Medication Unit Conversions: mg, mcg, %, mEq and units

At a glance

From	To	Do this
g	mg	× 1000
mg	mcg	× 1000
mcg	ng	× 1000
L	mL	× 1000
% (w/v)	mg/mL	× 10
1:1000	mg/mL	1 mg/mL (1:10,000 = 0.1 mg/mL)
mmol	mEq	× valence (1 for K⁺, Na⁺, Cl⁻; 2 for Ca²⁺, Mg²⁺)
mg/h	mcg/min	× 1000 ÷ 60
units	mg	no fixed conversion

The idea underneath every row: it is best to convert before you calculate, so that the dose and the stock share the same unit.

Mass: g, mg, mcg, ng

The metric mass units used in drug doses each step by a factor of 1000:

1 g = 1000 mg
1 mg = 1000 mcg (µg)
1 mcg = 1000 ng

Moving to a smaller unit makes the number bigger; moving to a larger unit makes it smaller. A quick sense check: 0.25 mg and 250 mcg are the same dose, and the number in mcg should be the larger one.

Writing “mcg” rather than “µg” is recommended in handwritten and printed orders, because a handwritten µ can be read as m, a 1000-fold error. Some infusion protocols use ng/kg/min; the same × 1000 step applies.

Volume: L and mL

1 L = 1000 mL. Infusion bags are labelled in mL; daily fluid totals are often discussed in litres. A “1.5 L” daily target is 1500 mL, or 62.5 mL/h over 24 hours. Note that 1 mL is the same as 1 cm³ (cc), but “cc” is on error-prone lists because it can be misread as “u”.

Units are not milligrams

Insulin, heparin, some vitamins, enzymes and biological products are dosed in units (or international units, IU). A unit defines a biological effect measured against a reference standard, not a mass, so:

there is no universal factor from units to mg;
the same number of units of two different substances can weigh very different amounts;
the only safe link to a volume is the units per mL on the product label.

Example: 2,500 units from a 5,000 units/mL vial is 2,500 ÷ 5,000 = 0.5 mL. The units to mL calculator does exactly this. Insulin is supplied in several concentrations (U-100 = 100 units/mL is the most common, but U-200, U-300 and U-500 exist), and insulin syringes are graduated in units for one specific concentration only. It is best to avoid writing “U” for units: it can be read as 0 or 4.

mEq and mmol

Electrolytes are prescribed in millimoles (mmol) or milliequivalents (mEq), depending on the country and the product. The relationship is simple:

mEq = mmol × valence (charge)

K⁺, Na⁺, Cl⁻, HCO₃⁻: 1 mmol = 1 mEq
Ca²⁺, Mg²⁺: 1 mmol = 2 mEq

Converting mg to mmol needs the molar mass of the exact salt. For example, 1 g of potassium chloride contains about 13.4 mmol of K⁺; 1 g of magnesium sulfate heptahydrate contains about 4.06 mmol of Mg²⁺ (about 8.1 mEq). Because hydrated and anhydrous salts have different molar masses, it is advisable to use the mmol or mEq printed on the label rather than calculating your own. Concentrated potassium is a high-alert medicine in most hospital protocols.

What do % and 1:1000 strengths mean?

Percentage (w/v) means grams per 100 mL:

0.9% sodium chloride = 0.9 g/100 mL = 9 mg/mL = 9 g/L (about 154 mmol/L of sodium)
5% glucose = 5 g/100 mL = 50 mg/mL, so a 500 mL bag contains 25 g
2% lidocaine = 20 mg/mL, so 5 mL contains 100 mg

Shortcut: % × 10 = mg/mL.

Ratio strength means grams per number of mL:

1:1000 = 1 g in 1000 mL = 1 mg/mL
1:10,000 = 1 g in 10,000 mL = 0.1 mg/mL
1:200,000 (adrenaline in some local anaesthetics) = 5 mcg/mL

Ratio strengths are a well-known source of tenfold errors, which is why many regulators and manufacturers have moved adrenaline labelling to mg/mL. When diluting from one concentration to another, the dilution calculator applies C1 × V1 = C2 × V2. Its fields are labelled mg/mL and mL, but it works with any unit as long as both concentrations share it.

