 mg or 200 mg. Use the custom field if you know the exact number:

Typical caffeine per serving (USDA / FDA ranges)
Drink	Serving	Caffeine
Brewed coffee	8 oz	95 mg
Brewed coffee	12 oz	140 mg
Espresso	1 shot	63 mg
Cold brew	12 oz	200 mg
Black tea	8 oz	47 mg
Green tea	8 oz	28 mg
Energy drink	8–16 oz	80–160 mg
Cola	12 oz	34 mg
Pre-workout	1 scoop	150–300 mg
Dark chocolate	1 oz	23 mg
Decaf coffee	8 oz	2–15 mg

What Is a Safe Daily Caffeine Limit by Weight and Age?

Health agencies publish a daily cap, not a target. A second check is 3–6 mg per kg of body weight. Single doses are best kept under 200 mg.

Daily caffeine guidance used by this calculator
Who	Daily limit	Source
Healthy adult	400 mg	FDA, EFSA
Pregnancy	200 mg	ACOG
Teen (12–17)	100 mg	AAP-style guidance
Per kg (adult)	3–6 mg/kg	Sports / sensitivity check

Example: a 70 kg adult at 6 mg/kg could theoretically “afford” 420 mg, but the FDA cap still wins — use 400 mg. The same person at 3 mg/kg should stay near 210 mg if they get jittery. Children under 12 are generally advised to avoid caffeine.

How Long Before Bed Should I Stop Drinking Caffeine?

Caffeine blocks adenosine. Drake et al. (2013) found a dose 6 hours before bed still cut total sleep. EEG work often treats ~50 mg as the point where sleep architecture starts to change. Some sleep researchers (including Matthew Walker’s public guidance) prefer close to 0 mg at lights-out. This tool reports both: time to drop under 50 mg, and time to drop under 20 mg.

Latest drink time to be under 50 mg at 10 pm (normal 5 h half-life)
Dose	Hours needed	Cutoff for 10 pm bed
95 mg (8 oz coffee)	4.6 h	~5:20 pm
140 mg (12 oz)	7.4 h	~2:35 pm
200 mg (strong / cold brew)	10.0 h	~12:00 pm
300 mg (pre-workout)	12.9 h	~9:05 am

Hours = half-life × log₂(dose ÷ 50). Slow metabolizers need several extra hours. Tolerance can hide how you feel — it does not fully protect deep sleep.

Does Caffeine from Several Drinks Stack?

Yes. Each dose adds to whatever is still circulating:

Caffeine(t) = Σ dose_i × 0.5^{((t − t_i) / half-life)}

Two 95 mg coffees at 7 am and 10 am is not “95 mg at 10 am.” At 10 am you still have ~64 mg from the first cup (5 h half-life), so the new peak is about 159 mg. Fatcalc-style stacking is why logging every drink matters more than a single-cup timer.

Can You Overdose on Caffeine?

You can get very sick. Death from coffee itself is rare. Typical warning signs above a few hundred milligrams: jitters, anxiety, fast heartbeat, insomnia, stomach upset, headache. Severe toxicity is more often caffeine powder, pills, or several energy drinks in a short window, not three drip coffees over a morning.

Stay under your profile cap above. A single sitting over 200 mg is already aggressive for many people. If you have heart disease, panic disorder, or take medications that slow CYP1A2, use a clinician’s limit — not this page. This is not an “overdose challenge” tool.

Does Caffeine Help Workouts?

Yes, for many people. A common sports dose is 3–6 mg per kg, 30–60 minutes before training. For 70 kg that is 210–420 mg — the top end bumps the FDA daily cap, so time it as your main dose, not an add-on to three coffees. Habitual users get less kick; a short taper can restore some effect. For training fuel around caffeine, use the macro calculator or a personalized meal plan.

What Happens If I Quit Caffeine?

Withdrawal often starts in 12–24 hours: headache, fog, irritability. It usually peaks at 1–2 days and fades within a week. Cut about 25% every few days instead of stopping from 400 mg overnight.

How Much Caffeine by Weight?

A caffeine calculator by weight uses 3–6 mg per kg as a practical band. 3 mg/kg is a cautious daily cap if you get jittery. 6 mg/kg is a common pre-workout dose — and it can already hit the 400 mg adult limit:

Daily caffeine from body weight vs the 400 mg FDA cap
Weight	3 mg/kg	6 mg/kg	Use this cap
55 kg (121 lb)	165 mg	330 mg	330 mg
70 kg (154 lb)	210 mg	420 mg	400 mg
85 kg (187 lb)	255 mg	510 mg	400 mg
100 kg (220 lb)	300 mg	600 mg	400 mg

Open More options in the calculator to enter your weight. The FDA 400 mg line still wins when 6 mg/kg is higher.

What Are Signs You’ve Had Too Much?

You do not need a lab test. If several of these show up after coffee or an energy drink, you are over your personal line even if you are under 400 mg:

Jitters, shaky hands, or a racing heart
Anxiety or a crash a few hours later
Trouble falling asleep even when you “feel fine”
Headache, stomach upset, or extra bathroom trips

Front-load caffeine in the morning and skip the 3 pm top-up if bedtime caffeine in the calculator stays above 50 mg.

Where Else Is Caffeine Hiding?

Decaf still has 2–15 mg. Dark chocolate is about 23 mg per ounce. Some headache pills, pre-workouts, and “healthy” sodas add another 30–200 mg. If the bedtime number looks high and you only logged coffee, add those extras with Other (enter mg).

Key Takeaways

Average half-life is 5 hours (fast ~3 h, slow ~7 h).
Healthy adults: 400 mg/day. Pregnancy: 200 mg. Teens: ~100 mg.
An 8 oz coffee is about 95 mg; cafe sizes and cold brew run much higher.
Stop caffeine 8–10 hours before bed if you want under ~50 mg at lights-out.
Later drinks stack on what is still in your blood — log every cup.

Time caffeine around food, not only the clock. Get a meal plan that matches your sleep and training, check a bedtime with the sleep cycle calculator, or check water with the water intake calculator.