Up next
mg → mL
Work out the volume to give from the prescribed dose and the medicine amount/volume in the available strength.
MedMaths Learn pathway
Core Medication Calculations
Build confidence with the everyday medication calculations nurses use most often, from mg to mL through to drawing up and weight-based dosing.
4 lessonsabout 1 hourProgress saved in this browser
Continue learning
mg → mL
Pick up where you left off, or start with the next lesson in this topic.
Topic pathway
Work through Core Medication Calculations
Follow the order below for a structured pathway, or open any lesson when you need that skill.
Not started
Tablets & Capsules
Calculate the number of dosage units and handle fractional or awkward results without automatic rounding.
Not started
Weight-Based Dosing
Doses written per kilogram, including per-dose and per-day orders, then converting a liquid dose to mL when needed.
Not started
Drawing Up Medication
Calculate the required mL, then reproduce that volume on an illustrative graduated syringe.
Need something else?
Keep moving without losing your place
PractiseTest this skillOpen the practice library and choose the lesson or mixed set you need.Practice library CalculateNeed the answer now?Jump to the clinical calculator hub without opening a new tab.Browse calculators UnderstandRead the explanationUse Guides for formulas, worked examples, limitations and references.Browse Guides
Lesson summaries
What each Core Medication Calculations lesson teaches
A short recap of every lesson in this topic: the key idea, a worked example and the mistake students make most often. Open any lesson for the full step-by-step teaching and its four levels of practice.
mg → mLWhen a dose is prescribed in milligrams but the medicine is a liquid, you need the volume that contains that many milligrams. Every method — the formula method, ratio and proportion, or dimensional analysis — uses the same relationship between the dose wanted and the strength available, and all three give the same answer.
When a dose is prescribed in milligrams but the medicine is a liquid, you need the volume that contains that many milligrams. Every method — the formula method, ratio and proportion, or dimensional analysis — uses the same relationship between the dose wanted and the strength available, and all three give the same answer.
Formula
volume = (dose wanted ÷ amount in the available strength) × volume of that strength
Often written as Desired ÷ Have × Quantity.
Worked example
375 mg is prescribed. The available liquid is labelled 250 mg in 5 mL.
- 1Both values are already in milligrams, so no conversion is needed.
- 2375 ÷ 250 = 1.5
- 31.5 × 5 = 7.5
- 4Sense check: 375 mg is more than 250 mg, so the volume should be more than 5 mL.
7.5 mL
Common mistake
Placing the available strength on top and the prescribed dose underneath. The dose you want always goes above the amount you have.
Match the units first, then ask how many times the available strength fits into the dose.
Tablets & CapsulesA tablet calculation asks how many dosage units make up the prescribed dose. The arithmetic is the same relationship as a liquid dose, but the answer is counted in whole units — and an awkward result is information, not something to round away.
A tablet calculation asks how many dosage units make up the prescribed dose. The arithmetic is the same relationship as a liquid dose, but the answer is counted in whole units — and an awkward result is information, not something to round away.
Formula
number of units = dose prescribed ÷ strength of one unit
Worked example
150 mg is prescribed. The available tablets are 50 mg each.
- 1150 ÷ 50 = 3
- 2Sense check: three 50 mg tablets total 150 mg.
3 tablets
Common mistake
Rounding a result such as 1.4 tablets to the nearest whole number. A fractional result means the numbers need re-checking, not smoothing.
If a tablet calculation does not resolve to a sensible count, stop and re-read the prescription and the label.
Weight-Based DosingA dose written per kilogram is a rate, not a total. Multiplying it by the supplied body weight converts it into a dose in milligrams, which is then treated like any other dose. This makes weight-based dosing a two-stage calculation: first find the dose, then find the volume.
A dose written per kilogram is a rate, not a total. Multiplying it by the supplied body weight converts it into a dose in milligrams, which is then treated like any other dose. This makes weight-based dosing a two-stage calculation: first find the dose, then find the volume.
Formula
dose = dose per kg × body weight · then volume = dose ÷ concentration
Worked example
A prescription supplies 4 mg/kg for a patient weighing 55 kg. The available liquid is labelled 100 mg in 5 mL.
- 1Stage 1 — dose: 4 × 55 = 220 mg
- 2Stage 2 — concentration: 100 ÷ 5 = 20 mg/mL
- 3Stage 2 — volume: 220 ÷ 20 = 11
220 mg, given as 11 mL
Common mistake
Dividing by the concentration before multiplying by weight, or stopping at the dose in milligrams when the question asked for a volume.
Finish stage one and write the dose down before starting stage two.
Drawing Up MedicationA calculated volume still has to be measurable. Syringes are graduated in fixed steps, so the practical question is whether the number you calculated falls on a marking you can actually read — which is a different question from whether the arithmetic is right.
A calculated volume still has to be measurable. Syringes are graduated in fixed steps, so the practical question is whether the number you calculated falls on a marking you can actually read — which is a different question from whether the arithmetic is right.
Worked example
A calculation gives 0.65 mL, to be measured in a 1 mL syringe graduated in 0.02 mL steps.
- 10.65 ÷ 0.02 = 32.5
- 2The value falls between the 32nd and 33rd graduation.
- 3The arithmetic is correct; the volume does not sit on a marking.
Correct arithmetic, but not directly measurable on this syringe
Common mistake
Assuming any calculated decimal can be drawn up. Calculated precision and measurable precision are not the same thing.
Check your answer against the graduations of the syringe you are actually holding.