Liquid Medication Dose Calculator
Find the millilitres to draw up for an ordered dose of a liquid medicine from its concentration, using the desired-over-have-times-quantity rule.
How it works
Volume = (desired dose ÷ strength on hand) × reference volume · Concentration = strength ÷ volume
Liquid medicines are labelled as an amount per a volume — for example 125 mg per 5 mL. To get the millilitres for a different dose, divide the ordered dose by the strength on hand and multiply by that reference volume (the “desired over have times quantity” rule). The tool also reports the plain mg-per-mL concentration so you can sanity-check the number. Always draw the volume with a calibrated oral syringe.
Worked example
For an order of 250 mg where the bottle reads 125 mg per 5 mL: (250 ÷ 125) × 5 = 2 × 5 = 10 mL. The concentration is 125 ÷ 5 = 25 mg/mL, so 250 ÷ 25 also gives 10 mL as a check.
Frequently asked questions
Why not just divide dose by concentration?
You can — dose ÷ (mg per mL) gives the same millilitres. The desired-over-have-times-quantity form is taught because drug labels state a strength per a stated volume rather than a per-millilitre figure, so it maps directly onto what is printed on the bottle.
How should I measure the volume?
Use an oral/dosing syringe or the calibrated cup supplied with the medicine. Household teaspoons vary widely and are a known cause of dosing errors, especially for children. For volumes that fall between markings, round only as the label or pharmacist allows.
What if the dose needs an odd volume like 3.7 mL?
Draw it accurately with a syringe graduated finely enough to measure it. If the volume is impractical or the syringe cannot measure it, ask the pharmacy about a more suitable concentration rather than rounding to a convenient number.
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Educational estimate only — not medical advice, and no substitute for a clinician, pharmacist or the product label. Every dose, rate and volume must be independently verified against the actual drug, order and patient before administration. Do not use this tool to self-treat or to bypass an institution’s double-check policy.