Dosage Calculation
Part of Pharmacy and Apothecary
Mathematical methods for determining correct medicine amounts based on patient weight, age, and concentration of the preparation.
Why This Matters
Underdosing fails the patient — the infection persists, the pain is not controlled, the fever does not break. Overdosing harms or kills. The difference between a medicine and a poison is often just the quantity given. Without reliable dosage calculation, your apothecary is guessing — and guessing kills people at a predictable rate.
Modern pharmacists have reference tables, computer software, and standardized commercial preparations whose concentrations are precisely known. You will have none of these things. You will have plants of variable potency, preparations made by hand, and patients ranging from infants to elderly adults. The mathematics of dosage is not complicated, but it must be done correctly and consistently every time.
The most dangerous moments are when someone is in pain or dying and the impulse is to give more, faster. Good calculation discipline prevents panic decisions. A practitioner who can show their work — who can say “I calculated this dose to be safe based on weight and concentration” — is also better protected from blame if an outcome is poor despite correct care.
Concentration and Strength
Before you can calculate a dose, you must know how strong your preparation is. Concentration is expressed as amount of active substance per unit volume or weight.
Weight per volume (w/v): grams per 100 mL, or milligrams per mL. A 2% solution contains 2 grams of active substance per 100 mL, or equivalently 20 mg/mL.
Weight per weight (w/w): grams per 100 grams. A 5% ointment contains 5 grams of active substance per 100 grams of ointment base.
Ratio notation: 1:100 means 1 gram per 100 mL (which is 1%). Tinctures are often expressed this way — a 1:5 tincture has 1 gram of herb per 5 mL of liquid.
When you make a preparation, calculate and record its concentration. Example: you extracted 15 grams of dried willow bark in 150 mL of water. Your decoction concentration is 15g/150mL = 0.1 g/mL = 100 mg/mL.
Basic Dose Calculation
The fundamental formula:
Volume to give = Desired dose ÷ Concentration
Example: You want to give 500 mg of willow bark extract. Your decoction is 100 mg/mL. Volume = 500 mg ÷ 100 mg/mL = 5 mL
Always check: does this volume make practical sense? 5 mL is a teaspoon — reasonable. If your calculation produces 200 mL for a single dose, check your math and your concentration.
For ointments and salves, you rarely calculate by volume. Instead, you standardize application: “apply a layer 1-2 mm thick to the affected area.” The dose is controlled by the concentration of the preparation and the surface area treated.
Weight-Based Dosing
Many medicines have doses expressed per kilogram of body weight (mg/kg). This is especially critical for children. To apply a weight-based dose:
- Determine patient weight (estimate if scale unavailable — see Weight-Based Dosing)
- Multiply weight by dose per kg
- Divide by concentration to get volume
Example: Dose is 10 mg/kg. Patient weighs 20 kg. Concentration is 50 mg/mL.
- Required dose = 10 mg/kg × 20 kg = 200 mg
- Volume = 200 mg ÷ 50 mg/mL = 4 mL
Children are not small adults. Weight-based dosing accounts for the fact that a 10 kg child needs proportionally less total drug than a 70 kg adult, but often needs a higher dose per kg due to faster metabolism. Always verify pediatric doses against your pharmacopoeia tables.
Age-Based Approximations
When you cannot weigh a patient, age-based rules provide estimates. These are less precise than weight-based dosing but far better than guessing.
Young’s Rule (for children 2-12): Child dose = (Age in years ÷ (Age + 12)) × Adult dose
Example: Adult dose is 500 mg. Child is 6 years old. Child dose = (6 ÷ 18) × 500 mg = 167 mg
Clark’s Rule (weight-based approximation): Child dose = (Child weight in lbs ÷ 150) × Adult dose
Fried’s Rule (for infants under 2): Infant dose = (Age in months ÷ 150) × Adult dose
These rules are approximations. They work reasonably well for typical patients but break down for very ill, malnourished, or developmentally unusual patients.
Frequency and Total Daily Dose
A dose must be specified not just as an amount but as a frequency. The total daily dose (TDD) is amount per dose multiplied by number of doses per day.
| Frequency | Abbreviation | Times per Day | Hours Between Doses |
|---|---|---|---|
| Once daily | qd | 1 | 24 |
| Twice daily | bid | 2 | 12 |
| Three times daily | tid | 3 | 8 |
| Four times daily | qid | 4 | 6 |
| Every 6 hours | q6h | 4 | 6 |
| Every 8 hours | q8h | 3 | 8 |
Many natural medicines have narrow therapeutic windows — the gap between an effective dose and a toxic dose is small. For these, maintain consistent dosing intervals. A q8h medicine should genuinely be taken at 8-hour intervals, not “morning, noon, and night” if those are only 5 hours apart and then 14 hours apart.
Adjustments for Special Populations
Elderly patients: Generally need reduced doses. As a rule of thumb, reduce adult dose by 25-30% for patients over 65. Elderly patients have reduced kidney and liver function, which means drugs clear more slowly and accumulate.
Severely malnourished patients: Also need reduced doses. Malnourished patients have less protein in their blood, which means more “free” drug circulates and affects the body.
Pregnant patients: Many medicines are harmful to the fetus. When in doubt, do not treat pregnant patients with potent medicinal preparations unless the condition itself threatens her life. Default to safest possible interventions.
Patients with visible jaundice (yellow skin/eyes): Liver function is compromised. Reduce doses of any preparation metabolized by the liver.
Patients with swollen ankles and reduced urine output: Kidney function may be compromised. Reduce doses of anything cleared by the kidneys.
Worked Calculation Examples
Example 1: Simple oral dose Preparation: elderberry syrup, 200 mg of extract per 5 mL (40 mg/mL) Required dose: 400 mg three times daily Volume per dose: 400 ÷ 40 = 10 mL (two teaspoons) Total daily dose: 1,200 mg in 30 mL
Example 2: Pediatric calculation Patient: 8-year-old child, estimated 25 kg Medicine: willow bark decoction, 80 mg/mL Adult dose: 1,000 mg every 8 hours Young’s Rule child dose: (8 ÷ 20) × 1,000 = 400 mg Volume per dose: 400 ÷ 80 = 5 mL Give 5 mL every 8 hours.
Example 3: Cross-checking with weight-based Same patient, 25 kg Standard dose range: 10-15 mg/kg per dose Dose range: 250-375 mg Young’s Rule gave 400 mg — slightly above weight-based range. Use 350 mg = 4.4 mL. Round to 4.5 mL for practical measurement.
When two methods give similar results, you have confidence. When they diverge widely, use the lower estimate and monitor the patient carefully.
Recording Calculations
Every calculation should be written in the dispensing log or patient record. Show your work. Write the formula used, the numbers plugged in, and the result. This takes thirty extra seconds and serves three purposes: it catches arithmetic errors, it allows another practitioner to verify your work, and it documents that you calculated rather than guessed.
Never perform dosage calculations from memory alone when dealing with potent or dangerous medicines. Write it out each time.