Randomized mL problems from real oral suspensions, solutions and injection strengths: amoxicillin, acetaminophen, morphine, ondansetron, lorazepam and more. Answers are marked with the volume rounding rule most programs use: tenths above 1 mL, hundredths below.
Divide the ordered dose by the amount of drug in the stated volume, then multiply by that volume: D ÷ H × Q. Order 650 mg of acetaminophen with an oral solution of 160 mg per 5 mL: 650 ÷ 160 × 5 = 20.31, which rounds to 20.3 mL.
The Q is the step people drop. A label that says 250 mg per 5 mL means 250 mg is in 5 mL, not in 1 mL. Stop at D ÷ H and you have the number of 5 mL portions, not the number of millilitres.
| Order | Setup | Answer |
|---|---|---|
| Amoxicillin 0.5 g PO; 250 mg/5 mL | 500 ÷ 250 × 5 | 10 mL |
| Acetaminophen 650 mg PO; 160 mg/5 mL | 650 ÷ 160 × 5 = 20.31 | 20.3 mL |
| Lactulose 20 g PO; 10 g/15 mL | 20 ÷ 10 × 15 | 30 mL |
| Ondansetron 4 mg IV; 2 mg/mL | 4 ÷ 2 × 1 | 2 mL |
| Morphine 3 mg IV; 4 mg/mL | 3 ÷ 4 × 1 | 0.75 mL |
| Hydromorphone 0.5 mg IV; 2 mg/mL | 0.5 ÷ 2 × 1 | 0.25 mL |
The usual rule: volumes over 1 mL round to the nearest tenth, and volumes under 1 mL round to the nearest hundredth. The rule follows the syringe. A 3 mL syringe is marked in tenths, while a 1 mL syringe is marked in hundredths, so 0.75 mL can be drawn up exactly but 20.31 mL cannot.
Round only at the end. If you round 650 ÷ 160 to 4 before multiplying by 5, you get 20 mL instead of 20.3 mL, and on an exam that requires the tenth that is a wrong answer.
Teaspoons and tablespoons still show up in older exam questions, and you should know 1 tsp = 5 mL and 1 tbsp = 15 mL. In practice, oral liquids are ordered and measured in mL only, with an oral syringe or a marked cup. The American Academy of Pediatrics recommends mL-only dosing because household spoons vary and "tsp" and "tbsp" get confused.
Injections add one more check. Look at the volume you calculated and ask whether it fits the route. An IM injection of 12 mL, or an IV push of 0.02 mL, is a sign that the concentration or the setup is wrong. Several drugs here are high-alert medications, including opioids and subcutaneous heparin, and need an independent double-check under most facility policies.
Use D ÷ H × Q: the ordered dose divided by the drug amount on the label, multiplied by the volume that amount comes in. 500 mg ordered from 250 mg per 5 mL is 500 ÷ 250 × 5 = 10 mL.
Most programs round volumes over 1 mL to the nearest tenth and volumes under 1 mL to the nearest hundredth. Round only the final answer, and follow your own program if its rule differs.
You are probably dividing D by H and stopping. If the label is 250 mg per 5 mL, multiply by the 5 mL. D ÷ H alone tells you how many 5 mL portions to give, not how many millilitres.
No. Oral liquids are measured in mL with an oral syringe or a marked cup, which is what the AAP recommends. Know that 1 tsp is 5 mL for older exam questions, but dose in mL.