You cannot go from mg to mL without a density or a concentration
A milligram is a unit of mass. A millilitre is a unit of volume. They measure different things, so there is no universal factor between them the way there is for kg to lb. To cross from one to the other you have to know how much a given volume of the substance weighs.
For water the density is 1000 mg/mL, so 1 mL weighs 1000 mg and the numbers happen to divide cleanly. That single case is where the "1 mg = 1 mL" myth comes from — and it is only ever true for water.
By substance: how much one millilitre weighs
Use the mg-per-mL value to convert. Divide your milligrams by this number to get millilitres, or multiply millilitres by it to get milligrams.
| Substance | Density (mg/mL) | 500 mg is |
|---|---|---|
| Water | 1000 | 0.50 mL |
| Whole milk | 1035 | 0.48 mL |
| Cooking oil | 916 | 0.55 mL |
| Ethanol | 789 | 0.63 mL |
| Glycerin | 1261 | 0.40 mL |
| Honey | 1420 | 0.35 mL |
Oil is lighter than water so it floats and takes up more volume per mg; honey is denser so it takes up less. That is why the same 500 mg spans 0.35 to 0.63 mL here.
For medicine, use concentration — not density
Liquid medicines are labelled with a concentration such as "100 mg / 5 mL". That means 5 mL of the liquid contains 100 mg of the active drug, so the concentration is 20 mg/mL. Divide the dose by that number to find the volume to give.
| Medicine | Label | Concentration |
|---|---|---|
| Children's ibuprofen | 100 mg / 5 mL | 20 mg/mL |
| Infant ibuprofen drops | 50 mg / 1.25 mL | 40 mg/mL |
| Children's acetaminophen | 160 mg / 5 mL | 32 mg/mL |
| Amoxicillin | 250 mg / 5 mL | 50 mg/mL |
- Never assume 1 mg = 1 mL for a drug. Real concentrations run around 20 to 80 mg/mL, not 1000, so treating them as water would overdose by a huge margin.
- Infant and children's strengths differ. Infant ibuprofen (40 mg/mL) is twice as concentrated as the children's version (20 mg/mL); reading one bottle's dose off the other doubles the dose.
- Measure with a syringe, not a kitchen spoon. A "teaspoon" from a drawer holds anywhere from 4 to 7 mL, which is why the FDA recommends dosing only with a marked syringe or cup.
Common questions
Is 1 mg the same as 1 mL?
No. Milligrams measure mass and millilitres measure volume, so there is no fixed conversion between them. They only line up for water, where 1 mL weighs 1000 mg (1 gram). For anything else you need its density or, for medicine, its concentration.
How many mL is 500 mg?
It depends on the substance. For water, 500 mg is 0.5 mL. For a medicine at 50 mg/mL it is 10 mL. For cooking oil (about 916 mg/mL) it is roughly 0.55 mL. Without knowing the substance the question has no single answer.
How do I convert a dose in mg to mL of liquid medicine?
Divide the prescribed dose in mg by the concentration printed on the bottle in mg/mL. A 200 mg dose of a syrup labelled 100 mg per 5 mL (which is 20 mg/mL) is 200 divided by 20, so 10 mL.
