Last updated: June 2026 · Reviewed June 2026 · Built by the InjectBuddy team
Simple Injection Math Examples (8 fully worked)
Every injection-math problem uses one formula: volume (mL) = dose ÷ concentration, then for a U-100 syringe units = mL × 100. Below are eight fully worked examples across Testosterone (TRT), peptide, HCG and GLP-1 vials so you can match your own scenario and check the arithmetic line by line.
Key takeaways
- Two steps every time: divide the dose by the concentration to get mL, then multiply mL by 100 to get U-100 units.
- Keep the mass unit identical on top and bottom: mg with mg/mL, or mcg with mcg/mL. Never mix mg with mcg/mL.
- The same dose draws a different volume on a different vial strength — concentration, not dose, sets the mark.
- Plug your Testosterone (TRT) numbers into the Testosterone (TRT) dose calculator to confirm the volume and unit mark.
How this is calculated
A dose is the amount of active drug your prescriber specifies — measured in mg, mcg or IU. The volume is how much liquid carries that dose, measured in mL. Concentration is the bridge: it states how much drug sits in each mL of solution. The "desired over have" formula taught in clinical dosage-calculation references is exactly this division — the dose you want over the strength you have (StatPearls, 2023).
The arithmetic is two lines. First, mL = dose ÷ concentration. Then, because a U-100 syringe is printed so that 100 units fill 1 mL, units = mL × 100. The only trap is units of measurement: 1 mg equals 1,000 mcg, so before dividing you must express the dose and the concentration in the same mass unit. Mixing mg and mcg is the single most common cause of a 1,000-fold error, which is why dosing mistakes are among the most preventable medication errors (StatPearls, 2024). See mg vs mcg vs mL vs units if that conversion is unfamiliar.
For ready-made vials such as testosterone cypionate, the concentration is printed on the label — for example 200 mg/mL (DailyMed). For powders like peptides, HCG or compounded GLP-1, you create the concentration yourself by reconstituting: concentration = vial strength ÷ bacteriostatic water added. Get that step right and every example below follows the same two lines.
Multi-scenario summary table
Eight scenarios, each worked in full below. Skim the table to find the row closest to your vial, then read the matching worked example for the arithmetic.
| Scenario | Dose | Concentration | Volume (mL) | U-100 units |
|---|---|---|---|---|
| Testosterone (TRT) weekly | 100 mg | 200 mg/mL | 0.50 mL | 50 |
| Testosterone (TRT) on 250 mg/mL | 100 mg | 250 mg/mL | 0.40 mL | 40 |
| Testosterone (TRT) split dose | 50 mg | 200 mg/mL | 0.25 mL | 25 |
| Semaglutide start | 0.25 mg | 10 mg/mL | 0.025 mL | 2.5 |
| Semaglutide 1 mg | 1 mg | 2.5 mg/mL | 0.40 mL | 40 |
| Tirzepatide | 5 mg | 20 mg/mL | 0.25 mL | 25 |
| HCG | 500 IU | 1,000 IU/mL | 0.50 mL | 50 |
| BPC-157 | 250 mcg | 2,000 mcg/mL | 0.125 mL | 12.5 |
Testosterone (TRT) worked examples
Example 1 — Testosterone (TRT) 100 mg/week on a 200 mg/mL vial
Concentration from the label is 200 mg/mL (DailyMed lists testosterone cypionate at 200 mg/mL). Dose 100 mg ÷ 200 mg/mL = 0.50 mL. On a U-100 syringe, 0.50 mL × 100 = 50 units.
Example 2 — same 100 mg dose on a 250 mg/mL vial
Switch the vial to 250 mg/mL and the dose is unchanged. 100 mg ÷ 250 mg/mL = 0.40 mL. That is 0.40 mL × 100 = 40 units. Identical dose, smaller draw — proof that concentration, not dose, sets the mark.
Example 3 — splitting 100 mg/week into two 50 mg shots
A split dose of 50 mg twice weekly on the 200 mg/mL vial: 50 mg ÷ 200 mg/mL = 0.25 mL, which is 0.25 mL × 100 = 25 units per injection. The weekly total stays 100 mg.
