Dose-math explainer
mcg vs mg: the thousand-fold line
Short answer
The thousand-fold line: a milligram holds exactly a thousand micrograms. Written out, 1mg = 1,000mcg, with no rounding and no exceptions by compound.
Both directions: milligrams to micrograms, multiply by 1,000. Micrograms to milligrams, divide by 1,000. Either way, the decimal point slides three places, and that is the entire skill.
Why both units exist: the two spellings suit different sizes. At 1mg and above the milligram spelling carries the fewer digits; below 1mg the microgram spelling does, which is why 250mcg turns up in place of 0.25mg.
The same amount twice: 0.5mg and 500mcg are one amount in two spellings, not two different doses.
The expensive misread: 5mg next to 500mcg is a tenfold gap, 5,000mcg against 500mcg, and two lost decimal places turn it into a hundredfold one.
The scope: one conversion between two units of mass, and nothing wider. The rule, the table, and the mistakes, with no dose attached to anyone; the rest of the section sits on the peptide dose math index.
On this page
The one rule
The rule: one milligram equals one thousand micrograms, 1mg = 1,000mcg, in both directions, for every substance on earth.
The prefixes: milli marks a thousandth of a gram and micro marks a millionth, so the step between the two units is a clean thousand.
Forwards: milligrams into micrograms multiplies by 1,000. 2.5mg becomes 2,500mcg, and 0.1mg becomes 100mcg.
Backwards: micrograms into milligrams divides by 1,000. 500mcg becomes 0.5mg, and 250mcg becomes 0.25mg.
The only moving part: a decimal point traveling three places. Get the direction right and the rest is arithmetic a phone can check.
What there is to cite: nothing. These are unit definitions rather than study results, and every figure on this page re-derives from the one rule above.
1,000
micrograms inside each milligram, exactly
0.001
milligrams inside each microgram, exactly
3
decimal places between the two units, either direction
1
letter separates mg from mcg on a label: the c
Unit definitions only: milli is a thousandth of a gram, micro a millionth. Nothing here needed a source.
Why peptides live in both units
The split: both units measure the same masses, and which spelling gets used comes down to which one keeps the number shortest at that size.
Milligram ground
What sits here: amounts of 1mg and up, where the milligram spelling carries the fewer digits. A stamp reading 5mg or 10mg is one or two characters of number; the identical amounts written the other way are 5,000mcg and 10,000mcg.
Where it stops helping: below a milligram. There the milligram spelling opens with a zero and a decimal point, and the digits that carry the amount sit one to three places behind them.
Microgram ground
What sits here: amounts under 1mg, the range the BPC-157 guide works in. 250mcg and 500mcg are whole numbers; the identical amounts spelled in milligrams are 0.25mg and 0.5mg, each one opening on that same zero.
Where it stops helping: above a milligram, symmetrically. 10mg spelled in micrograms is 10,000mcg, five characters of number where two carried the same amount.
The peptide itself: unchanged by any of this. A molecule does not know which unit spelled its amount; only the notation moves, and how much of it rides in each milliliter is a separate question.
The decimal mistakes
The pattern: three ways the two units cross, and each one shows up in the arithmetic. The same amount read as two, the digits matched while the unit slipped, or the decimal parked in the wrong place.
Move one, the phantom second amount: 0.5mg and 500mcg on the same label are one amount written twice. Counted as two and summed, they come to 1mg, exactly twice what the label states.
Move two, the digit match: 5 against 500 looks close. In units it is 5mg, or 5,000mcg, against 500mcg, a tenfold gap wearing matching digits.
Move three, the parked decimal: converting 250mcg and stopping one place early lands on 2.5mg instead of 0.25mg, ten times the amount on paper. Stopping two places early lands on 25mg, a hundred times.
What the missing letter is worth: mg and mcg differ by one character, and that character is a factor of a thousand. Lose it in either direction and the amount is out by three decimal places.
| On the label | What it means | Read as | The result |
|---|---|---|---|
| 0.5mg | 500mcg | 500mcg | correct, same amount in two spellings |
| 500mcg | 0.5mg | 5mg | 10x the intended amount |
| 250mcg | 0.25mg | 2.5mg | 10x, decimal parked one place early |
| 250mcg | 0.25mg | 25mg | 100x, decimal parked two places early |
The conversion table
How to read it: pick a milligram amount on the left and its microgram spelling sits on the right. Backwards works too: find the micrograms, read off the milligrams.
| Milligrams | Micrograms |
|---|---|
| 0.1mg | 100mcg |
| 0.25mg | 250mcg |
| 0.5mg | 500mcg |
| 1mg | 1,000mcg |
| 2mg | 2,000mcg |
| 2.5mg | 2,500mcg |
| 5mg | 5,000mcg |
| 10mg | 10,000mcg |
The Decadewise briefing
One conversion, one chart, or one unit trap like this, every week. Free.
Education only. Unsubscribe anytime.
How many mcg are in 1 mg?
Exactly 1,000: one milligram is one thousand micrograms, so 1mg and 1,000mcg are two spellings of a single amount. The prefixes set the ratio, milli for a thousandth of a gram and micro for a millionth, and it holds for every compound, not only peptides.
Is 500 mcg the same as 5 mg?
No, and the gap is tenfold: 500mcg is 0.5mg, half a milligram, while 5mg is 5,000mcg. The near-matching digits are the whole trap, since 5 and 500 sit comfortably side by side on a label, a chart, or a forum post.
Why do some doses use mcg and others mg?
The unit follows the size of the number. At 1mg and above the milligram spelling carries the fewer digits; below 1mg the microgram spelling does, which is why 250mcg turns up where 0.25mg would otherwise sit. Nothing about the amount changes when the spelling does, because the thousand-to-one rule joins the two exactly.
How do you convert mcg to mg?
Divide the micrograms by 1,000, which slides the decimal three places left: 500mcg becomes 0.5mg, 250mcg becomes 0.25mg, and 100mcg becomes 0.1mg. In reverse, milligrams to micrograms, multiply by 1,000 and the same decimal walks three places right, leaving the milligram figure the units to mg converter works in.
More dose math
Where this page sits: one conversion, with the pages that keep using it on either side:
- The peptide dose math index: every explainer and calculator in the cluster, gathered on one page.
- Concentration vs dose: dose is a mass and concentration is a mass per volume, and every barrel reading rests on the difference.
- Units to mg converter: syringe marks to milligrams and back, on whatever vial, water, and syringe get typed in.
- BPC-157 dosage guide: the library's microgram native, a compound whose numbers live in mcg from the first section.
The disclaimer
Who checks a page: the review and sourcing standard this site holds itself to, including who reviews a page before it ships and why no individual reviewer is named, is published on the methodology page.
Every page is reviewed by medical professionals before it ships, and written with longtime biohackers who were doing this before it was a trend. Reviewed still does not mean prescribed: nothing here is medical advice.
It is research, trial data, and reported use, with the numbers intact so you can check them. For decisions about your body, see a doctor who can look at your labs.
The weekly briefing
Get the next explainer.
New explainers, new tables, and the unit math behind the next misread label. One clear email a week.
Education only, never medical advice. Unsubscribe anytime.