Half-Life Calculator

Initial QuantityEnter a valid quantity.

Half-Life (time units)Enter a valid half-life greater than 0.

Elapsed Time (same units)Enter a valid elapsed time.

Remaining Quantity

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How to Use This Tool

Enter the initial quantity of the substance.
Enter its half-life and the elapsed time, using the same time units for both.
Click Calculate to see the remaining quantity.

The Half-Life Decay Formula

Remaining Quantity = Initial Quantity × (0.5)^(Elapsed Time ÷ Half-Life)

What Half-Life Means

Half-life is the time it takes for exactly half of a radioactive (or otherwise exponentially decaying) substance to decay. This is a constant property of a given substance — for example, Carbon-14 has a half-life of about 5,730 years, meaning any sample of it loses half its radioactivity every 5,730 years, regardless of the starting amount.

Common Applications

Radiocarbon dating: Estimating the age of organic archaeological samples
Nuclear medicine: Calculating appropriate dosing and timing for radioactive tracers and treatments
Pharmacology: Estimating how a drug's concentration decreases in the body over time (biological half-life)
Nuclear physics and engineering: Understanding radioactive waste decay timelines

Important Note

Make sure your half-life and elapsed time are both expressed in the same time units (both in years, both in hours, etc.) — mixing units will give an incorrect result.

Frequently Asked Questions

Does a substance ever fully decay to zero?

Mathematically, exponential decay approaches zero but never reaches it exactly — after enough half-lives (commonly cited as around 7-10), the remaining amount becomes negligibly small for practical purposes, even though it's technically never precisely zero.

Can I use this for drug half-life in the body?

Yes, the same exponential decay math applies to a drug's biological half-life (how long it takes the body to eliminate half of a substance), commonly used in pharmacology to plan dosing schedules.

This calculator is provided for general informational and estimation purposes only. Results should not be treated as professional financial, tax, legal, medical, or engineering advice. Always verify critical calculations with a qualified professional or official source before making decisions.