Dosage Calculation: Phenobarbital Tablets from Grams

Question

PROBLEM

Phenobarbital 0.03 g po q1 available: phenobarbital 15 mg per tablet. How many tablets will the nurse administer?

Skills This Problem Builds

Unit conversion mastery — Converting between metric units (grams to milligrams) accurately and efficiently
Dimensional analysis — Using systematic factor-label method to track units through calculations
Medication safety awareness — Understanding that precise dosing requires careful unit management
Clinical calculation fluency — Applying the fundamental dosage formula used in healthcare settings
Critical verification skills — Checking that calculated doses make practical sense for patient safety

Picture This

phenobarbital 0.03 g po q1 available: phenobarbital15 mg per tablet. how many tablets will the nurse administer.

Every medication calculation starts with this flow: identify what's ordered, see what's available, convert to matching units, then calculate the quantity to administer.

Solution: Method 1 — Unit Conversion Bridge

Step 1 — Identify the unit mismatch

The doctor ordered phenobarbital in grams (0.03 g), but the pharmacy stocks tablets measured in milligrams (15 mg per tablet). Before we can calculate how many tablets to give, we need both quantities in the same unit.

Step 2 — Convert grams to milligrams

Use the metric conversion: 1 g = 1000 mg

0.03 g × (1000 mg / 1 g) = 0.03 × 1000 = 30 mg

So the ordered dose is 30 mg of phenobarbital.

Step 3 — Apply the dosage formula

Now that both values are in milligrams, we can use the standard dosage calculation:

Number of tablets = Required dose ÷ Dose per tablet Number of tablets = 30 mg ÷ 15 mg/tablet = 2 tablets

Step 4 — Interpret the result

The nurse should administer 2 tablets of phenobarbital to deliver the ordered 0.03 g dose.

Solution: Method 2 — Dimensional Analysis Chain

Step 1 — Set up the conversion chain

Start with the ordered dose and arrange conversion factors so unwanted units cancel out, leaving only tablets:

0.03 g × (1000 mg/1 g) × (1 tablet/15 mg) = ? tablets

Step 2 — Check unit cancellation

Watch how the units cancel: grams cancel with grams, milligrams cancel with milligrams, leaving only tablets.

0.03 [g] × (1000 [mg]/1 [g]) × (1 tablet/15 [mg]) = 0.03 × 1000 × 1 / (1 × 15) tablets

Step 3 — Calculate the final result

= (0.03 × 1000) / 15 tablets = 30 / 15 tablets = 2 tablets

This one-step calculation gives us the same answer: 2 tablets.

The nurse will administer 2 tablets

Verification

Let's verify by working backwards from our answer:

2 tablets × 15 mg/tablet = 30 mg 30 mg ÷ 1000 mg/g = 0.03 g ✓

Perfect! Two tablets deliver exactly 0.03 g of phenobarbital, matching the doctor's order.

Clinical sense check: Two tablets is a reasonable quantity for oral administration — not too many to swallow, not an unusual fraction that would require tablet splitting.

Watch Out For These

✗ Forgetting the unit conversion
Calculation: 0.03 ÷ 15 = 0.002 tablets
This treats 0.03 g as if it were 0.03 mg, resulting in a 1000-fold underdose. In clinical practice, this error could be life-threatening.

✗ Converting in the wrong direction
Calculation: 0.03 g × 0.001 = 0.00003 mg
Using "divide by 1000" instead of "multiply by 1000" when going from grams to milligrams. Remember: smaller units mean bigger numbers.

✗ Flipping the dosage formula
Calculation: 15 mg/tablet ÷ 30 mg = 0.5 tablets/mg
This gives tablets per milligram instead of total tablets needed. Always put the required dose in the numerator.

How to Spot This Problem Type

You're looking at a medication dosage calculation when you see:

"Available:" or "On hand:" followed by a concentration (mg per tablet, mg/mL, etc.)
Mixed units: The ordered dose and available concentration use different metric units
"How many [units] will the nurse administer?" This signals you need to calculate a quantity
Medical abbreviations: "po" (by mouth), "q1" (every 1 hour), "bid" (twice daily) — these indicate clinical context
Drug names: Phenobarbital, acetaminophen, digoxin — real medication names point to dosage calculations

NCLEX Alert: These problems appear frequently on nursing licensing exams. The test may include extra clinical information (route, frequency) that doesn't affect the calculation — focus on dose and concentration.

