DoseDrill

Lessons · Oral liquids

Dosage calculation

Oral liquid (suspension) dosage calculation

Liquid medications are labeled as a strength per volume, such as 250 mg per 5 mL. The same D/H × Q rule applies, but Q is the volume that carries the labeled strength, so the answer comes out in milliliters.

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The formula

Volume (mL) = (Desired dose ÷ dose On hand) × Quantity (mL)

How to solve it

  1. Read the concentration from the label: strength per volume (e.g. 250 mg per 5 mL).
  2. Set up dimensional analysis: ordered mg × (labeled mL ÷ labeled mg).
  3. Cancel mg and multiply/divide to get milliliters.
  4. Round to a volume the syringe can measure (usually tenths of a mL).

Common mistakes

  • Using the strength (mg) as the answer volume instead of multiplying by the mL the strength comes in.
  • Dropping the leading zero on volumes under 1 mL - write 0.5 mL, never .5 mL.
  • Not matching units before dividing.
Safety check: Always write volumes under 1 mL with a leading zero (0.5 mL). A missing zero is a classic ten-fold dosing error.

Worked examples

See it solved

The prescriber orders 500 mg of azithromycin oral suspension. The bottle is labeled 200 mg per 5 mL.

Ordered dose500 mg
Concentration200 mg per 5 mL

How many milliliters should the nurse give?

Answer: 12.5 mL

Step-by-step solution

1. Identify what you have and what you want
Have: 200 mg per 5 mL · Want: 500 mg
2. Set up dimensional analysis so mg cancels
500 mg × ( 5 mL ÷ 200 mg )
3. Cancel mg and solve
= ( 500 × 5 ) ÷ 200 mL
= 12.5 mL

The prescriber orders 300 mg of azithromycin oral suspension. The bottle is labeled 200 mg per 5 mL.

Ordered dose300 mg
Concentration200 mg per 5 mL

How many milliliters should the nurse give?

Answer: 7.5 mL

Step-by-step solution

1. Identify what you have and what you want
Have: 200 mg per 5 mL · Want: 300 mg
2. Set up dimensional analysis so mg cancels
300 mg × ( 5 mL ÷ 200 mg )
3. Cancel mg and solve
= ( 300 × 5 ) ÷ 200 mL
= 7.5 mL

The prescriber orders 20 mg of prednisolone oral solution. The bottle is labeled 15 mg per 5 mL.

Ordered dose20 mg
Concentration15 mg per 5 mL

How many milliliters should the nurse give?

Answer: 6.7 mL

Step-by-step solution

1. Identify what you have and what you want
Have: 15 mg per 5 mL · Want: 20 mg
2. Set up dimensional analysis so mg cancels
20 mg × ( 5 mL ÷ 15 mg )
3. Cancel mg and solve
= ( 20 × 5 ) ÷ 15 mL
= 6.7 mL