Time conversions for infusion rates

Doses may be ordered per minute, per hour or per day, while pumps run in mL/h.

per minute → per hour: × 60
per hour → per day: × 24
mg/h → mcg/min: × 1000 ÷ 60

Worked example: an infusion of 400 mg in 250 mL contains 400 × 1000 ÷ 250 = 1,600 mcg/mL. At 5 mcg/kg/min for a 70 kg patient, the dose is 350 mcg/min, or 21 mg/h. The pump rate is 5 × 70 × 60 ÷ 1,600 = 13.125, shown as 13.13 mL/h. The mcg/kg/min calculator shows these same steps; forgetting the × 60 makes the rate 60 times too small, and mixing up mg and mcg makes it 1000 times wrong.

The 10× and 1000× errors

Tenfold and thousand-fold errors tend to follow a few patterns:

Decimal point errors (10×): a missing leading zero (.5 read as 5) or a trailing zero (5.0 read as 50).
Unit errors (1000×): mg entered where mcg was meant, or g confused with mg.
Per-kg errors: a mg/kg dose given as a total dose, or a total dose multiplied by weight again.
Concentration errors: using the mg in the whole vial instead of the mg per mL.

Paediatric care is especially exposed, because nearly every dose is calculated from weight and the volumes are small. A five-year review of tenfold errors in a Canadian children’s hospital found that they were rarely exotic: dose calculation, the way decimal points were written and confusion over zeros were frequent causes, and intravenous formulations, paper orders and infusion pumps often made them possible.

Safe notation

These are the habits we recommend, in line with the ISMP list:

A leading zero: 0.5 mg rather than .5 mg.
No trailing zero: 5 mg rather than 5.0 mg (except where a laboratory value requires precision).
Units in full or in standard form: “units”, not U or IU; “mcg”, not µg; “mL”, not cc.
Thousands separators for large numbers: 5,000 units or 100,000 units, to avoid miscounting zeros.
A space between the number and the unit: 10 mg, not 10mg, which can read as 100 mg.

Common pitfalls

Calculating before converting, so that mg and mcg end up in the same fraction.
Assuming units can be turned into mg “to make the maths work”.
Reading 1:1000 and 1:10,000 as similar strengths: one is ten times the other.
Forgetting that % w/v is per 100 mL, not per mL.
Dropping the × 60 when moving between per-minute doses and mL/h.
Copying a dose with a trailing zero or without a leading zero.

For a structured way to set up the calculations once the units match, see the dosage calculation methods guide. This guide is educational: we recommend following product labels and local protocols, which take precedence.

Reading the concentration on the label

The total number of units (here 25,000).
The volume (here 5 mL).
25,000 ÷ 5 = 5,000 units/mL, the number the calculator asks for.

Writing “units” in full is generally recommended: a handwritten “U” or “IU” can be misread as a 0 or a 10.

Frequently asked questions

How many micrograms are in a milligram?

1 mg = 1000 mcg. To go from mg to mcg multiply by 1000; from mcg to mg divide by 1000. So 0.25 mg is 250 mcg and 62.5 mcg is 0.0625 mg. The number in mcg is always the larger one. Writing “mcg” rather than “µg” is advisable, because a handwritten µ can be read as m.

Can I convert units to mg?

Not with a fixed factor. A unit measures biological activity, and the mass that gives one unit differs between substances and sometimes between products of the same substance. It is advisable to work in units throughout, using the units per mL stated on the label.

What does a 2% solution mean in mg/mL?

2% is 20 mg/mL. A percentage strength (w/v) means grams per 100 mL, so 2% is 2 g in 100 mL. The quick rule is % × 10 = mg/mL: 5 mL of 2% lidocaine contains 100 mg, and 0.9% sodium chloride is 9 mg/mL.

What is 1:1000 adrenaline in mg/mL?

1 mg/mL. The ratio 1:1000 means 1 g in 1000 mL; 1:10,000 is ten times weaker, 0.1 mg/mL. Ratio strengths are a well-known source of tenfold errors, so many labels now state the strength in mg/mL to avoid confusion between the two.

What is the difference between mEq and mmol?

mEq takes the ion’s charge into account: mEq = mmol × valence. For monovalent ions such as potassium, sodium and chloride, 1 mmol = 1 mEq; for divalent ions such as calcium and magnesium, 1 mmol = 2 mEq. So 10 mmol of magnesium is 20 mEq.

Why is a trailing zero dangerous?

Because the decimal point can be missed. “5.0 mg” misread as 50 mg is a tenfold overdose. It is advisable to write 5 mg, and to put a leading zero before a decimal point, as in 0.5 mg, so that .5 is not read as 5. Both rules are on the ISMP list of error-prone dose designations.

Medication unit conversions explained