The Pattern Behind This

All medication dosage calculations follow this universal structure:

Quantity to give = (Desired dose) / (Dose per unit) × (Conversion factor)

The conversion factor equals 1 when units already match. When they don't match:

Grams to milligrams: multiply by 1000
Milligrams to micrograms: multiply by 1000
Kilograms to grams: multiply by 1000
Going the other direction: divide by 1000

The safety principle: This same calculation structure works whether you're giving tablets, liquid medications, or IV dosages. Master the pattern once, apply it everywhere.

What If?

1

Different Tablet Strength

The order is still phenobarbital 0.03 g po, but now the available tablets are 30 mg each instead of 15 mg. How many tablets should the nurse administer?

Step 1 — Convert the order to milligrams

0.03 g × 1000 mg/g = 30 mg

Step 2 — Apply the dosage formula

Number of tablets = 30 mg ÷ 30 mg/tablet = 1 tablet

Step 3 — Verify

1 tablet × 30 mg/tablet = 30 mg = 0.03 g ✓

Answer: 1 tablet

2

Reverse the Unknown

A nurse administers 4 tablets of phenobarbital. Each tablet contains 15 mg. What is the total dose administered in grams?

Step 1 — Calculate total milligrams

4 tablets × 15 mg/tablet = 60 mg

Step 2 — Convert to grams

60 mg × (1 g / 1000 mg) = 60/1000 = 0.06 g

Step 3 — Verify

0.06 g × 1000 = 60 mg ÷ 15 mg/tablet = 4 tablets ✓

Answer: 0.06 g

3

Daily Dosing Schedule

The order is phenobarbital 0.03 g po q6h (every 6 hours) for 24 hours. Available: 15 mg tablets. How many tablets total will be administered in one day?

Step 1 — Find doses per day

q6h means every 6 hours. In 24 hours: 24 ÷ 6 = 4 doses per day

Step 2 — Calculate tablets per dose

0.03 g = 30 mg. Tablets needed: 30 mg ÷ 15 mg/tablet = 2 tablets per dose

Step 3 — Find total daily tablets

2 tablets/dose × 4 doses/day = 8 tablets per day

Step 4 — Verify

8 tablets × 15 mg/tablet = 120 mg = 0.12 g total daily dose

4 doses × 0.03 g/dose = 0.12 g ✓

Answer: 8 tablets per day

4

Liquid Formulation

The order is phenobarbital 0.045 g po. The pharmacy supplies a liquid suspension with 20 mg/5 mL. How many milliliters should the nurse administer?

Step 1 — Convert order to milligrams

0.045 g × 1000 mg/g = 45 mg

Step 2 — Identify the concentration

20 mg/5 mL = 4 mg/mL

Step 3 — Apply the liquid dosage formula

Volume needed = Required dose ÷ Concentration

Volume = 45 mg ÷ 4 mg/mL = 11.25 mL

Step 4 — Verify

11.25 mL × 4 mg/mL = 45 mg = 0.045 g ✓

Answer: 11.25 mL

Frequently Asked Questions

Use the conversion factor: 1 gram = 1000 milligrams. To convert grams to milligrams, multiply by 1000. In this problem, 0.03 g × 1000 = 30 mg. Always convert to the same units before calculating tablet quantities.

Number of tablets = Required dose ÷ Dose per tablet. Both doses must be in the same units. Here, we need 30 mg and each tablet contains 15 mg, so 30 ÷ 15 = 2 tablets.

Medication errors can be life-threatening. Converting 0.03 g to 0.03 mg (instead of 30 mg) would result in a 1000-fold underdose. Always double-check unit conversions and verify that your final answer makes clinical sense.

NJ

Neven Jurkovic, PhD

Professor of Computer Science, Palo Alto College, Alamo Colleges District, San Antonio, TX

Developer of Algebrator

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This solution was prepared with AI assistance and reviewed by Dr. Jurkovic for mathematical accuracy and pedagogical clarity.

2026-08-25