Caffeine half-life calculator

How much caffeine is still in your system, and when it drops low enough for sleep.

Caffeine leaves your body on a predictable curve. It has an average half-life of about 5 hours, so roughly half is gone every 5 hours. Enter what you drank and when, and this free calculator estimates how much is still active now and how it fades through the day. These are population-average estimates for reference only, not medical advice.

How the calculation works

The math is one half for every half-life:

• Caffeine usually peaks 30 to 60 minutes after you drink it.
• After about 5 hours, roughly half remains.
• After about 10 hours, roughly a quarter remains.
• After about 15 hours, roughly an eighth remains.

So a 200 mg cold brew at noon is about 100 mg at 5 pm, about 50 mg at 10 pm, and about 25 mg at 3 am. That lingering tail is why an afternoon coffee can still affect sleep.

Caffeine in common drinks

Amounts vary a lot by brand, bean, and brew. Treat them as starting points and adjust to what you actually drink.

What changes how fast you clear caffeine

The 5-hour half-life is an average. It can be much longer if you are pregnant or nursing (often several times longer), have liver or kidney conditions, or take certain medications (some antibiotics, antidepressants, and oral contraceptives). It can be shorter if you smoke. Genetics also play a big role. If any of this applies to you, do not rely on these numbers, and talk to a doctor or pharmacist.

Using this for sleep

If you want to protect your sleep, work backward from your bedtime. Many people aim to have only a small amount of caffeine left when they go to bed. Our guide on when to stop drinking coffee walks through how to set a personal cutoff time.

Keep reading