The arithmetic
Nearly every dosing mistake with injectable testosterone comes from one of three places - confusing millilitres with syringe units, forgetting the ester has weight, or not realising how much levels swing between shots.
The core conversion is simple. Divide your weekly dose by how many injections you do that week to get milligrams per shot, then divide by the concentration on the vial to get millilitres.
mL per shot = (mg per week ÷ shots per week) ÷ mg per mL
At 100 mg a week of 200 mg/mL cypionate split into two shots, that's 50 mg per shot, which is 0.25 mL, which is 25 units on an insulin syringe. The calculator does this and the three other things that are easy to get wrong.
Why the ester changes the number
Injectable testosterone isn't pure testosterone. It's testosterone with a fatty acid chain attached, which makes it oil-soluble and slows absorption from the injection site. That chain has mass, and the milligram figure on the vial covers the whole molecule.
| Ester | Molecular weight | Testosterone | 100 mg gives |
|---|---|---|---|
| Propionate | 344.49 | 83.7% | 83.7 mg |
| Enanthate | 400.59 | 72.0% | 72.0 mg |
| Cypionate | 412.61 | 69.9% | 69.9 mg |
| Undecanoate | 456.70 | 63.2% | 63.2 mg |
Testosterone itself has a molecular weight of 288.42, so each percentage above is just 288.42 divided by the ester's molecular weight. This is why switching esters at the same milligram dose doesn't deliver the same amount of hormone - moving from 100 mg of cypionate to 100 mg of propionate is roughly a 20% increase in actual testosterone.
Why frequency matters more than people expect
Testosterone clears on an exponential curve. Between injections your level rises to a peak, then falls to a trough right before the next shot. How far it falls depends entirely on how the gap between shots compares to the ester's half-life.
Inject at intervals much shorter than the half-life and the swing is small. Inject at intervals near or beyond the half-life and you get a sawtooth - supraphysiological for a couple of days, then dropping off before the next dose. That's the mechanism behind people reporting they feel good early in the week and flat by the end of it.
The calculator shows this as a peak-to-trough percentage for every common schedule at once, so you can see what changing frequency would actually do. Note it deliberately doesn't estimate your serum level in ng/dL. Doing that honestly needs your volume of distribution and absorption rate, which nobody knows without bloodwork, and any calculator that gives you a confident ng/dL number is guessing.
Testosterone dosage questions
Why doesn't this recommend a dose?
Because a safe dose depends on your bloodwork, symptoms, haematocrit, age, and what your prescriber is targeting - none of which a web page can see. Dose is something you enter here, not something you get out. Anything that tells you what to inject based on a form is selling something.
What's a U-100 syringe and why do people use them?
It's an insulin syringe calibrated so 100 units equals 1 mL. TRT volumes are usually small - often a quarter of a millilitre - and that's hard to measure accurately on a 3 mL syringe where it barely clears the first mark. On an insulin syringe the same dose is 25 clearly numbered units.
Does splitting my weekly dose change how much I'm taking?
No. The weekly total is identical. What changes is the shape of the curve - more frequent, smaller injections mean a lower peak and a higher trough, so your level stays in a narrower band. The calculator's swing table shows exactly how much narrower.
Why is the half-life editable?
Because the published figures genuinely disagree. Cypionate is cited between about 7 and 9 days, enanthate anywhere from 4.5 to 8, and undecanoate from about 20 to 36. Rather than pick one and present it as fact, the default is editable so you can model whichever figure your clinician uses. It only affects the swing estimate - your millilitres don't change.
How long does a vial last?
Multiply the vial size by its concentration to get total milligrams, then divide by your weekly dose. A 10 mL vial at 200 mg/mL holds 2,000 mg, which is 20 weeks at 100 mg a week. The calculator shows this, though bear in mind most manufacturers specify discarding a multi-dose vial a set period after first puncture regardless of what's left.
Does this work for gel, patches, or pellets?
No. This models intramuscular and subcutaneous oil injections only. Transdermal gels have daily application and absorption rates in a completely different range, and pellets release over months on a curve this model doesn't describe.