Understanding mg/mL: How to Measure Liquid Medicine for Children
If your child's prescription says 200 mg/5 mL and you need to give 150 mg, how many mL is that? This guide explains suspension concentrations, volume math, and how to measure accurately.
> **Quick Answer:** mg/mL tells you how much drug is in each milliliter of suspension. To find the volume to give: divide the dose in mg by the concentration in mg/mL. Example: 150 mg ÷ 40 mg/mL (which is 200 mg/5 mL) = 3.75 mL. Use an oral dosing syringe for accuracy; never use kitchen spoons.
One of the most confusing parts of filling a pediatric prescription is translating the mg dose your child needs into the mL volume you actually put in a syringe. This article explains how to do that and why getting it right matters.
What mg/mL Means
Most liquid medications are suspensions — the drug is dissolved or suspended in a liquid vehicle. The concentration tells you the ratio of drug to liquid:
- **200 mg/5 mL** means 200 milligrams of drug in every 5 milliliters of liquid
- **This is the same as 40 mg/mL** (just divide both numbers by 5)
The label on the bottle may show one or both formats. "200 mg/5 mL" and "40 mg/mL" mean identical concentrations. If you see a discrepancy — the bottle says one thing and the prescription says another — call the pharmacy before measuring anything.
Common Concentrations for Pediatric Medications
Different drugs come in different concentrations, and even the same drug often comes in multiple concentrations:
| Medication | Common Concentrations |
|---|---|
| Augmentin (amoxicillin-clavulanate) | 125 mg/5 mL, 200 mg/5 mL, 250 mg/5 mL, 400 mg/5 mL |
| Cefdinir | 125 mg/5 mL, 250 mg/5 mL |
| Azithromycin | 100 mg/5 mL, 200 mg/5 mL |
| Prednisolone | 15 mg/5 mL, 25 mg/5 mL |
| Cephalexin | 125 mg/5 mL, 250 mg/5 mL |
| Ondansetron | 4 mg/5 mL |
| Omeprazole | 2 mg/mL |
The same mg dose from different concentrations requires different volumes. A 200 mg dose of cefdinir from a 125 mg/5 mL bottle (25 mg/mL) = 8 mL. From a 250 mg/5 mL bottle (50 mg/mL) = 4 mL. Same drug, same mg dose, but you give twice as much liquid with the lower concentration. This is why you can't assume a standard volume — you must know the concentration.
The Volume Calculation
Once you know the concentration in mg/mL, the math is:
**Volume (mL) = Dose needed (mg) ÷ Concentration (mg/mL)**
Examples:
- Need 300 mg of Augmentin, have 250 mg/5 mL (50 mg/mL) suspension: 300 ÷ 50 = **6.0 mL**
- Need 150 mg of azithromycin, have 200 mg/5 mL (40 mg/mL) suspension: 150 ÷ 40 = **3.75 mL**
- Need 30 mg of prednisolone, have 15 mg/5 mL (3 mg/mL) liquid: 30 ÷ 3 = **10.0 mL**
The pediatric dose calculators on this site do this automatically once you enter the weight and select the concentration. The [Augmentin pediatric dose calculator](/augmentin-pediatric-dose-calculator), for example, shows the exact mL volume based on which formulation you have.
Why Not Just Use the "teaspoon" on the Label?
Some older prescription labels still use "teaspoons" as a unit. A pharmaceutical teaspoon is defined as exactly 5 mL — but a kitchen teaspoon is not. Studies measuring household teaspoons found volumes ranging from 2.5 mL to 7.3 mL. This variation is harmless when you're measuring cooking ingredients, but it's not acceptable for medication.
If a label says "give 1.5 teaspoons," that means 7.5 mL — measured with an oral syringe, not a kitchen spoon. Some pharmacists now write "give 7.5 mL (1.5 tsp)" precisely to avoid this confusion.
Always use an oral dosing syringe. They come with most prescription liquid medications. If yours didn't include one, ask the pharmacist for a syringe before you leave.
How to Measure Accurately With an Oral Syringe
1. Hold the syringe vertically (plunger pointing down, tip pointing up)
2. Pull the plunger back to slightly more than the target volume
3. Hold the syringe at eye level and push the plunger until the bottom of the curved liquid surface (the meniscus) sits exactly at the target marking
4. Keep the syringe vertical when administering to avoid air gaps that can cause the child to spit it back out
For small volumes (under 2 mL), a 1 mL syringe is more accurate than a 5 mL syringe — the graduations are finer. Most pharmacies have both sizes.
What to Do If the Numbers Don't Add Up
If you're looking at a prescription that says "give 6 mL twice daily" but the bottle in your hand has a different concentration than you expected, recalculate before giving the first dose.
Run the weight through the appropriate dose calculator, select the actual bottle concentration, and see if the calculated mL matches the label. If they match: proceed. If they differ by more than 20%: call the pharmacist.
A prescription written for a 250 mg/5 mL suspension that was filled with a 125 mg/5 mL suspension produces a 50% underdose if you follow the written volume. This type of concentration substitution error is a known dispensing issue. Recalculating before the first dose catches it.
Reconstituted vs. Pre-Mixed Suspensions
Most pediatric antibiotic suspensions come as a dry powder that the pharmacy adds water to before dispensing — this is "reconstitution." The reconstituted suspension has a limited shelf life (typically 10–14 days refrigerated) and must be shaken before each use. Antibiotic particles settle to the bottom of the bottle, and an unshaken dose from the top delivers less drug than intended.
Shake the bottle vigorously for 5–10 seconds before each dose. The label will typically remind you, but it's easy to skip in a tired 2 AM moment.
Pre-mixed liquid medications (omeprazole, ondansetron) don't require reconstitution but still need proper storage — usually refrigerated, depending on the product. Check the label.
Calculating With a Dose Calculator
If the arithmetic feels error-prone, use the tools on this site. Each calculator takes weight and concentration as inputs and outputs the per-dose mL directly. The [cefdinir dosage calculator](/cefdinir-dosage-calculator), the [azithromycin calculator](/azithromycin-pediatric-dose-calculator), and every other calculator here applies the guideline dose, applies the max-dose cap, and converts to mL in one step. The math is the easy part — accurate inputs (correct weight, correct concentration) are what make the output reliable.