How to Calculate Grams & Milligrams

A gram (g) is the basic metric unit of weight. Calculating grams and milligrams (mg) is important for understanding medication doses, nutritional supplements, recipes, and similar calculations. In the US, where ounces (oz), the basic imperial unit of weight, are used, g and mg calculations must start by converting oz to g. One g contains 1,000 mg, and one oz contains 28.3495 g (usually rounded to 28, 28.3, etc., depending on the degree of precision you desire).

Syringes with milligram measurements (Image: Alison Roscoe/iStock/Getty Images)

Grams to Milligrams

Step 1

Write, or enter into your calculator, the number of g which you want to convert to mg.

Step 2

Multiply the number of g by 1,000. Example: 2.25 g X 1,000 = 2,250 mg.

Step 3

Reverse the process to convert mg to g. Divide the number of milligrams by 1,000. Example: 1,250 mg divided by 1,000 = 1.25 g.

Grams and Milligrams from Ounces

Step 1

If you are given only oz but need to calculate g and mg, write, or enter into your calculator, the number of oz.

Step 2

Multiply the oz by 28 if you want an answer in g. Example 2.25 oz X 28 = 63 g.

Step 3

Use more decimal places for the value of g if you want a precise answer including mg. Example: 1.5 oz X 28.3495 = 42.5243 g, which can be read as 42 g and 524 mg.

Things You'll Need

  • Pen or pencil

  • Paper or calculator


Most medications and nutritional supplements are measured in metric units because it is the universal standard for scientific calculations.

The metric system is also used for all everyday, household measurements and calculations throughout most non-English speaking nations.


If you are making gram and milligram calculations to adjust or record medication dosages please verify your calculations with a pharmacist, physician, or other trained practitioner. Miscalculations can lead to significant under- or over-dosages.

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