The calculator does three pieces of arithmetic and then checks the answer against the device you are measuring with. All of it is written out here so you can check any result by hand. How pages are sourced and reviewed is on the editorial process page.
The three formulas
- Concentration. The amount in the vial divided by the volume of liquid added to it.
concentration (mg/mL) = vial amount (mg) / diluent volume (mL) - Draw volume. The dose divided by the concentration.
draw volume (mL) = dose (mg) / concentration (mg/mL) - Device reading. The draw volume converted to the marks on the device. On a U-100 insulin syringe one unit is 0.01 mL, so units = draw volume (mL) x 100. A peptide pen dials in 0.01 mL clicks, so clicks = draw volume (mL) x 100 as well.
Worked example. A 5 mg vial with 2 mL of bacteriostatic water is 5 / 2 = 2.5 mg/mL. A 250 mcg dose is 0.25 mg, and 0.25 / 2.5 = 0.1 mL, which is 10 units on a U-100 syringe or 10 clicks on a pen. Halve the water to 1 mL and the concentration doubles to 5 mg/mL, so the same dose becomes 0.05 mL, or 5 units.
Units: mg, mcg, mL, IU
- mg and mcg. 1 mg is 1,000 mcg. Vial labels are usually in mg and doses in mcg, which is where most hand calculations go wrong. The calculator converts a mcg dose to mg by dividing by 1,000 before anything else, and switching the dose control between mg and mcg keeps the same amount.
- mL and syringe units. Because a U-100 syringe is marked in hundredths of a milliliter, 100 units is 1 mL, 50 units is 0.5 mL and 10 units is 0.1 mL. The Original calculator lets you enter the water volume in units for that reason: 100 IU on an insulin syringe is 1 mL.
- IU. An international unit is a measure of biological activity defined separately for each substance, so there is no universal conversion between IU and mg. For a vial labeled in IU, the calculator's IU potency task takes the label's IU figure, the water volume and a target dose in IU and works in IU per mL; it never invents a mg equivalent.
Syringes, pens and sprays
U-100 insulin syringes
Three barrels are offered: 0.3 mL (30 units), 0.5 mL (50 units) and 1 mL (100 units). The capacity check uses the barrel's unit count, and the draw is reported in whole units because that is what the barrel is marked in.
Peptide pens
A pen dials fixed 0.01 mL clicks regardless of any syringe scale. Cartridges of 1.5 mL and 3 mL are offered, and a maximum dial of 30, 60 or 80 clicks per injection. When the calculator counts how many injections a cartridge gives, it deducts 1.5 clicks per injection for priming, and it applies that loss inside each cartridge fill separately, so a partial final fill cannot borrow leftover liquid from the previous one.
Nasal sprays
A metered spray delivers a fixed volume per pump, offered as 0.05, 0.1 or 0.2 mL, into a 5 mL or 10 mL bottle. The amount per spray is the bottle concentration multiplied by the spray volume; the number of sprays in a bottle is the bottle volume divided by the spray volume, rounded down. The intranasal calculator works the other way around as well: from a desired amount per use, the number of uses and the spray volume, it gives the bottle concentration to prepare.
Rounding and capacity rules
- Whole marks. The headline reading is rounded to the nearest whole unit or click, because that is what can actually be set on the device. It is not always rounded up: rounding up every time would systematically increase the measured volume.
- Capacity is checked before rounding. A draw of 100.4 units on a 100-unit barrel is flagged as over capacity even though it would display as 100. A draw above 85 percent of the device's capacity is flagged as near capacity.
- Tiny draws. A syringe draw under 6 units, or under 5 percent of the barrel, is flagged as hard to measure repeatably; the usual fix is more water. When the calculator suggests a water volume for a pen it aims for at least 4 clicks for the same reason. A positive dose whose draw rounds to no whole mark reads "less than 1" rather than 0.
- Comfortable range. Draws between 10 and 30 units are treated as ideal for reading the barrel, and up to 50 units as comfortable. These thresholds guide the water volumes the calculator suggests; they never change a result you typed.
- Suggested water. When the calculator proposes a water volume, it stays at or below 3 mL for a standard vial and 10 mL for a larger one.
The reverse calculator
The reverse calculator starts from the reading you want to draw and solves for the water to add. Because a pen dials whole clicks and a syringe is read in whole units, it does not simply divide: it walks candidate water volumes in 0.25 mL steps up to the vial's limit, keeps the ones that land the dose on a whole number of marks between 2 units and 95 percent of the device's capacity, and returns the largest of those draws, because a larger draw is easier to measure precisely.
Doses per vial and supply
Doses per vial is the vial amount divided by the dose, rounded down. The supply calculator and the workspace schedules turn a frequency such as "every other day" or "twice a week" into a day count, multiply by the doses available, and report a run-out date; for pens the priming deduction above is applied first. The calculator does not track how long a mixed vial has been open; the storage guide covers the in-use window of bacteriostatic water.
How the compound library records amounts
A compound page does not recommend an amount. It records the amounts that appear in its sources and shows them as a ladder: a starting range, the most commonly reported range and an upper range, each with the kind of source it came from (clinical, preclinical, community-reported) and the context the source gave (duration, frequency, study model). When you choose a compound in the calculator, it can fill in a vial size, water volume and dose recorded in the library as a starting point for the arithmetic. Those are recorded examples, not recommendations, and every field stays editable.
Identity data (CAS number, molecular formula, molar mass, amino acid count, sequence) is taken from PubChem or the approved product's label. Evidence tiers and source labels are explained on the editorial process page.
How the math is tested
All of the arithmetic above lives in small functions with no screen, network or storage attached, and an automated test suite exercises them on every change: the worked examples on this page, the rounding and capacity edge cases, pen priming across several cartridge fills, the reverse solver, and the agreement between the Original calculator and the guided Basic and Advanced modes on identical inputs. A result the server reports is never trusted; the browser recomputes it from the inputs.
What the calculator does not do
- It does not tell you what dose to use. It converts a dose you enter into a device reading.
- It does not know what is in your vial. It trusts the amount you type, which should be the amount printed on the label.
- It does not account for dead space in a syringe, liquid left in a vial, or evaporation. Its figures are the arithmetic, and real draws run slightly short of it.
- It is an educational tool for research contexts and is not medical advice.