GLP-1 worked examples (compounded vials)
Example 4 — semaglutide 0.25 mg starting dose
Reconstitute a 10 mg vial with 1 mL bacteriostatic water → 10 mg ÷ 1 mL = 10 mg/mL. The 0.25 mg start dose (the lowest Wegovy step on the DailyMed label) = 0.25 mg ÷ 10 mg/mL = 0.025 mL, which is 2.5 units. A 0.3 mL syringe makes this tiny draw readable.
Example 5 — semaglutide 1 mg on a thinner mix
Reconstitute a 5 mg vial with 2 mL water → 5 mg ÷ 2 mL = 2.5 mg/mL. A 1 mg dose = 1 mg ÷ 2.5 mg/mL = 0.40 mL = 40 units. Same drug as Example 4, very different unit mark — because the concentration is different.
Example 6 — tirzepatide 5 mg on a concentrated mix
Reconstitute a 10 mg vial with 0.5 mL water → 10 mg ÷ 0.5 mL = 20 mg/mL. A 5 mg dose = 5 mg ÷ 20 mg/mL = 0.25 mL = 25 units. Less water means a higher concentration and a smaller, harder-to-read draw.
HCG and peptide worked examples
Example 7 — HCG 500 IU twice weekly
Reconstitute a 5,000 IU vial with 5 mL bacteriostatic water → 5,000 IU ÷ 5 mL = 1,000 IU/mL. A 500 IU dose = 500 IU ÷ 1,000 IU/mL = 0.50 mL = 50 units. IU behaves exactly like mg in the formula: keep the same unit top and bottom.
Example 8 — BPC-157 250 mcg (watch the mg↔mcg step)
Reconstitute a 5 mg vial with 2.5 mL water → 5 mg ÷ 2.5 mL = 2 mg/mL. Convert to mcg so it matches the dose: 2 mg/mL = 2,000 mcg/mL. A 250 mcg dose = 250 mcg ÷ 2,000 mcg/mL = 0.125 mL = 12.5 units. BPC-157 is an investigational peptide with no established human dose — this is arithmetic only, not a dosing recommendation.
Common mistakes these examples expose
The biggest trap is copying someone else's unit mark. Example 4 draws 2.5 units for 0.25 mg, but on the Example 5 mix the same 0.25 mg would need a different volume — the unit number only means a dose once you fix the concentration. Always re-derive your own mark.
The second trap is the mg-to-mcg jump in Example 8. Dividing 250 mcg by 2 mg/mL without converting gives 125 mL — an impossible draw, and a clear signal that a unit was mismatched. When an answer looks absurd, check that the mass units agree before anything else.
Finally, dead space and rounding matter most when the draw is tiny (Examples 4 and 8). A new sterile needle and syringe for every injection, and disinfecting the vial septum before each entry, keeps correct math from becoming a contamination problem (CDC).
So, how do you calculate an injection dose?
Every injection-dose calculation is the same two steps: volume (mL) = dose ÷ concentration, then units = mL × 100 for a U-100 syringe. Match your units (mg with mg/mL, mcg with mcg/mL) and the rest is arithmetic. Work the examples above against your own vial, or let the dosage calculator do the division for you.
FAQs
What is the basic injection math formula?
How do I convert mL to syringe units?
Why does the same dose draw a different volume?
Is this page medical advice?
What if my vial is labelled in mcg/mL, not mg/mL?
Sources
- DailyMed (FDA). Testosterone Cypionate Injection label — 200 mg/mL. DailyMed label.
- DailyMed (FDA). Wegovy (semaglutide) injection label — dose strengths 0.25–2.4 mg. DailyMed label.
- CDC. Safe Injection Practices to Prevent Transmission of Infections to Patients. CDC injection safety guidance.
- Toney-Butler TJ, Wilcox L. Dose Calculation (Desired Over Have Formula Method). StatPearls, 2023. NCBI Bookshelf NBK493162.
- Tariq RA, et al. Medication Dispensing Errors and Prevention. StatPearls, 2024. NCBI Bookshelf NBK519065.
This guide is for general educational purposes only and does not constitute medical advice. The worked examples are arithmetic illustrations, not dose recommendations. Always follow your prescriber's specific instructions and the product label.