Unit Conversion
Overview
Unit conversion is the first step of nearly every dosage calculation: the prescriber writes one unit and the pharmacy supplies another, and no formula works until both sides match. The metric ladder moves in steps of 1,000 — 1 g = 1,000 mg and 1 mg = 1,000 mcg — and volume follows the same pattern with 1 L = 1,000 mL. Two other equivalences appear constantly on the floor: 1 mL equals 1 cc, and 1 kg equals 2.2 lb. Because the metric steps are thousand-fold, a conversion done in the wrong direction is not a small error; it is a thousand times the intended dose.
Interpretation
Technique
Worked conversion: 0.5 g ordered, 250 mg tablets supplied
- Name both unitsorder in g, supply in mg
- Pick the directiong to mg is a smaller unit
- Multiply by 1,0000.5 x 1,000 = 500 mg
- Sanity-checkthe number got bigger, as it should
- Then calculate500 / 250 = 2 tablets
During — Monitoring
Patient Teaching
Clinical Pearl
Smaller unit, bigger number. If you converted grams to milligrams and the number got smaller, you divided when you should have multiplied.