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Free caffeine half-life tool: model elimination with an adjustable half-life, see estimated remaining milligrams after time, and optionally time to a target amount. Pair with our nap duration calculator for rest planning.
Last updated: April 18, 2026
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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.
Often quoted
~3–7 hours
Not fixed—enzyme genetics and context shift it.
C = C₀ × 0.5^(t / t½)
First-order elimination sketch; educational use.
Labels vary
Check each product
Brew strength and bean change caffeine per cup.
Planning aid
Not sleep guarantee
Stress, screens, and disorders dominate sleep beyond caffeine math.
Ask your clinician
Limits differ
Some drugs alter caffeine clearance; do not rely on generic defaults.
Caffeine line
Sum labeled mg
This tool does not model other ingredients—only caffeine mg you enter.
Default: 100 mg dose, 5 h half-life, 3 h elapsed → about 66 mg remaining (66% of original) in this model.
We apply a standard one-compartment exponential decay with a half-life you can edit. That mirrors how many textbooks introduce drug elimination, while acknowledging that real pharmacokinetics include absorption phases and person-to-person scatter. For more fitness & lifestyle tools, see sleep, hydration, and nutrition calculators on the same hub.
Remaining = dose × 0.5^(hours_since / half_life_hours)
Hours_to_target = half_life × ln(dose / target) / ln(2) — when 0 < target < dose
Logarithms use natural log equivalently; the calculator uses the same relationship in code.
Tracking rest windows? Try the sleep cycle calculator.
Get Custom Calculator for Your PlatformShare it with anyone timing caffeine around sleep or travel.
Suggested hashtags: #Caffeine #Sleep #Health #Calculator #Coffee