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Free caffeine half-life calculator: estimate how long caffeine lasts in your body using adjustable elimination half-life (often 3–7 hours). See remaining milligrams, percent left, and optional time to a target mg for sleep or sensitivity planning.
Last updated: May 24, 2026
Daily caffeine limit from weight? Coffee calculator
Example: ~95 mg per 8 oz brewed coffee (varies); sum multiple sources.
Often ~3–7 h in healthy adults; pregnancy and some medicines can lengthen it.
If set below your dose, we show estimated hours from intake to reach that level (same model).
Estimate
About 66 mg caffeine remains after 3 h (66% of the original 100 mg), using a 5 h half-life. From intake, reaching ~25 mg would take about 10 h in this model (first-order decay).
Remaining now
66 mg
% of original
66%
Half-lives elapsed
0.6
Time to ~target from intake
About 10 h to reach ~25 mg (same half-life assumption).
Not medical advice. Anxiety, heart rhythm issues, pregnancy, and drug interactions change risk—ask a clinician about safe caffeine use.
66 mg
66% of dose left
0.6
One half-life = 50% remaining
10 h
Modeled first-order clearance
100 mg · 3 h
66 mg left
80 mg espresso · 6 h
34.8 mg
200 mg · 2 h
151.6 mg
100 mg · 6 h · 3 h t½
25 mg (fast)
Remaining = dose × 0.5^(hours_since ÷ half_life)
Hours to target = t½ × ln(dose ÷ target) ÷ ln(2)
Default: 100 × 0.5^(3/5) = 66 mg. To reach 25 mg from 100 mg at 5 h t½: ~10 h total from intake.
| Hours since intake | Remaining (mg) | % of dose |
|---|---|---|
| 0 h | 100 mg | 100% |
| 1 h | 87.1 mg | 87.1% |
| 2 h | 75.8 mg | 75.8% |
| 3 h | 66 mg | 66% |
| 4 h | 57.4 mg | 57.4% |
| 5 h | 50 mg | 50% |
| 6 h | 43.5 mg | 43.5% |
| 8 h | 33 mg | 33% |
| 10 h | 25 mg | 25% |
| 12 h | 18.9 mg | 18.9% |
| Half-lives elapsed | Approx. left | At 5 h t½ (clock time) |
|---|---|---|
| 1 | 50% | 5 h — One half-life elapsed |
| 2 | 25% | 10 h — Two half-lives — common “quarter left” checkpoint |
| 3 | 12.5% | 15 h — Three half-lives — small residual for many people |
| 4 | 6.25% | 20 h — Four half-lives — trace for planning only |
| Source | Typical mg | Note |
|---|---|---|
| 8 oz brewed coffee | 80–100 | Often ~95 mg in references; dark roast ≠ always more caffeine |
| Single espresso (1 oz) | 60–80 | Less volume but concentrated per ounce |
| 8 oz black tea | 40–70 | Steep time and leaf type shift mg |
| 12 oz cola | 30–40 | Check label — diet versions may differ |
| 8 oz energy drink | 80–160 | Some cans list 200+ mg total |
| Caffeine tablet | 100–200 | Enter exact label dose |
| Factor | Typical effect | Detail |
|---|---|---|
| Typical healthy adult | 3–7 h | Default teaching range; calculator default 5 h |
| CYP1A2 slow metabolizers | Longer (often >7 h) | Genetics; caffeine lingers |
| CYP1A2 fast metabolizers | Shorter (~3 h) | May tolerate late coffee better |
| Pregnancy (3rd trimester) | Up to ~15 h cited | Use clinician limits, not this tool alone |
| Oral contraceptives | Often prolonged | Can roughly double half-life in some studies |
| Smoking | Often shortened | Induces metabolism — levels fall faster |
| Liver disease | Often prolonged | Clearance impaired — medical supervision |
| Audience | Often cited | Note |
|---|---|---|
| Many healthy adults (FDA context) | ~400 mg/day | Not a personal prescription — sensitivity varies |
| Pregnancy (ACOG often cited) | <200 mg/day | Follow your obstetric team |
| Adolescents | Lower caution | Pediatric guidance differs by age |
See FDA — Caffeine and your body and StatPearls — Caffeine (NIH) for authoritative background.
Share it with anyone timing caffeine around sleep, work, or travel.
Suggested hashtags: #Caffeine #Sleep #Coffee #Health #Calculator