🧮 Dosage Calculations · Lesson 2 of 5 · 6 min
Desired Over Have, Times Quantity
💊 The order is 500 mg. The bottle says 250 mg per 5 mL. One formula answers every question of this type, and the exam can give you several of them.
💡 Key idea
Desired ÷ Have × Quantity, with matching units. If the answer looks absurd, the conversion is wrong.
🧠 Why it matters
The formula: DESIRED dose ÷ HAVE (the dose on hand) × QUANTITY (the volume or form it comes in) = amount to give. Order 500 mg, have 250 mg per 5 mL: 500 ÷ 250 × 5 = 10 mL. Order 0.25 mg, have 0.125 mg tablets: 0.25 ÷ 0.125 × 1 = 2 tablets. The units of desired and have must MATCH, so convert first: order 1 g, have 500 mg tablets, convert 1 g to 1,000 mg, then 1,000 ÷ 500 = 2 tablets. Dimensional analysis gives the same answer by lining up units so they cancel. Sanity-check the result: a dose of 15 tablets or 0.02 mL almost always means a conversion error. For injectables, choose the syringe that measures the volume accurately: a 1 mL syringe for small doses, and round to the syringe's gradations. Split only scored tablets, and never split unscored or coated tablets.
🏥 In the real world
🧴 Order: amoxicillin 400 mg PO. Available: 250 mg per 5 mL suspension. 400 ÷ 250 = 1.6; 1.6 × 5 = 8 mL. The nurse measures 8 mL in an oral syringe. A classmate who forgot to multiply by the quantity gives 1.6 mL, one fifth of the dose. The formula has three parts because all three matter.
📌 Takeaways
- Desired over have, times the quantity on hand
- Convert so the units match before dividing
- Absurd answers signal a conversion mistake
📖 Terms in this lesson
Desired over have: The dose calculation formula: desired dose divided by the dose on hand, times the quantity it comes in.
✅ Test yourself
The order is for 750 mg of an antibiotic. The suspension available is 500 mg per 5 mL. How many mL should the nurse give?
- 5 mL
- 10 mL
- 7.5 mL
- 15 mL
Answer: C · 7.5 mL
750 ÷ 500 = 1.5; 1.5 × 5 mL = 7.5 mL.
The order is for 0.5 g of a medication. Tablets are available as 250 mg. How many tablets should the nurse give?
- 2 tablets
- 1 tablet
- 5 tablets
- 0.5 tablet
Answer: A · 2 tablets
0.5 g = 500 mg; 500 ÷ 250 = 2 tablets.
An order calls for 60 mg of an injectable medication. The vial contains 80 mg per 2 mL. What volume should the nurse draw up?
- 0.75 mL
- 1.5 mL
- 2.5 mL
- 3 mL
Answer: B · 1.5 mL
60 ÷ 80 = 0.75; 0.75 × 2 mL = 1.5 mL.
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