Half-life and steady-state calculator
How long a compound takes to clear, how much it builds up when you dose it repeatedly, and how far the level swings between doses — for 50 PK-modelled compounds. Free, no account.
A half-life is the time for a level to fall by half, and on its own it answers less than people expect. What it actually determines are three things: how long one dose stays meaningfully present, how much repeated doses accumulate, and how long until that accumulation stops changing — steady state. All three fall out of the same number and the interval between doses.
Worked through with testosterone cypionate, whose modelled half-life is 6 days: injected twice weekly, the interval is 84 hours, so each dose lands with roughly 67% of the previous one still there. Levels therefore stack to about 3.01× what a single dose reaches, plateau after about 30 days, and then oscillate between a trough and a peak only about 1.2 times higher. Move the same weekly total to a single injection and the accumulation falls while the swing roughly doubles.
How the math works
one dose: level(t) ∝ e^(−ke·t) − e^(−ka·t) (one-compartment, ka matched to time-to-peak)
repeated dosing: add one curve per dose, offset by the interval
accumulation ratio = 1 ÷ (1 − 2^(−interval ÷ half-life))
time to steady state ≈ 5 × half-life (~97% of the way there)
fraction left after one interval = 2^(−interval ÷ half-life)
The curve is the app's own pkCurve function — a one-compartment model
with absorption and elimination, where the absorption rate is fitted so the peak lands at
the compound's published time-to-peak. It is normalised so a single dose peaks at 1, which
is why the vertical axis reads in multiples rather than in ng/dL: the shape and the ratios
transfer between people, the absolute concentrations do not.
"Five half-lives to steady state" is the same arithmetic read from the other end: each half-life closes half the remaining gap, so after five you are about 97% of the way there. It is also why a bloodwork panel drawn two weeks into a change measures a level still travelling.
Where these numbers come from
Every half-life and time-to-peak here is the figure the TherapyLog app carries for that compound, drawn from published pharmacokinetic literature. 14 of the 50 are marked estimated in the compound list above and everywhere else they appear: that means the human PK data is limited or absent and the figure is inferred. Most research peptides are in that category, and treating an estimated four-hour half-life as though it were a measured one is the most common way this kind of calculation goes wrong.
The app also lets you override a half-life per compound, in the Levels tab under Adjust — useful when you have reason to prefer a different published figure. This page uses the defaults.
Questions people actually ask
How long until a compound is completely out of my system?
Practically, four to five half-lives leaves a few per cent, and most people treat that as cleared. Mathematically it never reaches zero. Two cautions: "cleared from serum" is not the same as "its effects have resolved" — suppression of your own hormone axis outlasts the drug that caused it — and detection windows for testing purposes are a different question again, with different answers.
Why does the level keep rising if I am taking the same dose?
Because each dose lands on what is left of the last one. If half the previous dose is still present when the next arrives, the level climbs until the amount cleared per interval equals the amount given per interval. That plateau is steady state, and the accumulation ratio above says how much higher it sits than a single dose.
Does a longer half-life mean a stronger compound?
No. It means a smoother and more persistent one. Half-life describes how fast something leaves, not how much it does while it is there. Two compounds with the same half-life can differ enormously in potency, and a very short half-life is why some compounds are dosed daily rather than being weaker.
What is the peak-to-trough ratio for?
Choosing a cadence. It is the modelled peak divided by the modelled trough at steady state, so 1.2 is a fairly flat line and 4 is a sawtooth. Whether a flatter curve feels better is individual and this page cannot tell you — what it can tell you is which cadences produce which shape, which is the part that is actually calculable.
Per-compound pages with the curve already drawn, the provenance, the monitoring panel and the interaction rules that name it are linked from the tools hub. Related: testosterone dose calculator.
In the app this curve is drawn from the doses you actually logged, with your own dates, and projects seven days forward from your last one.
Save this dose to your logThis calculator does the arithmetic you typed and nothing else. It does not know what is actually in your vial, whether the label is accurate, or anything about you. Confirm the vial strength and the diluent volume on your own label before you draw, and take dosing decisions to a qualified provider.