Pharmacy Math Formula Sheet
Every calculation on the exam, on one page. Each formula links to the lesson that teaches it. Print it, or save it as a PDF for the night before.
Unit conversionslearn →
- Metric: 1 kg = 1000 g · 1 g = 1000 mg · 1 mg = 1000 mcg · 1 L = 1000 mL
- Weight: 1 kg = 2.2 lb · 1 lb = 454 g · 1 oz = 28.4 g
- Volume: 1 tsp = 5 mL · 1 tbsp = 15 mL · 1 fl oz = 30 mL · 1 pt = 480 mL · 1 gal = 3840 mL
- Apothecary: 1 grain (gr) = 65 mg
- °F = (°C × 9/5) + 32 · °C = (°F − 32) × 5/9e.g. 37 °C → (37 × 9/5) + 32 = 98.6 °F
Days supply & quantitylearn →
- Days supply = total quantity ÷ amount used per daye.g. 60 tabs, 1 tab TID → 60 ÷ 3 = 20 days
- Quantity to dispense = amount per day × dayse.g. 5 mL TID × 10 days = 150 mL
- Insulin: U-100 = 100 units/mL (10 mL vial = 1000 units)
Weight-based dosinglearn →
- Dose = mg/kg × weight in kg (convert lb → kg by ÷ 2.2)e.g. 25 mg/kg × (44 ÷ 2.2 = 20 kg) = 500 mg/day
- Per dose = daily dose ÷ number of doses per daye.g. 500 mg/day ÷ 3 ≈ 167 mg/dose
Concentration & strengthlearn →
- % w/v = g per 100 mL · % w/w = g per 100 g · % v/v = mL per 100 mL
- 1% = 1 g/100 mL = 10 mg/mLe.g. 500 mL of D5W (5%) = 5 g/100 mL × 500 = 25 g
- Ratio strength 1:N = 1 g in N mL → as %: (1 ÷ N) × 100e.g. 1:200 → (1 ÷ 200) × 100 = 0.5%
- Percent error = |actual − desired| ÷ desired × 100
Dilution & alligationlearn →
- Dilution: C1 × V1 = C2 × V2e.g. 2% × 500 mL from 10% → V1 = (2 × 500) ÷ 10 = 100 mL, qs to 500
- Alligation: parts of each strength = difference across the diagonale.g. 70% & 20% → 40%: parts 20 : 30 = 2 : 3
- Specific gravity = weight (g) ÷ volume (mL) (water = 1)
IV flow & infusion rateslearn →
- Flow rate (mL/hr) = total volume ÷ time in hourse.g. 1000 mL ÷ 8 hr = 125 mL/hr
- Drip rate (gtt/min) = (volume mL × drop factor gtt/mL) ÷ time in minutese.g. (125 × 15) ÷ 60 ≈ 31 gtt/min
- Infusion time (hr) = total volume ÷ flow ratee.g. 1000 mL ÷ 125 mL/hr = 8 hr
Round only at the final step. Carry units through every line so they cancel — the units are your error-check.