Calculate how much medication to give using the proportion method
The Setup
Most dosage problems come down to one proportion: What you have / What it contains = What you give / What is ordered
Have (on hand) / Amount = Give (x) / Desired dose
Or equivalently: Desired dose ÷ Have (on hand) × Quantity = Amount to give
Examples
Order: 500 mg. Available: 250 mg tablets. How many tablets?
250 mg / 1 tablet = 500 mg / x tablets
250x = 500
x = 2 tablets
Order: 150 mg. Available: 100 mg per 5 mL. How many mL?
100 mg / 5 mL = 150 mg / x mL
100x = 750
x = 7.5 mL
Safety Check
Always verify:
• Does the answer make sense? (Ordering more mg → give more tablets/mL)
• Are units consistent? (Both sides in mg, both sides in mL)
• Is the dose within a reasonable range?
💡 Common formula: D/H × Q = Amount to give. D = desired dose, H = have (on hand), Q = quantity (volume or tablets per dose on hand).
🎯 Practice Problems
1. Order: 750 mg. Available: 250 mg tablets. How many tablets?
A) 1
B) 2
C) 3
D) 4
2. Order: 0.5 g. Available: 250 mg tablets. How many tablets? (1 g = 1,000 mg)
A) 0.5
B) 1
C) 2
D) 5
3. Order: 200 mg. Available: 125 mg per 5 mL. How many mL?
A) 4 mL
B) 6 mL
C) 8 mL
D) 10 mL
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🎯 Bonus Practice
1. Order: 60 mg. Available: 20 mg per mL. How many mL?
A) 1 mL
B) 2 mL
C) 3 mL
D) 0.33 mL
2. Order: 1.5 mg. Available: 2 mg per mL. How many mL?
A) 0.25 mL
B) 0.5 mL
C) 0.75 mL
D) 3 mL
3. Order: 400 mg. Available: 200 mg per 5 mL. How many mL?
A) 5 mL
B) 8 mL
C) 10 mL
D) 20 mL
4. Order: 0.25 mg. Available: 0.5 mg tablets. How many tablets?
A) 0.5 tablet
B) 1 tablet
C) 2 tablets
D) 0.25 tablet
5. Order: 80 mg. Available: 40 mg/mL in a 2 mL vial. How many mL? Is one vial enough?