How to Convert Teaspoons to Milligrams

Teaspoons and milligrams describe different physical quantities. A teaspoon measures volume, while a milligram measures mass. The missing link is density: the mass packed into each milliliter of volume.

Choose the teaspoon definition

The calculator supports two common definitions:

• One US teaspoon is 4.92892159375 mL.
• One metric teaspoon is 5 mL.

The difference is small, but it changes the result. Always use the definition intended by your source.

Find density in g/mL

Use a reliable density for the exact substance. Water is often approximated as 1.000 g/mL. Powders are more difficult because bulk density changes with packing, moisture, particle size, and measuring technique.

Apply the formula

For a US teaspoon:

mg = tsp x 4.92892159375 x density(g/mL) x 1000

For a metric teaspoon:

mg = tsp x 5 x density(g/mL) x 1000

For example, one US teaspoon of water is:

1 x 4.92892159375 x 1.000 x 1000 = 4928.92159375 mg

Rounded for display, that is approximately 4,928.92 mg.

Why 5,000 mg is not universal

One metric teaspoon of water is 5,000 mg under the density assumption above. Another substance can have a lower or higher mass in the same volume. A statement such as “one teaspoon always equals 5,000 mg” is therefore incorrect.

Use the teaspoons to milligrams calculator to choose the system and density explicitly.

Medication labels use concentration

Medicine products commonly state concentration as mg per 5 mL. That is product-specific information, not a generic density conversion. Follow the product label and qualified medical guidance rather than using this calculator to determine a dose.

Primary references

• NIST Special Publication 811, Appendix B.9 — lists the US teaspoon-to-milliliter conversion factor under volume.
• BIPM SI Brochure — the official reference for SI units, prefixes, and unit symbols used in the calculation.