Half-life calculator

Enter a half-life and how long it has been, and this works out the fraction still in you. Add a dosing interval and it also works out how much builds up when you take the next dose before the last one has gone.

What a half-life actually tells you

A half-life is the time it takes for half of what is in you to be gone. After one half-life, 50% remains; after two, 25%; after three, 12.5%. By five half-lives you are at about 3%, which is the convention for calling something effectively cleared.

The useful thing about that curve is that it is the same shape every time, so a single number tells you the whole timeline. The dangerous thing is what people read into it.

Three things this cannot tell you

Not how long the effect lasts. Effect and concentration are different curves. Plenty of medications keep working long after most of the dose has gone, and some stop working while nearly all of it is still there.

Not when it is safe to take another dose, drive, or drink. That depends on the specific medication and on you, and it is a question for the leaflet or a pharmacist. Do not read a number off this page and treat it as a green light.

Not your half-life. The value you type in is a population average. Liver and kidney function, age, other medications, pregnancy and genetics all move it, sometimes by a factor of two or more.

Where the model does not apply

This assumes first-order kinetics, which is what most substances do: a constant fraction leaves per unit time. Alcohol does not work that way — it clears at a roughly constant amount per hour, so a half-life calculation gives the wrong answer for it. Phenytoin and a handful of others switch behaviour at higher concentrations too.

Why accumulation matters more than the single dose

If you take the next dose before the last one has cleared, the amount in you climbs until intake and clearance balance. That plateau is steady state, it arrives after about five half-lives, and it is why a medication can feel different in week two than on day one.

The build-up factor the calculator shows is why: dosing at exactly the half-life doubles your steady-state level compared with a single dose, and dosing twice as often as the half-life roughly triples it. If you are tracking how a new medication feels over its first fortnight, Doz keeps the dose log that makes the pattern visible.

Common questions

How long until a drug is completely out of my system?

Effectively cleared is usually taken as five half-lives, which leaves about 3% of the dose. Mathematically it never reaches zero, which is why the convention exists. For anything where the answer matters — a test, a procedure, starting another medication — ask a pharmacist rather than relying on the arithmetic.

Does a shorter half-life mean the effect wears off sooner?

Not reliably. Concentration and effect are different curves. Some medications bind so tightly to their target that the effect outlasts the drug, and some produce active breakdown products with half-lives of their own.

Why does my medication feel different after two weeks?

If you dose more often than the half-life, the amount in you keeps climbing until intake and clearance balance out. That plateau is called steady state and it takes about five half-lives to arrive, so the level in week two can be several times what it was after the first dose.

Can I use this for alcohol?

No. Alcohol clears at a roughly constant amount per hour rather than a constant fraction, so half-life arithmetic gives a wrong and potentially dangerous answer for it.

One thing this cannot do

This is an arithmetic tool, not medical advice. It works out numbers from what you type in — it does not know your medication, your prescription, or what you should do if something goes wrong. Anything about changing a dose, spacing doses differently, or handling a missed dose is a question for your doctor or pharmacist.