Methodology

Every number on Vial to Units comes from three formulas: concentration = amount in vial ÷ water added; volume = amount ÷ concentration; syringe units = volume × 100. All calculations assume a U-100 insulin syringe, on which 100 units is 1 mL, and results are shown to at most four significant figures.

This page gives the formulas, the assumptions, the rounding rules and a table of test values you can reproduce.

Key facts

Concentration (mg/mL) = amount in vial (mg) ÷ water added (mL).
Volume to draw (mL) = amount (mg) ÷ concentration (mg/mL).
Syringe units = volume (mL) × 100, because every calculation assumes a U-100 syringe: 100 units = 1 mL.
Results are shown to at most 4 significant figures, with trailing zeros removed: 10 ÷ 3 is shown as 3.333.
Test value: a 10 mg vial with 2 mL of water and an amount of 1 mg gives 5 mg/mL, 0.2 mL, 20 units.

The core calculation

concentration (mg/mL) = amount in vial (mg) ÷ water added (mL) volume to draw (mL) = dose (mg) ÷ concentration (mg/mL) syringe units = volume (mL) × 100 (U-100 syringe: 100 units = 1 mL)

Concentration. Divide the amount printed on the vial by the volume of water added. 10 mg ÷ 2 mL = 5 mg/mL.
Volume. Divide the amount to be drawn by the concentration. 1 mg ÷ 5 mg/mL = 0.2 mL.
Syringe units. Multiply the volume in mL by 100. 0.2 mL × 100 = 20 units on a U-100 insulin syringe.

The example uses round numbers to show the arithmetic. It is not a suggestion of an amount to use. Amounts can be in mg or in IU, as long as the vial and the amount to be drawn use the same one. An amount entered in mcg is divided by 1,000 before the calculation.

All formulas

Calculation	Formula	Used by
Concentration	amount in vial ÷ water added	Every calculator and chart
Volume to draw	amount ÷ concentration	Reconstitution calculator, compound pages
Syringe units	volume (mL) × 100	Every calculator and chart
Amount in a draw	units ÷ 100 × concentration	Units → mg mode, units to mg charts
Amounts per vial	amount in vial ÷ amount per draw	Reconstitution calculator, vial supply calculator
Water for a chosen draw	amount in vial × units ÷ (amount × 100)	Bacteriostatic water calculator
Days per vial	whole amounts per vial ÷ injections per week × 7	Vial supply calculator
Blend component	component mg ÷ water (mL) × units ÷ 100	Blend calculator
Mass	1 g = 1,000 mg; 1 mg = 1,000 mcg	Unit converter
Syringe scales	mL = units ÷ 100 (U-100); mL = units ÷ 40 (U-40)	Syringe units chart
IU and mg	mg = IU ÷ (IU per mg from the label)	Unit converter

Assumptions

The syringe is U-100. 100 units = 1 mL, so 1 unit = 0.01 mL. A U-40 or U-500 syringe gives a different volume for the same number. Only the syringe units chart handles U-40.
The final volume equals the water added. The dry powder takes up a small volume of its own, which is ignored. Where a product label states the concentration after mixing, the label figure takes priority.
The vial holds the labeled amount and all of the measured water went in.
Nothing is lost. Liquid left in the needle, the syringe tip or the vial is not subtracted, so counts of amounts per vial are upper limits.
1 mL = 1 cc, and 1 mg = 1,000 mcg.
IU per mg is never assumed by the unit converter; it is entered from the product label. The HGH IU to mg chart uses 3 IU per mg, the figure in FDA-approved somatropin labeling, and says so.

Rounding and display rules

Calculations run at full floating-point precision. Rounding happens once, when a number is displayed.
Displayed values have at most 4 significant figures. Trailing zeros are dropped, so 5.000 is shown as 5 and 0.0500 as 0.05.
Values of 1,000 or more are shown as whole numbers, and charts add thousands separators.
The unit converter keeps up to 10 significant figures, because it converts between exact units.
Whole amounts per vial are rounded down; vials needed for a period are rounded up.
Chart cells above 100 units are marked "over 1 mL", because a 1 mL syringe stops at 100 units. Cells under 2 units are marked as too small to read reliably.
Rounding is for display only. It is not a statement about how precisely a syringe can be read.

