Dosage Practice

Oral Liquid Dosage Calculation: How Many mL to Give

A liquid's label gives a concentration, such as 125 mg per 5 mL. Divide the dose ordered by the milligrams in that concentration and multiply by its milliliters: desired ÷ have × volume.

The Formula

mL to give = (desired mg ÷ mg on label) × mL on label

D ÷ H × Q, where Q is the volume the label's dose comes in.

How to work it out

  1. 1 Read the label as a pair: the amount of drug and the volume it comes in, for example 125 mg in 5 mL.
  2. 2 Make sure the order and the label use the same unit. Convert grams to milligrams if needed.
  3. 3 Divide the ordered dose by the label's drug amount, then multiply by the label's volume.
  4. 4 Round to the nearest tenth of a milliliter unless the dose is under 1 mL, when hundredths are used with a 1 mL syringe.

Worked Examples

Example 1

Ordered: amoxicillin 250 mg by mouth. Available: 125 mg per 5 mL. How many mL?

  1. 250 mg ÷ 125 mg = 2.
  2. 2 × 5 mL = 10 mL.

Answer: 10 mL

Example 2

Ordered: 0.4 g by mouth. Available: 200 mg per 5 mL. How many mL?

  1. 0.4 g = 400 mg.
  2. 400 mg ÷ 200 mg × 5 mL = 10 mL.

Answer: 10 mL

Try five free problems

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Problem 1 of 3No sign-up needed

Ordered: 500 mg by mouth. Available: 250 mg per 5 mL. How many mL will you give?

Round to the nearest tenth

mL

Common Mistakes

  • Forgetting to multiply by the label's volume

    250 ÷ 125 is 2, but the answer is not 2 mL. The label says 125 mg comes in 5 mL, so 2 × 5 = 10 mL.

  • Using the bottle size instead of the concentration

    A 100 mL bottle of 125 mg/5 mL is still 125 mg per 5 mL. The total volume in the bottle does not enter the calculation.

  • Mixing grams and milligrams

    If the order is in grams, convert it before dividing. 0.4 g is 400 mg.

Questions Students Ask

How do I measure 7.5 mL at home?

With an oral syringe or a proper medicine cup, never a kitchen spoon. One and a half teaspoons is 7.5 mL, but kitchen teaspoons vary too much to trust.

When do I round to hundredths?

For doses under 1 mL drawn up in a 1 mL syringe, which is marked in hundredths. Everything else is usually rounded to tenths.

Keep practicing in lesson 5: Liquid Oral Doses

Fresh numbers every time. Ten correct answers finish the lesson. The practice round and the first five problems of lesson 1 are free.

Open lesson 5