Frequently Asked Questions
What is the half-life formula?
N = N₀ · (1/2)^(t / t½). Equivalently N = N₀·e^(−λt) with decay constant λ = ln2 / t½. The tool solves for any one unknown.
What is the decay constant and mean lifetime?
λ = ln2 / t½ is the per-unit-time decay probability; the mean lifetime τ = 1/λ = t½ / ln2 ≈ 1.4427 · t½.
How many half-lives until something is "gone"?
After n half-lives, (1/2)ⁿ remains: ~50%, 25%, 12.5%… After 10 half-lives under 0.1% remains. It never reaches exactly zero mathematically.
Does this apply to medication?
The same math models drug elimination (plasma half-life), but dosing is clinical - this is educational, not medical advice.
What is the difference between physical and biological half-life?
Physical (radiological) half-life is fixed by the isotope and never changes: it is how long the radioactive decay itself takes. Biological half-life is how long the body takes to clear a substance through metabolism and excretion, regardless of any radioactivity. For a radioactive tracer inside the body, the effective half-life combines both and is always shorter than either: 1/t_eff = 1/t_physical + 1/t_biological.
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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.