Dosage Calculations: From Simple to Advanced

Basic Tablet Dosage

• Formula: \text{Dose} = \left(\frac{\text{Desired Dose}}{\text{Available Dose}}\right) \times \text{Quantity}
• Example:
  • Order: 500 mg
  • Available: 250 mg tablets
  • Calculation:
    \text{Dose} = \left(\frac{500\,\text{mg}}{250\,\text{mg}}\right) \times 1\,\text{tablet} = 2\,\text{tablets}
  • Answer: Administer 2 tablets
• Important notes:
  • Ensure units match (mg vs mg, etc.)
  • If the calculation yields a non-integer number of tablets, follow clinic/protocol guidance for rounding or splitting tablets

Liquid Medication Dosage

• Formula: \text{Volume} = \left(\frac{\text{Desired Dose}}{\text{Available Dose}}\right) \times \text{Volume Available}
• Example:
  • Order: 75 mg
  • Available: 100 mg per 5 mL
  • Calculation:
    \text{Volume} = \left(\frac{75\,\text{mg}}{100\,\text{mg}}\right) \times 5\,\text{mL} = 3.75\,\text{mL}
  • Answer: Administer 3.75 mL
• Notes:
  • Always confirm the same mass unit (mg) before applying the ratio

Weight-Based Dosage

• Formula: \text{Dose} = \text{Weight (kg)} \times \text{Dose per kg}
• Example:
  • Order: 5 mg/kg
  • Patient Weight: 18 kg
  • Calculation:
    18\,\text{kg} \times 5\,\frac{\text{mg}}{\text{kg}} = 90\,\text{mg}
  • Answer: Administer 90 mg
• Notes:
  • Check weight units; use kg for weight in the calculation

IV Drip Rate (mL/hr)

• Formula: \text{Rate} = \frac{\text{Total Volume (mL)}}{\text{Time (hr)}}
• Example:
  • Order: 1000 mL over 8 hours
  • Calculation:
    \frac{1000\,\text{mL}}{8\,\text{hr}} = 125\,\text{mL/hr}
  • Answer: Set IV pump to 125 mL/hr
• Notes:
  • Use consistent time units; convert hours to minutes only when needed for other formulas

IV Drip Rate (gtt/min)

• Formula: \text{Rate} = \frac{\text{Volume} \times \text{Drop Factor}}{\text{Time (min)}}
• Example:
  • Order: 500 mL over 4 hours
  • Drop Factor: 15 gtt/mL
  • Time: 4 hours = 240 min
  • Calculation:
    \text{Rate} = \frac{500\,\text{mL} \times 15\,\frac{\text{gtt}}{\text{mL}}}{240\,\text{min}} = 31.25\,\text{gtt/min}
  • Answer: Round to 31 gtt/min
• Notes:
  • Round to the nearest whole drop as per policy
  • Ensure correct drop factor (gtt/mL) for the IV set

Advanced: Unit Conversion

• Example:
  • Order: 0.5 g
  • Available: 250 mg tablets
• Conversion:
  • Convert grams to milligrams: 0.5\,\text{g} = 500\,\text{mg}
  • Determine tablets: \frac{500\,\text{mg}}{250\,\text{mg}} = 2\,\text{tablets}
• Answer: Administer 2 tablets
• Steps (summary):
  • Convert all units to a common base (e.g., mg)
  • Apply the tablet-dose formula using the converted values
  • Report final quantity with appropriate unit

Tips for Success

• Always double-check units (mg vs g, mL vs L)
• Round carefully—especially for IV rates
• Use a calculator for precision
• Know your drop factors (common: 10, 15, 20 gtt/mL)
• Remember to check for consistency in dose form (tablet vs liquid) before calculating

Real-world connections & principles

• Dosage calculations rely on proportional reasoning and unit cancellation
• Dimensions must align (units must reduce to the desired output unit)
• Small errors in rounding IV rates can have clinical impact; follow local rounding policies
• This content supports safe prescribing, administration, and monitoring in clinical settings