Introduction
Every drug clears from the body at a characteristic exponential rate described by its half-life — the time for its concentration to fall by 50%. Knowing the remaining amount helps time re-dosing or predict washout.
How to use the Half-Life Time of Medicine Calculator
- Enter the initial dose or concentration in mg.
- Enter the drug's known half-life in hours.
- Enter how many hours have elapsed since the dose.
- Click Calculate to see the remaining amount and its decay curve.
Formula and Calculation Method
Remaining amount = Initial dose × 0.5(Time elapsed ÷ Half-life)
Visual breakdown
REFERENCE
≈4–5 half-lives ≈ 94–97% eliminated (clinically negligible)
Why use this calculator
Half-life decay is exponential, not linear, so mental estimates are frequently wrong by a wide margin. This calculator computes the precise remaining amount and plots the full decay curve out to four half-lives.
FAQs
How many half-lives until a drug is essentially gone?
By convention, about 4–5 half-lives (94–97% eliminated) is considered clinically negligible.
Does this account for changing elimination rate?
No, it assumes constant first-order elimination, which holds for most drugs at therapeutic concentrations.
Can I use this for radioactive decay too?
Yes, the same exponential formula applies to any first-order decay process.