Testosterone half-life & steady-state calculator. See the accumulation, not just the number.

What this model shows

Each injection is modelled with a two-compartment first-order (Bateman) curve — an absorption ramp off the oil depot followed by exponential elimination — and repeated doses are summed by superposition. Because a long ester is still releasing when the next shot lands, levels accumulate for the first several weeks before settling into a repeating steady-state rhythm, conventionally reached after roughly four to five elimination half-lives.

The maths, stated plainly

• Single dose: A(t) = D · (ka / (ka − ke)) · (e^−ke·t − e^−ka·t)
• Rate constants: ke = ln2 / t½_elimination and ka = ln2 / t½_absorption
• Absorption half-life defaults to 10% of the elimination half-life, clamped to 0.1–24 hours — so a long ester gets a multi-hour ramp rather than an instant spike.
• Multi-dose level at time t is the sum of every past dose’s contribution; steady-state peak and trough are found by summing the series at each phase across one interval.
• Where absorption is much faster than elimination (ka ≥ 20·ke) the model collapses to simple decay, which is the same simplification the app makes.

Worked example: 100 mg of cypionate a week, three ways

Holding the weekly milligrams constant and changing only the interval, with an 8-day elimination half-life:

Same milligrams, three different curves. More frequent injections flatten the line rather than raise it: the average is set by milligrams per week, the swing is set by the interval. That is a property of the model, not a recommendation — how often you inject is a decision for the clinician who prescribes it.

What the model deliberately ignores

• Bioavailability, volume of distribution and clearance — none of them are inputs.
• Individual absorption variability, injection site, depot volume and technique.
• SHBG, albumin and everything else that decides how much of a level is free rather than bound.
• Blend esters: Sustanon 250 is approximated as one composite curve rather than four separate ones.

Which is why the y-axis is relative and unlabelled. A calculator that hands you a serum concentration in ng/dL is claiming knowledge that pharmacokinetic constants alone cannot provide; the number that matters comes from a blood draw. Where in the interval that draw should sit is covered in the lab timing guide, and the full background is in the injection frequency guide.