Exact value	Displayed as
3.3333333333333335	3.333
0.016666666666666666	0.01667
0.05	0.05
2.5	2.5
33.333333333333336	33.33
1234.5	1235
12345.6	12350
0.000123456	0.0001235

Test values

Each row below is generated when the site is built by calling the same function the calculators use, so the table always shows what the code returns. The same functions are covered by an automated test suite that compares them with results worked by hand. The inputs are arbitrary test numbers, not doses.

Function	Inputs	Result
`reconstitute`	vial 10 mg, water 2 mL, amount 1 mg	5 mg/mL, 0.2 mL, 20 units, 10 such amounts per vial
`reconstitute`	vial 5 mg, water 2 mL, amount 0.25 mg	2.5 mg/mL, 0.1 mL, 10 units, 20 such amounts per vial
`reconstitute`	vial 10 mg, water 3 mL, amount 1 mg	3.333 mg/mL, 0.3 mL, 30 units, 10 such amounts per vial
`reconstitute`	vial 12 mg, water 2.4 mL, amount 0.6 mg	5 mg/mL, 0.12 mL, 12 units, 20 such amounts per vial
`reconstitute`	vial 10 IU, water 1 mL, amount 2 IU	10 IU/mL, 0.2 mL, 20 units, 5 such amounts per vial
`reconstitute`	vial 5,000 IU, water 5 mL, amount 500 IU	1000 IU/mL, 0.5 mL, 50 units, 10 such amounts per vial
`unitsToAmount`	vial 10 mg, water 2 mL, 20 units drawn	5 mg/mL, 0.2 mL, 1 mg
`unitsToAmount`	vial 5 mg, water 2.5 mL, 7 units drawn	2 mg/mL, 0.07 mL, 0.14 mg
`unitsToAmount`	vial 15 mg, water 3 mL, 100 units drawn	5 mg/mL, 1 mL, 5 mg
`waterForUnits`	vial 10 mg, amount 0.5 mg to fill 10 units	2 mL of water, 5 mg/mL, 0.05 mg per unit
`waterForUnits`	vial 5 mg, amount 0.25 mg to fill 20 units	4 mL of water, 1.25 mg/mL, 0.0125 mg per unit
`vialSupply`	vial 10 mg, amount 3 mg, 2 injections per week, 4 weeks	3 whole amounts per vial, 1 mg left over, 10.5 days per vial, 8 needed, 3 vials
`blendDraw`	vial 10 mg + 5 mg, water 2 mL, 10 units drawn	5 and 2.5 mg/mL; 0.5 mg and 0.25 mg in the draw
`convertMass`	250 mcg to mg	0.25 mg
`convertMass`	1.5 mg to mcg	1500 mcg
`mlToUnits`	0.25 mL on a U-100 syringe	25 units
`unitsToMl`	10 units on a U-100 syringe	0.1 mL
`unitsToMl`	10 units on a U-40 syringe	0.25 mL
`convertSyringeUnits`	10 units on U-40, read on U-100	25 units
`iuToMg`	12 IU at 3 IU per mg	4 mg
`mgToIu`	4 mg at 3 IU per mg	12 IU

To reproduce a row by hand, follow the three steps above, or use the guide to checking a result by hand.

What the site does not calculate

It does not state, suggest or imply a dose, a starting amount, a frequency or a schedule for any compound, and it does not model how any compound behaves in the body. The amount to draw is an input, taken from a prescription.

Regulatory status lines

Each compound page carries a one-line note on US regulatory status, the month it was checked, and a source. Statuses were last checked in November 2026.

"Not FDA-approved" means no product containing the substance was listed in Drugs@FDA, the FDA's database of approved drugs, when it was checked. "FDA-approved" lines name the brand and link to its label on DailyMed. The notes are facts about approval, not statements about safety, effectiveness or whether a substance may be compounded.

Sources

External links on this site go only to primary sources: the FDA, DailyMed drug labels, NIST, the CDC and USP. Each was read against the claim it supports.

Who publishes this

Vial to Units is published by the site itself. No named author, clinician or medical reviewer is credited, because none was involved: the content is arithmetic and label-reading, written and checked against the sources above. It is not medical advice. See the about page and the editorial policy.

Report an error

If a formula, a chart figure, a test value or a regulatory note is wrong, it should be corrected. This site does not yet publish a contact address; one will be listed here and on the editorial policy page when it is set up. A useful report gives the page address, the inputs used, the result shown and the result expected.

Last updated November 12, 2026