🧠 The theory: concentration is a bridge
A concentration in mg/mL is a conversion factor between an amount of drug and a volume of liquid. Once you know how many milligrams sit in one millilitre, you can go either way: divide a dose by it to get a volume, or multiply a volume by it to get an amount. This is why the very first move, every time, is to simplify the label to mg per mL.
concentration = amount of drug / volume
volume to give = dose / concentration
Worked example
Morphine oral solution is 10 mg in 5 mL. A 15 mg dose is required. What volume should be given?
- Step 1 — Find the concentration. There is 10 mg in 5 mL, so divide to get the amount in 1 mL: 10 / 5 = 2 mg/mL.
- Step 2 — Find the volume for the dose. You need 15 mg and each mL holds 2 mg, so divide the dose by the concentration: 15 / 2 = 7.5.
Answer: 7.5 mL
Worked example
Amoxicillin suspension is 250 mg in 5 mL. The dose is 200 mg. What volume is needed?
- Step 1 — Find the concentration: 250 mg in 5 mL means 250 / 5 = 50 mg/mL.
- Step 2 — Divide the dose by the concentration: 200 / 50 = 4.
Answer: 4 mL
Worked example
Furosemide oral solution is 20 mg in 5 mL. A 30 mg dose is prescribed. What volume is needed?
- Step 1 — Concentration: 20 / 5 = 4 mg/mL.
- Step 2 — Volume: divide the dose by the concentration, 30 / 4 = 7.5.
Answer: 7.5 mL
Worked example
Co-amoxiclav suspension is 125 mg in 5 mL. A 250 mg dose is needed. What volume?
- Step 1 — Concentration: 125 / 5 = 25 mg/mL.
- Step 2 — Volume: 250 / 25 = 10.
Answer: 10 mL
Harder: working backwards
Some questions give you a volume and ask for the dose, or ask how many doses are in a bottle. The same concentration does the work, you just use it the other way round.
Worked example
How much drug is in a 10 mL dose of a 250 mg in 5 mL suspension?
- Step 1 — Find the concentration: 250 / 5 = 50 mg/mL.
- Step 2 — This time multiply the volume by the concentration, because you want the amount: 50 x 10 = 500.
Answer: 500 mg
Worked example
A 150 mL bottle is given as 7.5 mL doses. How many doses does it contain?
- Step 1 — Each dose is 7.5 mL, so divide the total volume by the dose volume.
- Step 2 — 150 / 7.5 = 20.
Answer: 20 doses
💡 Practical check
Sense-check that the measured volume is practical and can be given with an oral syringe. An answer of 0.12 mL or 47 mL for a single oral dose should make you re-read the question.
Test yourself
Paracetamol suspension is 120 mg in 5 mL. A 180 mg dose is needed. What volume?
Answer: Concentration = 120 / 5 = 24 mg/mL; volume = 180 / 24 = 7.5 mL
A medicine is 250 mg in 5 mL. What volume contains 625 mg?
Answer: Concentration = 50 mg/mL; volume = 625 / 50 = 12.5 mL
How much drug is in 15 mL of a 100 mg in 5 mL solution?
Answer: Concentration = 20 mg/mL; amount = 20 x 15 = 300 mg
How many 5 mL doses are in a 100 mL bottle?
Answer: 100 / 5 = 20 doses