mg vs mcg: the 1,000× difference

The relationship

Both are units of mass in the metric system. A milligram (mg) is one thousandth of a gram. A microgram (mcg, sometimes written µg) is one thousandth of a milligram. So:

1 mg = 1,000 mcg  ·  1 mcg = 0.001 mg

To go from mg to mcg, multiply by 1,000 (move the decimal three places right). To go from mcg to mg, divide by 1,000 (three places left). That's the entire conversion — there are no edge cases.

Why peptide math mixes them

Vial labels are almost always printed in milligrams: a vial might say 5 mg or 10 mg. But individual doses are often discussed in micrograms, because typical amounts per dose are small — hundreds of mcg rather than whole mg. So a single calculation routinely uses both units, and every crossing between them is a chance to slip three decimal places.

A reliable habit: convert everything to micrograms first, do all the arithmetic in mcg, and only convert back at the end if you need to. One unit, no crossings, no 1,000× surprises. This is exactly what the peptide calculator does internally — and why its "show the math" steps display both units at each stage.

Conversion table

Three ways people get burned

Reading µg as mg. The µ symbol renders badly in some fonts and gets transcribed as "ug" or misread entirely. If a number looks suspiciously large or small, check which unit it was originally written in.

Typing the dose in the wrong box. Entering 250 into a field that expects mg, when you meant 250 mcg, produces an answer 1,000× too high. Any calculator worth using labels its units on every field and lets you switch them explicitly.

Trusting a memorised number of units. "My dose is 10 units" is only true for one specific concentration. Change the water volume at reconstitution and the same mcg dose is a different number of units. Recompute whenever the vial changes.

Related tools

• mg ⇄ mcg converter
• Peptide calculator