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.

Practice this nowMore worked examples

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 100 mg of cephalexin oral suspension. The bottle is labeled 200 mg per 1 mL.

Ordered dose100 mg
Concentration200 mg per 1 mL

How many milliliters should the nurse give?

Answer: 0.5 mL

Step-by-step solution

1. Identify what you have and what you want
Have: 200 mg per 1 mL · Want: 100 mg
2. Set up dimensional analysis so mg cancels
100 mg × ( 1 mL ÷ 200 mg )
3. Cancel mg and solve
= ( 100 × 1 ) ÷ 200 mL
= 0.5 mL

The prescriber orders 250 mg of cephalexin oral suspension. The bottle is labeled 250 mg per 1 mL.

Ordered dose250 mg
Concentration250 mg per 1 mL

How many milliliters should the nurse give?

Answer: 1 mL

Step-by-step solution

1. Identify what you have and what you want
Have: 250 mg per 1 mL · Want: 250 mg
2. Set up dimensional analysis so mg cancels
250 mg × ( 1 mL ÷ 250 mg )
3. Cancel mg and solve
= ( 250 × 1 ) ÷ 250 mL
= 1 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