Milligrams measure mass, teaspoons measure volume
You cannot convert one to the other without knowing the density of what you are measuring. The old shortcut "1 mg equals 1 mL" only holds for water — honey is 42% heavier, oil is 8% lighter.
The 5 is the millilitres in one metric teaspoon. (A US teaspoon is 4.929 mL — a 1.4% difference that is negligible in cooking but can matter in a pharmacy.) 500 mg of table salt at 1,100 mg/mL is 500 ÷ (1,100 × 5) ≈ 0.09 tsp.
Milligrams in one teaspoon, by substance
| Substance | Density (mg/mL) | mg per teaspoon (5 mL) |
|---|---|---|
| Water | 1,000 | 5,000 mg |
| Table salt (fine) | 1,100 | 5,500 mg |
| Kosher salt (flaky) | 550 | 2,750 mg |
| Granulated sugar | 800 | 4,000 mg |
| Baking powder | 800 | 4,000 mg |
| Vegetable oil | 916 | 4,580 mg |
| Honey | 1,420 | 7,100 mg |
Kosher salt weighs half what table salt does per teaspoon because its large flakes trap air — the same spoon holds far fewer crystals.
For medicine, use concentration, not density
Liquid drugs are labelled by concentration, such as "100 mg / 5 mL" — that is 20 mg per mL. Divide the prescribed dose by the concentration to get millilitres.
- Worked example. A 200 mg dose of children's ibuprofen at 100 mg/5 mL (20 mg/mL) is 200 ÷ 20 = 10 mL, which is 2 teaspoons.
- Measure in mL, not spoons. Kitchen teaspoons range from 2 to 9 mL; the FDA advises the marked oral syringe supplied with the medicine.
- Read the strength. Infant drops are often twice as concentrated as the children's suspension, so the same "teaspoon" can be a double dose. Always follow the label and a pharmacist.
Common questions
How do I convert mg to teaspoons?
You need the density of the substance, because milligrams measure mass and teaspoons measure volume. The formula is teaspoons equals mg divided by density in mg per mL, divided again by 5 (the mL in a teaspoon). There is no single answer that works for every material.
How many mg are in a teaspoon?
It depends entirely on what fills the spoon. A teaspoon of water is 5,000 mg, of fine table salt about 5,500 mg, of granulated sugar about 4,000 mg, and of honey about 7,100 mg.
Can I use a teaspoon to dose liquid medicine?
No. Ordinary kitchen teaspoons hold anywhere from 2 to 9 mL, so they are unsafe for dosing. Use the mL-marked oral syringe or cup that comes with the medicine, and follow the label concentration, not a teaspoon conversion.
