Handling and arithmetic
Measurement basics.
Mass, volume, concentration and device markings.
The calculator translates supplied mass and volume into concentration and syringe markings. It cannot choose a treatment, verify a vial, establish sterility or validate a preparation.
Why this matters. In July 2024 the FDA warned that people measuring compounded semaglutide from vials had taken 5 to 20 times the dose they meant to, for example 50 units instead of 5. Most mixed up milliliters, units and milligrams. Some were hospitalized.
The math.
1 mg = 1,000 mcg
Milligrams and micrograms differ a thousandfold. Doses are often written in mcg, vials in mg.
100 units = 1 mL
On a U-100 insulin syringe, each unit is one hundredth of a milliliter (0.01 mL).
Strength = mass ÷ final volume
5 mg in a final solution volume of 2 mL gives 2.5 mg/mL. The calculator assumes entered liquid volume equals final solution volume.
Worked example
A 5 mg vial mixed with 2 mL of water holds 2.5 mg per mL, which is 25 mcg per unit. A 250 mcg dose is 250 ÷ 25 = 10 units (0.1 mL).
Syringe calculator. Turn a dose into units on the syringe.
Arithmetic only. Inputs do not establish an appropriate dose, accurate product amount or a safe preparation. Initial numbers are a worked example.
Calculated markings reflect your inputs and selected device. They are not treatment or preparation validation.
Draw to 10 units
0.10 mL
2.5 mg per mL · 25 mcg per unitAbout 20 doses in the vial
250 mcg ÷ 25 mcg per unit = 10 units
Fill in every box to see the result.
This does arithmetic only. It doesn’t tell you what dose to take.
Historical handling notes and sources
These collected notes are not a universal validated preparation method. Exact product instructions, route, compatibility and sterility requirements differ. A clear-looking solution does not establish sterility. Original notes are preserved for review.
What you need.
The vial of powder
Check the amount on its label, for example 5 mg.
The liquid the label calls for
Bacteriostatic water or sterile water.
Alcohol swabs
A new U-100 insulin syringe for each dose
A sharps container
Bacteriostatic or sterile water.
Bacteriostatic water
Sterile water with 0.9% or 1.1% benzyl alcohol, a preservative that stops bacteria growing once the stopper has been pierced. Made for vials you’ll draw from more than once. Not for newborns: benzyl alcohol can harm them.
Sterile water
Sterile water with nothing added. It’s single-use: use it once and throw the rest away. Some medicines must be mixed with it, so follow the label.
Use only the liquid the label names. Tap, bottled and distilled water aren’t sterile, and even sterile water sold for irrigation is marked “not for injection”.
Mixing, step by step.
- Wash your hands and work on a clean surface.
- Check the vial: the powder intact, the seal unbroken, the date current.
- Wipe both rubber stoppers with an alcohol swab and let them dry.
- Draw up the amount of water you’ve planned.
- Push the needle through the stopper and let the water run slowly down the inside wall of the vial, not straight onto the powder.
- Swirl or roll the vial gently until the powder dissolves. Don’t shake it.
- Look at it: it should be clear, with no particles.
- Label the vial with the date and its strength, like “2.5 mg/mL”.
Mistakes that cause overdoses.
Mixing up mg and mcg. The dose is 1,000 times too big or too small.
Reading mL as units, or units as mL. 0.1 mL is 10 units; reading one as the other is a 100-fold error.
Using the wrong syringe. U-100 insulin syringes count 100 units to the mL. Tuberculin syringes are marked in mL only, and U-40 syringes count 40 units to the mL.
Changing the amount of water, or the vial size, without redoing the math. The same dose becomes a different number of units.
Reusing a needle or syringe. They are sterile only once.
Storing it.
Before mixing
Store the powder as its label says.
After mixing
It depends on the product, so follow its label. Serostim mixed with bacteriostatic water keeps in the fridge (2–8 °C, 36–46 °F) for up to 14 days. Egrifta WR stays at room temperature and is thrown away 7 days after mixing. Egrifta SV is used straight away.
Once mixed, none of them may be frozen.
A research vial comes with no tested storage limits: how long it stays good after mixing is unknown.
An opened vial of bacteriostatic water is thrown away 28 days after first use, unless its maker gives another date. That’s the CDC’s rule for multi-dose vials.
Sterile water is one use only. Throw away what’s left.
When not to use a vial.
It’s cloudy, has particles or has changed colour.
It was frozen, or kept warm, when its label says not to.
The seal or stopper is damaged, or the vial is cracked.
It’s past its date, or past the time its label allows after mixing.
You’re not sure what’s in it, or how strong it is.
Needles and disposal.
Use a new needle and syringe every time. Never share them, even with family.
Put used needles straight into an FDA-cleared sharps container. If you don’t have one, the FDA says a heavy-duty plastic household container can stand in, if it has a tight-fitting, puncture-resistant lid, stays upright and stable, doesn’t leak, and is labelled as hazardous waste. Replace it when it’s about three-quarters full.
Check your local rules for getting rid of a full container.
This guide explains handling and arithmetic. It isn’t medical advice, and it doesn’t recommend a dose: that comes from a prescriber.
Read this firstSources.
- FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products (opens in a new tab)
- Giving an insulin injection (opens in a new tab)
- Bacteriostatic Water for Injection label (opens in a new tab)
- Sterile Water for Injection label (opens in a new tab)
- Preventing Unsafe Injection Practices (opens in a new tab)
- How Do I Safely Use a Multi-Dose Vial? (opens in a new tab)
- Sharps Disposal Containers (opens in a new tab)
- Best Way to Get Rid of Used Needles and Other Sharps (opens in a new tab)
- Egrifta SV label (opens in a new tab)
- Egrifta WR label (opens in a new tab)
- Serostim label (opens in a new tab)
- Insulin syringes (opens in a new tab)
- Vetsulin label (opens in a new tab)