Log the coffee, tea, soda or energy drinks you had today and the time you finished each one. Using caffeine’s published half-life in adults, the calculator estimates a low and a high figure for what may still be circulating when you turn in.
Caffeine calculators
Caffeine half-life calculator
- FDA typical 113 to 247 mg
- FDA typical 71 mg
- Logged today
- 251 mg
- Estimated left at bedtime
- 16 to 73 mg
- 7-hour half-life
- 3-hour half-life
- the range between them
A general estimate built on the adult half-life range of 3 to 7 hours, not your personal half-life.[26] Preset amounts sit in the middle of the FDA’s typical range; use the label when your drink has one.[27]
Last-cup calculator
- Six hours before bed
- 4:30 PM
In a small trial, a large dose of caffeine still disrupted sleep when taken six hours before bed.[13] It isn’t a rule for everyone. Note your own last cup and how the night went.
Why a range, not one number
Caffeine clears at different speeds in different bodies. Rather than guess yours, the tool runs the day twice: once at the quick end of the adult range and once at the slow end.
Your real figure could sit outside both. If you want to learn your own pattern, note your last cup and how the night went for a week.
how it works
Halving, hour by hour.
Each drink starts a clock
A drink enters the sum at the time you finished it, at the milligrams you entered or the preset filled in.
Half goes every half-life
After one half-life half the dose is left, after two a quarter, after three an eighth. Doses simply add up.
Two speeds, one window
The sum is worked out with a 3-hour and a 7-hour half-life, and both are read at your bedtime.
left at bedtime = dose × 0.5 ^ (hours since the drink ÷ half-life), added across every drink
It treats each drink as fully absorbed the moment you log it. Real levels climb for a while first, peaking in the blood about 2 hours after a drink.[26]
what the research says
Caffeine, by the published numbers.
- 3 to 7 hours is the adult half-life given in a 2017 review. Caffeine peaks in saliva about 45 minutes after it is swallowed and in the blood after about 2 hours.[26]
- Oral contraceptives tend to double the half-life, pregnancy lengthens it, and smoking doubles the rate at which caffeine is cleared.[26]
- For most adults, the FDA has cited 400 mg a day as an amount not generally associated with negative effects, while noting wide variation in sensitivity and in how fast people eliminate it.[27]
- In a small trial of 12 healthy sleepers, 400 mg of caffeine disrupted sleep whether it was taken at bedtime, three hours before or six hours before.[13]
- Caffeine raised daytime cortisol in a study of 96 young adults. In regular users the response was smaller, though still present.[14]
Population figures and small studies. They can tell you how caffeine behaves on average, not how it behaves in you tonight.
Caffeine questions, answered with sources
What is the half-life of caffeine?
In adults it is roughly 3 to 7 hours, according to a 2017 review of caffeine safety. Oral contraceptives tend to double it, it runs longer during pregnancy, and smoking roughly doubles how fast caffeine is cleared.[26]
How long does caffeine stay in your system?
Each half-life removes about half of what is left. Take 200 mg at noon: by 10 PM, ten hours later, somewhere around 20 mg would remain with a 3-hour half-life and around 74 mg with a 7-hour one.[26] Small amounts linger well past the cup.
How much caffeine is too much in a day?
For most adults, the FDA has cited 400 mg a day as an amount not generally associated with negative effects. People vary widely in sensitivity, and pregnancy, some conditions and some medications change the picture, so that figure is not a personal limit.[27]
How late can I have coffee before bed?
There is no single cut-off. In a small trial, 400 mg of caffeine disrupted sleep even when it was taken six hours before bed.[13] The last-cup calculator on this page shows where that six-hour mark falls for you.
Why does caffeine hit some people harder than others?
How much caffeine is in a cup of coffee?
The FDA puts a typical 12 fl oz brewed coffee at 113 to 247 mg. A 12 fl oz black tea is about 71 mg, green tea about 37 mg, and decaf coffee usually 2 to 15 mg per 8 fl oz.[27]