A milligram is a tiny unit of weight in the metric system, equal to one-thousandth of a gram. To picture how small that is, imagine a grain of salt: it weighs about one milligram. A single raindrop weighs roughly 20 milligrams. The abbreviation is mg.

Milligrams matter most when measuring things that are extremely light but where precision is crucial. Pharmacists measure medicine in milligrams because the difference between 5 mg and 50 mg of a medicine can be the difference between helping someone and making them sick. Scientists use milligrams when weighing chemicals for experiments. Nutritionists list vitamins and minerals in milligrams on food labels.

The metric system makes milligrams easy to work with because everything connects by powers of ten: 1,000 milligrams equal one gram, and 1,000 grams equal one kilogram. This makes calculations simpler than remembering that 16 ounces make a pound or 5,280 feet make a mile. When you need to measure something incredibly light with great accuracy, milligrams give you that precision.