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Hong KongPercentage and ratio strength
A percentage strength in a liquid is grams per 100 mL. A ratio strength of 1 to 1000 is one gram in 1000 mL. Converting either to mg per mL makes them directly comparable.
Two notations, one underlying quantity, and most of the difficulty is in the translation rather than the arithmetic.
Percentage strength
Written on a liquid, it means grams per 100 mL. A 5 percent solution is 5 g in 100 mL.
Convert to mg per mL by multiplying by 10: 5 percent is 50 mg per mL. That shortcut is worth memorizing because it turns a two-step conversion into a glance. Always the same three.
Why multiplying by ten works
Five grams in 100 mL is 5000 mg in 100 mL, which is 50 mg in 1 mL. The thousand from grams to milligrams divided by the hundred from the volume leaves a factor of ten. Worth memorizing.
Knowing why means you can rebuild it if you doubt yourself mid-exam.
Ratio strength
A ratio of 1 to 1000 means one gram in 1000 mL for a liquid. That is 1000 mg in 1000 mL, so 1 mg per mL.
One to 10,000 is ten times weaker than one to 1000, which is the opposite of how percentages read. Candidates who convert everything to mg per mL before comparing never fall for it; candidates comparing the notations directly frequently do.
A comparison table
| Written as | mg per mL |
|---|---|
| 1 percent | 10 |
| 5 percent | 50 |
| 0.9 percent | 9 |
| 1 to 1000 | 1 |
| 1 to 10,000 | 0.1 |
Solids and semi-solids
For a solid preparation such as an ointment, a percentage means grams per 100 g rather than per 100 mL. The stem will tell you what the preparation is. Read what the preparation is.
Reading weight-in-weight as weight-in-volume is a subtle error and the exam includes it deliberately.
How much is in the bottle?
The standard question. A 500 mL bag of a 5 percent solution contains 50 mg per mL times 500 mL, which is 25,000 mg, or 25 g.
Convert to mg per mL, multiply by the volume, convert back if the options are in grams. Three steps, always the same three.
Common questions
What does a 5 percent solution mean?
5 g in 100 mL, which is 50 mg per mL.
How do I convert percentage to mg per mL?
Multiply by 10.
What does 1 to 1000 mean?
One gram in 1000 mL, which is 1 mg per mL.
Which is stronger, 1 to 1000 or 1 to 10,000?
1 to 1000. A bigger second number means a weaker solution.
Is a percentage always per 100 mL?
For liquids yes. For solids such as ointments it is grams per 100 g.