Milligrams of caffeine still in your body at bedtime, from one or two drinks, hour by hour, plus what moving the last cup to 8 hours before bed would change.
How long does 100 mg of caffeine stay in your system?
Milligrams of caffeine still in your body at bedtime, from one or two drinks, hour by hour, plus what moving the last cup to 8 hours before bed would change.
With a half-life of 5 hours, a planning figure inside the 3 to 6 hours a pharmacology review gives, 100 mg of caffeine drops to 50 mg after 5 hours, 25 mg after 10 and 13 mg after 15. Had at 8:00 AM, that leaves 13 mg at an 11 PM bedtime. A fast metabolism with a 3-hour half-life leaves about 3.1 mg after the same 15 hours, a slow one at 6 hours about 18 mg. The calculator below is filled in with the 8 AM drink, an 11 PM bedtime and the 5-hour figure; change them or add a second drink and press Calculate.
Parameters
Enter data for calculations
💡 Fill in all required fields to unlock the calculate button
Caffeine half-life, counted before the next cup
Is a 4 p.m. coffee really the problem, or is it the morning one? Two 12-ounce brewed coffees at 8:00 AM and 4:00 PM, 113 mg each, still leave about 57 mg in the body at an 11:00 PM bedtime with a 5-hour half-life. This caffeine half-life calculator adds up what is left of every drink at any hour, shows the amount at bedtime, and runs the same day again with the last drink moved to eight hours before bed, so you see what the change actually buys.
Going by feel
- "It wore off hours ago" after the jitters fade
- The morning cup counted as gone by lunch
- A second drink judged on its own, not added to the first
- One cut-off time for everyone, smokers and pregnant women included
Counting it
- Milligrams left at bedtime, from every drink together
- An hour-by-hour table from the first cup to after bedtime
- The same day with the last drink moved to 8 hours before bed
- A half-life that fits you: typical, smoker, pregnancy or your own
Seven entries, from the first cup to bedtime
- Caffeine in the first drink - in milligrams, from the label or the FDA figures further down, for example 113 for a 12-ounce brewed coffee at the low end.
- When you had it - in half-hour steps from 5:00 AM to 11:30 PM.
- Second drink and its time - optional. Leave both empty for a single drink.
- Bedtime - from 8:00 PM to 2:00 AM.
- Half-life - typical adult (5 hours), regular smoker (2.5), pregnancy (10) or your own.
- Your own half-life - only if you chose that option, from 2 to 12 hours.
- Read the result - caffeine at bedtime, the before and after comparison, the daily total against 400 mg, and the hourly table.
How the remaining caffeine is worked out
Caffeine leaves the body at a steady proportional rate: after one half-life, half of a dose is gone, after two, three quarters. A review in the Annals of Pharmacotherapy puts the half-life at 3 to 6 hours, the Sleep Foundation at anywhere from 2 to 12, and says smoking can shorten it by up to half while pregnancy doubles it. The calculator uses 5 hours as its planning figure for a typical adult and prints that assumption under every result.
The model counts each drink in full from the moment you have it. Real blood levels climb for 30 to 120 minutes after drinking first, so the hours right after a cup are overstated a little and the amount at bedtime is about right.
Which cup is still with you at night
The hourly table splits the bedtime amount by drink, and the split is rarely what people expect. Take the two 113 mg coffees from the opening. The breakfast cup has had 15 hours, three half-lives, and is down to 14 mg. The 4 p.m. cup has had only 7 hours and still adds about 43 mg, three times as much from the same drink. Together that is the 57 mg at 11:00 PM.
Stack a third or fourth coffee and the morning ones keep fading while the afternoon ones pile up, so the time of the last sizeable drink matters far more than the number of cups before noon. The same logic runs the other way for a slow metabolism: at a 10-hour half-life, the breakfast cup alone still leaves about 40 mg at bedtime, which is why the pregnancy row in the next table barely moves when the second drink is shifted.
What moving the last drink buys you
Each row is a day run through the calculator, then run again with the last drink moved to eight hours before bed, the cut-off the Sleep Foundation recommends. Bedtime is 11:00 PM except in the tea row.
| Day | At bedtime | Last drink 8 h before | Difference |
|---|---|---|---|
| 113 mg at 8 AM and 4 PM, typical | 57 mg | 51 mg | 5.5 mg |
| The same two coffees, smoker | 18 mg | 14 mg | 3.9 mg |
| The same two coffees, pregnancy | 110 mg | 105 mg | 4.7 mg |
| 247 mg at 8 AM, 246 mg at 5 PM | 138 mg | 112 mg | 26 mg |
| 400 mg at 5 PM | 174 mg | 132 mg | 42 mg |
| Black tea 71 mg at 7 AM and 3 PM, bed 10 PM | 36 mg | 32 mg | 3.5 mg |
| Green tea 37 mg at 8 PM | 24 mg | 12 mg | 12 mg |
The pattern is clear. Moving a moderate afternoon coffee by an hour changes very little, because most of what is left at bedtime is the sum of the whole day and of the half-life. The big gains come from the large late doses: 42 mg less for a 400 mg drink moved from 5 p.m. to 3 p.m. And in pregnancy, with a 10-hour half-life, two ordinary coffees still leave about 110 mg at bedtime, whatever time the second one is had.
One drink at 3 PM, left at 11 PM, by half-life
Eight hours, the recommended gap, and three doses run through the calculator. The spread shows why one cut-off time cannot fit everyone.
| Half-life | 95 mg | 200 mg | 400 mg |
|---|---|---|---|
| 2.5 h (smoker) | 10 mg | 22 mg | 44 mg |
| 3 h | 15 mg | 31 mg | 63 mg |
| 5 h (typical) | 31 mg | 66 mg | 132 mg |
| 6 h | 38 mg | 79 mg | 159 mg |
| 10 h (pregnancy) | 55 mg | 115 mg | 230 mg |
| 12 h | 60 mg | 126 mg | 252 mg |
Caffeine per 12-ounce drink, FDA figures
The FDA lists these amounts per 12 fluid ounces. Brewed coffee and energy drinks vary more than twofold, so a label value beats a guess whenever there is one.
| Drink, 12 fl oz | Caffeine |
|---|---|
| Brewed coffee | 113 to 247 mg |
| Energy drink | 41 to 246 mg |
| Black tea | 71 mg |
| Green tea | 37 mg |
| Caffeinated soft drink | 23 to 83 mg |
For most adults the FDA cites 400 mg a day as an amount not generally linked to negative effects. Pregnant or breastfeeding women are advised to ask their health care provider, and medical experts advise against energy drinks for children and teens.
Short answers about caffeine timing
Related tools
Sleep Cycle Calculator
Bedtimes for your alarm, or alarm times for your bedtime, in full cycles and checked against the hours for your age - See the calculator
Nap Calculator
The alarm time for a power nap, a coffee nap or a 90-minute nap, checked against your bedtime - See the calculator
Coffee Ratio Calculator
Grams of coffee and milliliters of water for every brewing method - See the calculator
Coffee Savings Calculator
What a daily coffee habit costs over the years - See the calculator
Sugar Content Calculator
Grams and teaspoons of sugar in drinks and snacks against the WHO limit - See the calculator
Water Intake Calculator
Daily water needs from body weight, activity and climate - See the calculator
Sobering Time Calculator
How many hours until your blood alcohol is back to zero - See the calculator
See also
Calculator verified by the LiczGrupa.pl team
Content, formulas and results have been reviewed for accuracy and relevance by our team of specialists.
Reviewed by: Natalia Skrzek