Methodology

Caffeine Intake Calculator

The intake calculator sums the caffeine from each beverage the user adds. Each beverage has a caffeineMg value (milligrams per serving). The total is:

Total caffeine (mg) = Σ (caffeineMg × servings)

The total is compared against a safe daily limit based on the user-selected demographic:

  • Healthy adults: 400 mg/day (FDA)
  • Pregnant or breastfeeding women: 200 mg/day (ACOG)
  • Adolescents (12–18): 100 mg/day (AAP)
  • Children under 12: 0 mg (avoid — AAP)

Caffeine Half-Life Calculator

Caffeine follows first-order pharmacokinetics — a constant fraction is eliminated per unit time. The decay formula is:

Remaining (mg) = Dose (mg) × 0.5^(hours ÷ half-life)

Default half-life: 5 hours (average for healthy, non-pregnant, non-smoking adults).

Sleep-safe threshold: 50 mg. Below this level, most adults can fall asleep without caffeine-related disruption.

Adjustable half-life: Users can override the default based on their individual factors:

  • Smokers: ~3 hours (CYP1A2 induced by smoking)
  • Pregnancy (3rd trimester): ~11 hours
  • Oral contraceptives: ~7–10 hours
  • Liver disease: up to 50+ hours

Limitations: The model assumes a single dose at a single point in time. Multiple doses throughout the day are not summed (the calculator does not model accumulation). Individual variation in CYP1A2 genetics is not accounted for.

Safe Limit by Body Weight

Sports nutrition research uses a body-weight-based dosing approach:

Recommended ceiling (mg) = min(bodyWeight (kg) × 6, 400)

The 3 mg/kg value represents the lower ergogenic (performance-enhancing) threshold; 6 mg/kg is the upper range. The result is capped at 400 mg per the FDA's safe daily limit for healthy adults.

Limitations: The FDA's flat 400 mg limit applies regardless of body weight. The body-weight calculation is based on sports nutrition research and may not be appropriate for caffeine-sensitive individuals or those with health conditions.

Coffee Brewing Ratio Calculator

The brew ratio is calculated as:

Ratio = water (ml) ÷ coffee (g)

Classification:

  • > 22: Weak / under-extracted
  • 17–19: Balanced (SCA golden ratio: 1:18)
  • 15–17: Strong
  • < 15: Very strong / espresso-style

Caffeine estimation: Uses approximate caffeine per gram of grounds — Arabica: 12 mg/g, Robusta: 22 mg/g, Blend: 17 mg/g. Assumes ~90% extraction efficiency for drip brew.

Estimated caffeine (mg) = coffee (g) × caffeinePerGram × 0.9

Caffeine Withdrawal Timeline

The withdrawal timeline is based on DSM-5 criteria for caffeine withdrawal. Stages:

  • 0–12 hours: No symptoms (caffeine still active)
  • 12–24 hours: Mild symptoms begin
  • 24–48 hours: Peak withdrawal
  • 2–4 days: Symptoms easing
  • 5–9 days: Resolution

Limitations: Individual symptoms may be milder or more severe. The timeline reflects typical patterns, not guarantees.

Cost Calculator

Simple multiplication:

Daily cost = drinksPerDay × pricePerDrink
Yearly cost = dailyCost × 365

Assumptions across all calculators

  • Caffeine values are typical averages — actual content varies by 20%+
  • Half-life is an average — individual genetics cause 3–11 hour range
  • Safe limits are population-level guidance, not personalized medical advice
  • The calculators do not account for caffeine tolerance, medications, or health conditions

Source data

See our data sources page for a complete list of where each caffeine value and safety limit comes from.