Milligrams to grams (mg to g)

One milligram is exactly 0.001 grams. That is the meaning of the metric prefixes, not a measurement: milli is a thousandth, centi a hundredth and kilo a thousand, so every conversion inside the metric system is a power of ten and nothing has to be looked up.

Worked through: 250 mg x 0.001 = 0.25 g.

Going the other way, one gram is 1,000 milligrams.

To convert without a calculator, multiply by 0.001. Rounding that to 0.001 is within 0% for most purposes.

Milligrams appear on nutrition labels, supplement panels and laboratory measurements, where quantities are small enough that the decimal point carries the meaning. Written conventions therefore require a leading zero (0.5 mg) and prohibit a trailing one (5.0 mg), because either error moves the decimal point and changes the quantity tenfold.

Precision is worth matching to the input. A measurement given as 1 mg carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.