How the math works
Four ideas cover everything our calculators do. The numbers below are made up for teaching — they are not suggestions for any substance.
1. Units: why mg vs mcg matters
A milligram (mg) is a thousandth of a gram. A microgram (mcg or µg) is a thousandth of a milligram. So 1 mg = 1,000 mcg — mixing them up gives an answer that's a thousand times off.
- 1 mg = 1,000 mcg
- 0.5 mg = 500 mcg
- 1 mL = 100 units on a U-100 syringe scale, so 1 unit = 0.01 mL
2. Concentration: how much is in each mL
Concentration tells you how much substance is in each millilitre. Divide the total amount by the total volume.
- Made-up example: 10 mg in 4 mL of solution.
- Concentration = 10 mg ÷ 4 mL
- = 2.5 mg/mL — every mL contains 2.5 mg.
3. Prescribed amount → volume
If your clinician has prescribed an amount, the volume that contains it is the amount divided by the concentration. Get both into the same units first.
- Made-up example: 500 mcg prescribed; the label says 2.5 mg/mL.
- Convert: 2.5 mg/mL = 2,500 mcg/mL.
- Volume = 500 mcg ÷ 2,500 mcg/mL
- = 0.2 mL of solution.
4. Comparing two solutions
To compare, put both in the same unit, then divide one by the other.
- Made-up example: A is 10 mg/mL; B is 2,500 mcg/mL.
- Convert A: 10 mg/mL = 10,000 mcg/mL.
- Ratio = 10,000 ÷ 2,500 = 4. A is 4× as concentrated as B.
Why no body-weight calculator? Working out an amount from someone's weight is a prescribing decision. Approved medicines that use weight-based dosing come with official instructions for clinicians; for unapproved compounds there is no established basis at all. So we don't do it.