The factor of a thousand
Not an arithmetic error. A unit error, which produces an answer that is out by a thousand and looks exactly as reasonable as a correct one.
Educational arithmetic only. Not medical advice and not a dosing protocol.
Why this error and not another
Two units are in constant use in this conversion and they differ by a factor of a thousand. Vials are labelled in milligrams: 5 mg, 10 mg. Amounts are usually written in micrograms: 250 mcg, 500 mcg. Every calculation on this site crosses between them.
What makes it dangerous is that the output does not look wrong. A volume of 0.02 mL and a volume of 0.2 mL are both draws, both fit a barrel and both look entirely plausible on a screen. Only one of them holds the amount you meant.
| In micrograms | In milligrams | At 2.5 mg/mL | On a U-100 |
|---|---|---|---|
| 100 mcg | 0.1 mg | 0.040 mL | 4 units |
| 250 mcg | 0.25 mg | 0.100 mL | 10 units |
| 500 mcg | 0.5 mg | 0.200 mL | 20 units |
| 1,000 mcg | 1 mg | 0.400 mL | 40 units |
| 2,500 mcg | 2.5 mg | 1.000 mL | 100 units |
Before you act on that number
This is educational arithmetic on figures entered by the reader. It is not medical advice, not a dosing protocol, and not a recommendation that anything be prepared, obtained or administered.
Nothing here states what quantity is appropriate for anyone, and there is no function in it that produces an amount to administer. A question about anyone’s health belongs with a licensed clinician who can examine the person asking.
Compounds of the kind this arithmetic is used on are largely research chemicals that are not approved by the FDA for human use. An exact calculation is not an accurate measurement: what is actually in a syringe depends on the preparation and the device, neither of which this page can see.
No qualified clinical reviewer is recorded against this site, and no credential is claimed anywhere on it. what this is not, at full length.
The three things that stop it here
The unit is part of the field
Every quantity on this site is entered as a number and a unit together, in one control. No field accepts a bare figure and assumes which unit it is in, because that assumption is the error itself. The selector sits against the input rather than above the form, so the unit gets read at the same time as the number.
Conversions are exact
A milligram is a thousand micrograms by definition, so the conversion is a multiplication by exactly 1,000 and nothing is approximated on the way through. Quantities are held as fractions of whole numbers and rounded once, when they are printed, so a chain of conversions cannot accumulate drift in the digit a reader acts on.
A dimension mismatch is refused
Micrograms into millilitres is not a conversion: it needs a strength, and inventing one would be worse than declining. So the amount unit page returns a refusal with the reason rather than a number.
The one arrangement that is safe to do in your head
A solution at exactly 1 mg/mL puts one microgram in every microlitre, so an amount in micrograms is the same number of microlitres. 250 mcg is 250 microlitres, which is 0.25 mL, which is 25 units on a U-100. No factor to apply and nothing to get backwards.
That is why 1 mg/mL appears in so many worked examples, and why 2 mg in 2 mL is the arrangement most of them use.