A percentage on a medication label is a concentration, and it always means an amount per 100. What that amount is measured in — grams, or millilitres — depends on the basis the label states. Read the basis first, because 1% w/v and 1% w/w do not describe the same preparation.
Answer first
1% w/v = 1 g in 100 mL = 10 mg/mL
Percent means per 100. The letters after it tell you what the 100 refers to.
Percent means “per 100”
Before it is anything clinical, a percentage is a fraction with 100 underneath. 25% is 25 out of 100, which is the fraction 25/100 and the decimal 0.25. Moving between the three forms is the skill everything else here rests on.
Percentage → decimal: divide by 100. 25% → 25 ÷ 100 = 0.25
Decimal → percentage: multiply by 100. 0.075 → 7.5%
Percentage → fraction: put it over 100 and simplify. 25% → 25/100 → 1/4
The three concentration bases
The FDA describes the standard strength expressions used on drug listings as follows: w/v is “mass (grams) of solute in a volume (100 mL) of solution, like oral liquids”; w/w is “mass (grams) of solute in mass (100 g) of solution, like topical creams and ointments”; and v/v is “volume (milliliters) of solute in a volume (100 mL) of solution, like alcohol”.1
StatPearls gives the same three definitions from the pharmacy calculations side: percent w/v is “the number of grams in 100 mL of solution”, percent w/w is “the number of grams in 100 grams of solution”, and percent v/v is “the number of milliliters in 100 mL of solution”.2
% w/v
grams per 100 mL
1% w/v = 1 g in 100 mL
% v/v
millilitres per 100 mL
1% v/v = 1 mL in 100 mL
% w/w
grams per 100 g
1% w/w = 1 g in 100 g
Converting % w/v into mg/mL
Most medication calculations want mg and mL, not a percentage. The conversion is two steps, and it is the same every time.
Step 1 — write the percentage as grams per 100 mL.
1% w/v → 1 g in 100 mL
Step 2 — convert grams to milligrams and divide by the volume.
1 g = 1000 mg, so 1000 mg ÷ 100 mL = 10 mg/mL
Because the second step is always “× 1000 then ÷ 100”, the shortcut is simply × 10: the percentage number multiplied by 10 gives mg/mL.
Worked examples
A 0.9% w/v preparation
0.9% w/v → 0.9 g in 100 mL
0.9 g = 900 mg
900 ÷ 100 = 9 mg/mL
How much medicine is in a stated volume?
Supplied: a 2% w/v preparation, and a volume of 250 mL.
2% w/v → 20 mg/mL
20 × 250 = 5000 mg, which is 5 g
Cross-check with the definition: 2 g per 100 mL, and 250 mL is 2.5 lots of 100 mL, so 2 × 2.5 = 5 g. Both routes agree.
Common mistakes
- Reading a percentage as mg/mL directly. 1% w/v is 10 mg/mL, not 1 mg/mL — a tenfold difference.
- Assuming the basis. A percentage with no stated basis has not told you whether it is w/v, v/v or w/w.
- Substituting a w/w figure into a volume calculation. Mass per mass and mass per volume are different quantities.
- Forgetting the gram-to-milligram step. The definition gives grams; medication calculations usually want milligrams.
- Treating percentage as a total. It is a concentration. The amount in the container depends on the volume as well.
What a percentage strength does not tell you
A percentage describes what is in the preparation. It says nothing about what dose is appropriate, how much should be given, or how quickly. Converting a percentage into mg/mL is arithmetic; every clinical decision around it comes from the prescription, the product information and local policy.
Remember
- Percent means per 100.
- The letters after the percentage tell you what the 100 refers to.
- 1% w/v = 1 g in 100 mL = 10 mg/mL.
- For any w/v percentage, mg/mL = percentage × 10.
- w/v, v/v and w/w are different bases and are never assumed.
Where to go next
- C₁V₁ = C₂V₂ Explained for Medication Dilution — using a percentage strength inside a dilution calculation.
- How to Read Medication Labels and Concentrations — the other ways strength appears on a label.
- Percentages in Medication Maths — learn percentage relationships and medication-strength notation step by step.
- Medication Percentage Practice Questions — four levels of percentage and concentration practice with worked rationales.
- mg to mL Calculator — once the concentration is in mg/mL.
Sources and references
- U.S. Food and Drug Administration. Strength Conversion in Drug Listing. Defines w/w, w/v and v/v strength expressions. FDA. Accessed 3 September 2026.
- Sharma P, Dunham A. Pharmacy Calculations. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. Defines percent w/v, w/w and v/v strength. NCBI Bookshelf. Accessed 3 September 2026.