How it works
Write what you have as a ratio (dose on hand : quantity), set it equal to what you want (dose ordered : x), and solve for x. With fractions: H / Q = D / x, so x = D × Q ÷ H, which is the same as desired over have.
Keep the same units in the same position on both sides.
Example: liquid
Order: cephalexin 375 mg PO. Available: 250 mg per 5 mL.
- 250 mg : 5 mL = 375 mg : x mL
- 250 × x = 5 × 375
- x = 1,875 ÷ 250 = 7.5 mL
Example: injection
Order: furosemide 40 mg IV push. Available: 10 mg/mL.
- 10 mg : 1 mL = 40 mg : x mL
- 10x = 40
- x = 4 mL
Frequently asked questions
Is ratio and proportion the same as desired over have?
It is the same math written differently, and it gives the same answer. Use the method your program teaches on exams.