Drug Half-Life Calculator

Calculate drug half-life, elimination rate, remaining concentration, and decay time for any medication.

Determine how long a drug stays in your system, when to take your next dose, and how much remains after a given time. Enter any three values to solve for the fourth: initial dose, remaining amount, time, or half-life.

Examples

Click on any example to load it into the calculator.

Standard Oral Dose

standard

A 500 mg drug with a 6-hour half-life. How much remains after 12 hours?

Initial Amount (C₀): 500 mg

Remaining Amount (C): mg

Elapsed Time (t): 12 hours

Half-Life (t₁/₂): 6 hours

Elimination Time

elimination

How long until a 200 mg drug (8-hour half-life) drops to 25 mg?

Initial Amount (C₀): 200 mg

Remaining Amount (C): 25 mg

Elapsed Time (t): hours

Half-Life (t₁/₂): 8 hours

Calculate Half-Life

halfLife

A drug drops from 1000 IU to 125 IU in 24 hours. What is its half-life?

Initial Amount (C₀): 1000 IU

Remaining Amount (C): 125 IU

Elapsed Time (t): 24 hours

Half-Life (t₁/₂): hours

Different Units

unitConversion

A 2 g drug (half-life 3 days) drops to 0.25 g after how many days?

Initial Amount (C₀): 2 g

Remaining Amount (C): 0.25 g

Elapsed Time (t): days

Half-Life (t₁/₂): 3 days

Other Titles
Understanding Drug Half-Life Calculator: A Comprehensive Guide
Master pharmacokinetics and medication timing. Learn how to calculate, interpret, and apply drug half-life for optimal therapy and safety.

What is Drug Half-Life?

  • Core Concepts and Definitions
  • Why Half-Life Matters
  • Pharmacokinetics Basics
Drug half-life (t₁/₂) is the time required for the concentration of a drug in the body to decrease by half. It is a fundamental pharmacokinetic parameter that determines how long a drug remains active, how often doses should be taken, and when a drug is considered eliminated from the system.
The Importance of Half-Life in Medicine
Understanding half-life helps patients and healthcare professionals optimize dosing schedules, avoid toxicity, and ensure therapeutic effectiveness. Drugs with short half-lives may require frequent dosing, while those with long half-lives can be dosed less often.
Pharmacokinetics: The Science Behind Drug Movement
Pharmacokinetics studies how drugs are absorbed, distributed, metabolized, and eliminated. Half-life is a key metric in this process, influencing drug accumulation, steady-state concentration, and withdrawal times.

Key Concepts:

  • Half-life: Time for drug amount to halve in the body
  • Elimination rate: Speed at which drug is removed
  • Therapeutic window: Range where drug is effective but not toxic

Step-by-Step Guide to Using the Drug Half-Life Calculator

  • Input Selection
  • Calculation Methodology
  • Result Interpretation
To use the calculator, enter any three of the following: initial amount, remaining amount, elapsed time, or half-life. The calculator will solve for the missing value using pharmacokinetic formulas.
1. Choose Your Units
Select consistent units for amount (mg, g, IU, etc.) and time (hours, days, minutes). All calculations require matching units for accuracy.
2. Enter Known Values
Input the three known values. For example, to find remaining amount, enter initial amount, elapsed time, and half-life. To find half-life, enter initial and remaining amounts plus elapsed time.
3. Calculate and Interpret Results
Click 'Calculate' to see the result. The calculator displays the missing value, elimination rate constant, and the decay formula used. Use these results to plan dosing, estimate clearance, or understand drug persistence.

Practical Scenarios:

  • Find how much drug remains after 12 hours
  • Calculate time to reach a safe level
  • Determine half-life from lab results

Real-World Applications of Drug Half-Life

  • Clinical Practice
  • Patient Safety
  • Pharmacology Research
Drug half-life calculations are used in clinical practice to adjust dosing intervals, prevent accumulation, and avoid adverse effects. Pharmacists use half-life to counsel patients on missed doses and withdrawal times.
Optimizing Dosing Schedules
Doctors use half-life to determine how often a drug should be administered. For drugs with narrow therapeutic windows, precise timing is critical to avoid toxicity or subtherapeutic levels.
Ensuring Patient Safety
Understanding half-life helps prevent drug accumulation, especially in patients with liver or kidney impairment. It also guides safe discontinuation and switching between medications.
Pharmacology and Research
Researchers use half-life data to design clinical trials, study drug interactions, and develop new medications with optimal pharmacokinetic profiles.

Clinical Use Cases:

  • Adjusting dose intervals for chronic medications
  • Estimating time to drug clearance before surgery
  • Designing research protocols for new drugs

Common Misconceptions and Correct Methods

  • Half-Life Myths
  • Calculation Pitfalls
  • Best Practices
A common myth is that a drug is completely eliminated after one half-life. In reality, it takes about 4-5 half-lives for a drug to be considered effectively cleared from the body.
Avoiding Calculation Errors
Always use consistent units and provide exactly three values. Double-check that remaining amount does not exceed initial amount, and that all time values use the same unit.
Best Practices for Safe Use
Consult healthcare professionals for personalized advice. Use the calculator as a guide, not a substitute for medical judgment.

Misconceptions Explained:

  • Drug is not gone after one half-life—about 97% is gone after 5 half-lives
  • Mixing units (mg vs. g) leads to errors
  • Incorrectly entering more than three values prevents calculation

Mathematical Derivation and Examples

  • Formulas Used
  • Worked Examples
  • Advanced Calculations
The calculator uses the exponential decay formula: C = C₀ × e^(-kt), where k = ln(2)/t₁/₂. You can rearrange this formula to solve for any variable if the other three are known.
Key Formulas
  • Remaining amount: C = C₀ × e^(-kt)
  • Half-life: t₁/₂ = ln(2)/k
  • Elimination rate: k = ln(2)/t₁/₂
  • Time: t = (ln(C₀) - ln(C)) / k
Worked Example
If you start with 500 mg, half-life is 6 hours, and 12 hours pass: C = 500 × e^(-ln(2)/6 × 12) = 125 mg.
Advanced Calculations
For drugs with multiple phases or non-linear kinetics, consult a healthcare professional. This calculator assumes first-order (exponential) elimination.

Mathematical Examples:

  • Calculate remaining amount after 3 half-lives: C = C₀ × (1/2)^3 = 12.5% of original
  • Find half-life from two concentrations and time interval
  • Estimate time to reach a target concentration