Reverse Dose Solver — Solve Concentration, Dose & Units

Last updated: June 2026

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

Step 1 — concentration

Vial total: 200 mg

BAC water added: 2 mL

Concentration = 200 ÷ 2 = 100 mg/mL

Step 2 — draw volume and units

Dose per injection: 25 mg

Draw volume = 25 ÷ 100 = 0.25 mL

U-100 units = 0.25 × 100 = 25 units

Step 3 — doses per vial

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.

Frequently asked questions

How many values do I need to enter?

Enter any two of the six linked variables — vial total mg, solvent mL, concentration mg/mL, dose mg, syringe units, or doses per vial — and the solver fills in the rest automatically. If you enter a third value it cross-checks the others. Leave the fields you want solved blank.

How are syringe units calculated?

Units are based on a U-100 insulin syringe, where 100 units equals 1 mL. The solver works out your draw volume as dose mg divided by concentration mg/mL, then multiplies that volume by 100 to give units. For example, a 0.25 mL draw equals 25 units.

What is concentration and how is it found?

Concentration is how much active compound sits in each millilitre of solution, in mg/mL. The solver divides the vial total mg by the solvent (BAC water) mL you added. A 200 mg vial reconstituted with 2 mL of water gives 100 mg/mL.

How do I work out doses per vial?

Divide the vial total mg by your dose per injection mg. A 200 mg vial dosed at 25 mg per injection gives 8 doses per vial. This is arithmetic only and does not account for waste or dead space in the syringe.

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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 labelling and confirm your protocol with your prescriber.