Ounces to Milligrams Converter

Ounces (oz). An ounce is exactly 28.349523125 grams, one sixteenth of a pound. It is used for food portions and postal weights, and precious metals use the different troy ounce of 31.1035 g.

Milligrams (mg). A milligram is one thousandth of a gram. It is used for medicine doses, laboratory work, water-quality readings and any measurement of trace amounts.

Metric conversions are exact. Results are displayed rounded, so values at the extreme ends of the scale — milligrams and tonnes — carry fewer significant digits than the middle of the range.

Between the metric units a weight conversion is a power of a thousand, so grams to kilograms and kilograms to tonnes are decimal shifts. The imperial units are all defined against the kilogram, which makes every metric-to-imperial pair a multiplication with the same factor at any size.

The dimension where this goes wrong is the ounce, because the same word covers two different weights. An avoirdupois ounce is one sixteenth of a pound and is the one used for food and parcels; a troy ounce is about 10% heavier (31.1035 g) and is used for precious metals. A price per ounce is meaningless until you know which ounce is meant.

To check a weight conversion, convert the result back, or go through the pound or the kilogram as a middle step. The commonest error is converting a figure stated in pounds into kilograms and then comparing it with a figure that was already in kilograms, which doubles or halves nothing and merely hides the mistake.