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Testosterone Enanthate Half-Life Calculator

Test E half life: testosterone enanthate's terminal half-life is about 6.9 days. See how injection frequency changes the peak-to-trough stability of your levels.

Active levels

Testosterone Enanthate

Half-life 6.9 days·tMax 96h

SourceModeled equal to cypionate (~6.9 days)
mg
0.000076.4152.8229.1305.5mg-equivStartW1W2W3W4W5W6W7W8
Estimated active level
Dose
Steady-state band
Steady-state peak
291.0mg
Trough
269.1mg
Trough as % of peak
92%
Time to steady state
~4.9wks
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Population-average pharmacokinetic model. Individual variation is significant. Not medical advice. See the methodology.

This tool is for informational purposes only, so always double-check your result with a healthcare provider. Levels shown are estimates from a population-average pharmacokinetic model, not a dosing recommendation, and individual variation is significant. We assume no liability for dosing errors.

Last updated October 2026 · Reviewed by the Regimen Team

The tool

How it works, what it tells you, what it doesn't.

Testosterone enanthate is a long-ester injectable testosterone with a terminal half-life of about 6.9 days (166 hours). Regimen models enanthate and cypionate at the same value: the two esters are near-identical in practice, and cypionate has the more rigorous measured figure, so anchoring both to it avoids implying a gap that isn't really there. Like other long esters, enanthate follows flip-flop kinetics where the depot release rate is what limits the curve. That produces a slow rise to peak (about 96 hours after injection) and a steady decline. The injection frequency you choose dramatically changes the peak-to-trough swing. Weekly produces wide swings; twice-weekly cuts that in half; daily essentially flattens the curve. This calculator uses the Bateman model with the L'Hôpital limit branch for flip-flop kinetics, matching the Regimen app's PK engine.

How to use it

How to use the testosterone enanthate half-life calculator

  1. 01

    Set your dose

    Common TRT doses: 100-200 mg per week split across one to three injections.

  2. 02

    Choose the schedule

    Weekly, twice weekly (every 3.5 days), or every 3 days. The schedule changes the curve shape more than the total weekly dose does.

  3. 03

    Set the simulation window

    8-12 weeks shows the climb to steady state plus stable oscillation.

  4. 04

    Compare schedules

    Run the same dose across weekly vs twice-weekly to see how stability improves.

Worked example: 200 mg/week, weekly vs twice weekly

Testosterone enanthate is an oil-based intramuscular or subcutaneous testosterone ester. The apparent serum half-life after a shot is about 6.9 days, governed by release from the oil depot rather than clearance of free testosterone (flip-flop kinetics). That release-limited rate is short enough that injection frequency still has a meaningful effect on the peak-to-trough variance you experience week to week.

Take a 200 mg per week protocol. Injected as a single weekly dose (200 mg on Monday), the curve climbs into a peak around day 2 to 3, then declines steadily across the following 4 to 5 days. The trough just before the next injection sits well below the peak, and the calculator above shows the exact ratio for your dose. That swing is what some users experience as a “low ebb” in the second half of the week.

Split the same 200 mg into two 100 mg injections (Monday and Thursday), and the curve compresses meaningfully. Peaks are lower, troughs are higher, and the trough-to-peak ratio rises substantially. The calculator plots both patterns side by side. Lower peaks mean less aromatization to estradiol from supraphysiological spikes. Higher troughs mean less symptomatic variance. This is why most modern TRT protocols default to twice-weekly injection at minimum, and some go to every-other-day or daily microdose protocols for the flattest possible levels.

Test E vs test C: are they actually different?

Practically, no. Testosterone enanthate and cypionate are near-identical esters, and both carry an apparent serum half-life of about 6.9 days, so the curve you see after a shot behaves the same way. You will sometimes see enanthate quoted around 4.5 days and cypionate around 8 days, but those older textbook figures imply a gap that does not hold up: the two are dosed interchangeably and clear the blood on essentially the same timeline. We anchor both to the rigorous measured pharmacokinetic data, which lands at about 6.9 days. So the same twice-weekly-vs-weekly math applies to either ester.

For dose math, open the TRT injection calculator. For a deeper dive into daily microdosing math, read the daily TRT insulin syringe dose guide.

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Questions

Frequently Asked Questions

What is the test enanthate half life?
About 6.9 days (166 hours). The Regimen app and this calculator use 166 hours as the modeling value, the same value used for cypionate.

Where does the 166-hour value come from?
It comes from testosterone cypionate, which has the more rigorous measurement. A population PK study (PMC5915615) puts cypionate's terminal half-life at about 6.9 days once natural testosterone production is suppressed, the situation during ongoing TRT. Enanthate and cypionate are near-identical esters dosed interchangeably, so Regimen models both at 6.9 days rather than implying a gap that the practical evidence doesn't support. There's no single FDA label for enanthate's half-life because the original Delatestryl label predates modern PK reporting.

Why is this different from the 'eight days' or 'four days' I see quoted elsewhere?
The 8-day figure is folklore from older TRT references, not a cited study. The roughly 4-day figure comes from a study of testosterone-naive men who still had active natural production, which pulls the apparent half-life down. The same model puts it at 6.9 days once natural production is suppressed, which is what applies during ongoing TRT. See our PK methodology page for the full sourcing.

Should I inject testosterone enanthate weekly or twice weekly?
Twice weekly (or more frequent) produces meaningfully more stable levels. Splitting the same weekly total across two injections raises the trough and lowers the peak, so the week-to-week swing narrows. The calculator above plots both patterns and shows the trough as a percentage of peak for each. Many clinicians now recommend at least twice-weekly to minimize estrogen spikes and mood swings.

How does testosterone enanthate compare to cypionate?
They behave near-identically. Regimen models both at the same 6.9-day half-life. People use them interchangeably and choose based on what their pharmacy stocks.

When does testosterone enanthate reach steady state?
Roughly five weeks at a fixed dose and schedule. The standard 4 to 5 half-lives rule applies, and at about 6.9 days per half-life that works out to around five weeks.

When should I draw blood for a testosterone level?
For the most meaningful steady-state reading, draw at trough: the morning before your next scheduled injection, at the bottom of your curve. Drawing at peak (24 to 48 hours post-injection on a weekly protocol) gives you an inflated number that doesn't represent your average.

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