core · Allied Health & Nursing
Desired-over-Have Dosage Algebra
Meet the worker
Nurse Kim — ER RN
trauma bay, code in progress
Provider shouts 'Push 0.5 mg epi IV now.' Vial is 1 mg / 10 mL. Kim has seconds.
What they'll need
- Pre-filled syringe
- Sharps bin
- Calculator (mental)
- Drug reference card on lanyard
How it's done — step by step
- 1
Desired over have
(Desired / Have) × Volume. (0.5 / 1) × 10 mL = 5 mL.
- 2
Sanity check the order of magnitude
If answer feels off by 10×, STOP. Re-read the vial.
- 3
Push at correct rate
Epi IV push: over 1 minute via running line. Flush with 20 mL NS.
- 4
Document immediately
Time, dose, route, your initials. ER charting can't wait.
Order: 75 mg morphine sulfate. Available: 50 mg/mL prefilled. Compute the volume to draw using the Desired/Have × Vehicle formula. Then re-run for a tablet order (250 mg ordered, 125 mg tablets) and a suspension (250 mg ordered, 125 mg/5 mL).
0 mL1.53 mL
Calculated dose: (75 / 50) × 1 = 1.50 mL
Tap Show next step to reveal the math one piece at a time.
Worked Example
Order 75 mg; have 50 mg / 1 mL
Given: Want 75 mg, vial 50 mg/mL
- 1
Want ÷ Have
75 ÷ 50 = 1.5
Worked Example
Order 250 mg; have 125 mg / 5 mL
Given: Suspension dosing
- 1
Want ÷ Have
250 ÷ 125 = 2