Liquid Medication Dose & Quantity Calculator (mg to mL)
Convert a milligram dose to millilitres (or millilitres back to milligrams) for an oral liquid or injectable from its concentration — mg per mL, mg per 5 mL, % w/v or common product strengths — with syringe rounding, the dose actually delivered, directions, course volume, bottles to dispense, beyond-use date check and an optional mg/kg check.
How it works
mL per dose = dose (mg) ÷ (concentration mg ÷ mL) · mg in a volume = mL × mg/mL · % w/v × 10 = mg/mL · delivered dose = rounded mL × mg/mL · course volume = mL × doses per day × days (+ extra %) · bottles = ⌈course volume ÷ bottle size⌉ · 1 tsp = 4.93 mL, 1 tbsp = 14.79 mL
Liquid medicines are prescribed in milligrams but measured in millilitres, and the bridge is the concentration on the label — always “so many milligrams in so many millilitres”, most often per 5 mL for children’s suspensions and per mL for drops and injectables. Dividing the dose by the milligrams per millilitre gives the volume, which is then rounded to what the syringe or cup can measure; the calculator shows the dose actually delivered after rounding, so a rounding step that changes the dose by more than 10% stands out. You can also work backwards from a volume to the milligrams it contains, enter a percentage strength (1% w/v = 10 mg/mL), or pick a common product strength. Kitchen teaspoons vary from about 3 to 7 mL and are a leading cause of paediatric dosing errors, so pharmacies dispense oral syringes. Multiplying the volume by doses per day and days gives the course volume, the bottles to dispense and the days the supply covers. For reconstituted suspensions the course should fit within the use-by period after mixing (often 10 or 14 days) — when it does not, the calculator says how many separate fills are needed. Add the patient’s weight to see the dose in mg/kg for comparison with the prescribing reference.
Worked example
A 250 mg dose of a 125 mg/5 mL suspension (25 mg/mL) is 10 mL — about two teaspoons, measured with a syringe or dosing cup. Three times a day for 10 days is 300 mL, so dispense three 100 mL bottles with nothing left over; the directions read “Take 10 mL by mouth three times daily for 10 days”. Going the other way, 7.5 mL of amoxicillin 400 mg/5 mL contains 600 mg.
Sources: ISMP — Recommendations to prevent errors with oral liquid medications (mL-only dosing, oral syringes) · DailyMed (NLM) — US product labels for amoxicillin, amoxicillin–clavulanate, azithromycin, cefalexin and cefdinir suspensions
Frequently asked questions
What if the concentration is a percentage?
Choose “Enter a percentage”. A w/v percentage is grams per 100 mL, so 1% = 10 mg/mL and 0.5% = 5 mg/mL; the calculator converts it for you.
How do I find how many mg are in a volume?
Switch “Calculate” to mL → mg and enter the volume per dose. For example, 7.5 mL of a 400 mg/5 mL suspension (80 mg/mL) contains 7.5 × 80 = 600 mg.
Why does the dose delivered differ from the dose I entered?
The volume is rounded to a measurable amount, which slightly changes the milligrams given. The calculator shows the delivered dose and the percentage difference; choose a finer rounding step or a smaller syringe if it is large.
Injectables in units or mEq?
The arithmetic is the same — divide the ordered units (or mEq) by the units (mEq) per mL on the vial, entering them in the mg fields. Insulin U-100 is 100 units/mL.
What if the course is longer than the bottle’s use-by period?
Reconstituted antibiotics are typically used within 10 or 14 days of mixing. If the course is longer, the calculator flags it and shows how many separate fills cover it; enter the exact period from the product label under “More options”.
Related calculators
For pharmacy staff and patients to sanity-check quantities — it does not replace the prescriber’s directions, the pharmacist’s judgement or payer rules.