Last updated: September 2026
Reverse Dose Solver
About the Reverse Dose Solver
The Reverse Dose Solver is a "I know X, solve for Y" reconstitution and dosing calculator. Instead of forcing you to work forwards from a fixed starting point, it lets you enter any two or three of six linked variables — vial total (mg), solvent / BAC water (mL), concentration (mg/mL), dose per injection (mg), syringe draw (U-100 units), and doses per vial — and it solves the rest for you instantly. Free, no login.
It is built for anyone who already knows some of their numbers but not others. For example, you know your vial is 200 mg in 2 mL and you want to know how many units to draw for a 25 mg dose — the solver fills in the concentration, draw volume, units, and how many doses your vial holds.
How to use it
Fill in any two or three fields you already know and leave the rest blank. As soon as there is enough information, the remaining fields solve automatically — there is no calculate button to press.
You do not have to start from the vial total. If you know your concentration and a target dose, the solver gives you the units. If you know units and concentration, it gives you back the dose in mg. Any consistent pair or triple works.
Entering a third value is optional — it lets the solver cross-check your inputs so you can spot a typo before you draw.
Worked example
Vial total: 200 mg
BAC water added: 2 mL
Concentration = 200 ÷ 2 = 100 mg/mL
Dose per injection: 25 mg
Draw volume = 25 ÷ 100 = 0.25 mL
U-100 units = 0.25 × 100 = 25 units
Vial total: 200 mg, dose: 25 mg
Doses per vial = 200 ÷ 25 = 8 doses
The math behind it
The solver is built on three plain equations that link all six variables:
- Concentration (mg/mL) = vial total (mg) ÷ solvent (mL)
- U-100 units = (dose (mg) ÷ concentration (mg/mL)) × 100
- Doses per vial = vial total (mg) ÷ dose (mg)
Because every field is connected through these three relationships, knowing any two values is usually enough to pin down the rest. The U-100 unit figure assumes a standard insulin syringe where 100 units equals 1 mL.
Reverse table: units and dose to water
For a 10 mg vial, these examples show how a target U-100 draw and dose determine concentration and water volume. They are arithmetic examples, not dosing recommendations.
| Target draw | Dose wanted | Required concentration | For a 10 mg vial, add water |
|---|---|---|---|
| 10 units (0.10 mL) | 250 mcg | 2,500 mcg/mL (2.5 mg/mL) | 4.0 mL |
| 20 units (0.20 mL) | 250 mcg | 1,250 mcg/mL (1.25 mg/mL) | 8.0 mL |
| 20 units (0.20 mL) | 500 mcg | 2,500 mcg/mL (2.5 mg/mL) | 4.0 mL |
| 25 units (0.25 mL) | 500 mcg | 2,000 mcg/mL (2.0 mg/mL) | 5.0 mL |
| 50 units (0.50 mL) | 1,000 mcg | 2,000 mcg/mL (2.0 mg/mL) | 5.0 mL |
The target volume comes from units ÷ 100. Concentration is dose ÷ volume, and water is vial content ÷ concentration. A different vial size changes the water in direct proportion while leaving the chosen dose and draw relationship intact.
Worked reverse examples
Each example holds the requested dose and syringe mark fixed, then solves backward. Keep mass units consistent: convert mg to mcg before dividing when the concentration is expressed in mcg/mL.
10 units ÷ 100 = 0.10 mL. 250 mcg ÷ 0.10 mL = 2,500 mcg/mL.
10,000 mcg ÷ 2,500 mcg/mL = 4.0 mL water.
20 units ÷ 100 = 0.20 mL. 250 mcg ÷ 0.20 mL = 1,250 mcg/mL.
10,000 mcg ÷ 1,250 mcg/mL = 8.0 mL water.
20 units ÷ 100 = 0.20 mL. 500 mcg ÷ 0.20 mL = 2,500 mcg/mL.
10,000 mcg ÷ 2,500 mcg/mL = 4.0 mL water.
25 units ÷ 100 = 0.25 mL. 500 mcg ÷ 0.25 mL = 2,000 mcg/mL.
10,000 mcg ÷ 2,000 mcg/mL = 5.0 mL water.
50 units ÷ 100 = 0.50 mL. 1,000 mcg ÷ 0.50 mL = 2,000 mcg/mL.
10,000 mcg ÷ 2,000 mcg/mL = 5.0 mL water.
For a 5 mg vial with the same concentration of 2,500 mcg/mL, 5,000 mcg ÷ 2,500 mcg/mL = 2.0 mL water. At 10 units, 0.10 mL × 2,500 mcg/mL = 250 mcg.
A 0.5 mg dose is 500 mcg. At 20 units (0.20 mL), 500 mcg ÷ 0.20 mL = 2,500 mcg/mL; for a 10 mg vial, 10,000 ÷ 2,500 = 4.0 mL water.
Common reverse-calculation mistakes
Mixing mg and mcg: 1 mg equals 1,000 mcg. Convert before dividing; using 250 as mg when it means mcg changes the result by a factor of 1,000. Confusing units and volume: on a U-100 syringe, 10 units equals 0.10 mL, not 10 mL. The conversion assumes U-100 calibration.
Ignoring dead space: syringe design and technique can leave liquid behind, so the amount delivered may differ slightly from the nominal barrel reading. Treating the math as a protocol: once dose and units are fixed, the required concentration and water are fixed too; if that water volume or concentration is impractical, choose different inputs with a qualified prescriber rather than altering the arithmetic. Follow product handling and beyond-use instructions; aseptic technique matters.
Frequently asked questions
Other calculators
Related guides
How this is calculated
This calculator links your concentration (vial total ÷ solvent added), draw volume for a target dose, the matching insulin-syringe units (volume in mL × 100 for a U-100 syringe), and doses per vial (vial total ÷ dose). It is unit-conversion arithmetic only — nothing is stored — and it is not medical advice. Always follow your supplier's labeling and confirm your protocol with your prescriber.