Steroid Half-Life Calculator

What is a half-life?

The half-life of a compound is the time it takes for its plasma concentration to drop by half. It is the foundational parameter of pharmacokinetics and it drives almost every practical decision around a cycle: injection frequency, when to draw blood work, when to start PCT, and how to plan ester overlap between compounds.

The formula this calculator applies is: residual = initial dose × (1/2)^(days / half-life), where days is the time elapsed and half-life is expressed in days. After roughly 5 half-lives, less than 3 % of the original dose is left in circulation — for most practical purposes, that is the moment the compound stops actively driving suppression and side effects.

Half-lives of common compounds

Approximate values, in days unless noted:

• Testosterone Propionate — about 0.8 day (≈ 20 h)
• Testosterone Enanthate (Test E) — about 4.5 days
• Testosterone Cypionate (Test Cyp) — about 5 days
• Testosterone Undecanoate (Nebido) — about 21 days
• Sustanon 250 — composite, effective half-life 15 to 18 days
• Trenbolone Acetate (Tren A) — about 1 day
• Trenbolone Enanthate (Tren E) — about 7 days
• Nandrolone Decanoate (Deca) — about 7 days
• Boldenone Undecylenate (EQ) — about 14 days
• Dianabol (Dbol) — 3 to 6 hours
• Anavar (Var) — about 9 hours
• Winstrol (Winny) — about 9 hours oral, about 24 hours depot

When to start PCT

The community rule of thumb is to wait 4 to 5 half-lives after the last injection of the longest ester in the stack before starting PCT. At that point the residual concentration is well below 3 % of the initial dose and the HPTA axis can actually begin to recover under the SERM. Starting PCT too early — while a long ester is still releasing meaningful amounts of testosterone — wastes the protocol and delays real recovery.

Worked example: a cycle ending with 500 mg of Test E (half-life ≈ 4.5 days) means significant plasma activity for roughly 18 to 22 days post-injection. A typical Nolvadex PCT therefore starts around day 14 to 18, not day 3. The calculator lets you plug in your exact compound and last dose to find the day where you cross under your chosen threshold.

Ester choice and overlap

Half-life is also why short and long esters are commonly stacked: a Test Prop or Dianabol kickstart fills the first 2 to 3 weeks while a long ester (Test E, Test Cyp) builds up to steady state. Steady state itself takes about 5 half-lives — so Test E does not fully "kick in" until weeks 3 to 4 of a cycle. Planning these overlaps is one of the most practical uses of this tool.

Half-life vs detection time

Two different concepts: half-life is when the compound stops being pharmacologically active in your body; detection time is when its metabolites can no longer be flagged by a drug test, which is typically much longer. Nandrolone, for example, has a 7-day half-life but a detection window of 12 to 18 months on a WADA panel. If you compete in a tested federation, look at detection times, not half-lives.

For deeper discussion of ester selection and PCT timing, the MESO-Rx ester comparison threads and r/steroids wiki are the most cited community references.