Last updated: June 2026
Ester Bioavailability & Half-Life Calculator
About the Ester Half-Life Calculator
The Ester Bioavailability & Half-Life Calculator is a free pharmacokinetic tool that models how an injectable steroid ester behaves in your bloodstream over time. Pick a compound and ester — testosterone propionate, acetate, enanthate, cypionate or undecanoate, nandrolone decanoate or phenylpropionate (NPP), trenbolone acetate, enanthate or hexahydrobenzylcarbonate, masteron propionate or enanthate, or boldenone undecylenate (EQ) — choose a route (IM or SubQ), set your dose in milligrams and how many days between injections, and the calculator returns your steady-state peak, trough, peak:trough ratio, a suggested frequency to keep that ratio near 1.5, and a 4-week steady-state line graph. Free, no login.
It uses a one-compartment first-order model built around each ester's half-life. The output is a set of relative serum levels, not lab readings — the point is to compare injection schedules and ester choices against each other so you can see which combination gives flatter, more stable levels before you commit.
How to use it
First, pick the ester. This is the single biggest lever — it sets the half-life that drives the whole curve. Then choose your route: IM gives a slightly sharper rise, while SubQ flattens the curve a little for the same dose.
Next, enter your dose in milligrams per injection and how often you inject — the number of days between shots. The calculator builds the steady-state curve and reads off the peak (just after a shot), the trough (just before the next) and the peak:trough ratio between them.
Finally, check the suggested frequency. If your ratio is high, the tool tells you roughly how often you would need to inject to bring it back toward 1.5 — a common stability target. Nothing here is a prescription; it is a planning layout you confirm with bloodwork and a clinician.
Worked example
Compound: testosterone propionate (half-life ~0.8 days)
Route: IM · Dose: 100 mg · Frequency: every 3.5 days
Result: a very high peak:trough ratio — big swings up and down between injections.
Because propionate clears so fast, injecting only every 3.5 days lets levels spike right after the shot and crash before the next one. Switching to every other day, or daily, smooths the curve out dramatically and pulls the ratio back toward 1.5. The same dose, injected more often, is far more stable.
Compound: testosterone enanthate (half-life ~4.5 days)
Route: IM · Dose: 100 mg · Frequency: every 3.5 days
Result: a fairly stable curve — the ratio sits close to the 1.5 target.
With enanthate's longer half-life, that same every-3.5-day schedule already produces gentle swings. This is exactly why short esters need frequent injections and long esters can be run less often for the same stability.
Ester half-lives at a glance
These are the approximate ester half-lives the model works from. They are reference figures, and individual clearance varies.
| Ester | Approx. half-life |
|---|---|
| Propionate | ~0.8 days |
| Acetate (incl. tren acetate) | ~3 days |
| NPP (nandrolone phenylpropionate) | ~2.7 days |
| Enanthate | ~4.5 days |
| Cypionate | ~8 days |
| Tren hexahydrobenzylcarbonate | ~14 days |
| Boldenone undecylenate (EQ) | ~14 days |
| Nandrolone decanoate | ~15 days |
| Testosterone undecanoate | ~21 days |
Peak, trough and injection frequency
The peak:trough ratio is the heart of this tool. It compares the highest steady-state level (just after a shot) to the lowest (just before the next). A ratio of 1.0 is a perfectly flat line; the higher it climbs, the bigger your swings. Most people aim for a ratio around 1.5 or lower, which is the stability target the calculator uses when it suggests a frequency.
The ester sets how fast that ratio climbs as you space injections out. Shorter esters need more frequent injections — propionate or acetate often want every-other-day or daily dosing to stay stable, whereas long esters like cypionate, decanoate or undecanoate can be run weekly or even less often for the same smoothness.
Route is the final lever. SubQ flattens the curve versus IM: the slower, steadier release from the fat layer lowers the peak and lifts the trough, nudging the ratio down. The effect is modest next to ester choice and injection gap, but it is one more way to smooth short esters out.
Frequently asked questions
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Important: read before you use this
This is a pharmacokinetic model only — it is NOT medical advice. The peak, trough and graph are relative serum levels, not lab readings, produced by a simplified one-compartment model. Individual metabolism, absorption and body composition vary, so your real numbers will differ. Use this tool to compare injection frequencies and ester choices, then confirm your actual levels with bloodwork and a qualified clinician who knows your full medical history. InjectBuddy is a maths and education tool, not a clinical service.