PTCB Guide to Clark’s Rule and Young’s Rule.

Clark’s Rule and Young’s Rule as a PTCB Exam Topic.

Clark’s rule and Young’s rule are formulae used to calculate pediatric doses based on the standard adult dose. For the PTCB exam, pharmacy technicians are required to understand when these rules are used and how to apply them correctly in dosage calculations.

Both rules estimate a child’s dose as a fraction of the adult dose, but they use different patient factors:

• Clark’s rule is based on the child’s weight (in pounds)
• Young’s rule is based on the child’s age (in years)

These two formulae were developed to help adjust adult doses for pediatric patients, since children generally require smaller doses due to lower body mass and differences in drug distribution and metabolism.

Clark’s rule is considered more precise than Young’s rule because weight is typically a better indicator of appropriate dosing than age alone. However, both methods are exam-relevant and may appear in PTCB math questions where you are given the adult dose and asked to calculate the appropriate pediatric dose.

It’s important to note that in modern clinical practice, weight-based dosing in kilograms (mg/kg) is preferred for accuracy and safety. Still, Clark’s rule and Young’s rule remain important for exam preparation and foundational pharmacy math knowledge, as they demonstrate how pediatric doses are adjusted to ensure safe and appropriate therapy.

Are these two math rules tested under the new 2026 syllabus?

Yes.

Both Clark’s rule and Young’s rule are examinable formulae under the new 2026 exam outline. Pharmacy technicians are not required to apply these formulae to complex examples.

Only standard applications of each formula can be tested.

Math formulae are tested under section 4.1 of the Order Entry and Processing knowledge domain.

Clark’s Rule.

Clark’s rule is written as:

Child’s dose = Adult dose × (Weight of child in pounds ÷ 150)

In simple terms, you multiply the standard adult dose by the child’s weight (in pounds) divided by 150. One of the most important exam tips: the child’s weight must be in pounds (lbs). If the PTCB question provides weight in kilograms, you must first convert kilograms to pounds using this formula:

kg × 2.2 = pounds

Practice Example 1.
Using Clark’s rule, what is the dose for a 12-year-old girl who weighs 31.7 kg if the average adult dose is 500 mg?

Step 1: Convert kilograms to pounds

31.7 kg × 2.2 = 69.74 lbs
Round appropriately → 70 lbs

Step 2: Apply Clark’s rule

Child’s dose = 500 mg × (70 ÷ 150)
Child’s dose = 500 mg × 0.47
Child’s dose = 235 mg

Young’s Rule.

Remember, the key difference between Clark’s rule and Young’s rule is that Clark’s rule uses weight whereas Young’s rule relates to age. A helpful memory tip: “Young” relates to age — so if you see Young’s Rule, think years.

The formula for Young’s rule is:

Child’s dose = Adult dose × [Age ÷ (Age + 12)]

This formula estimates the child’s dose as a fraction of the adult dose using the child’s age in years.

Practice Example 2.
To determine the appropriate dose for a 9-year-old child weighing 63 lbs, the adult dose of the medicine is 200 mg. Using Young’s rule, what dose should be administered?

Note that even though the child’s weight is given, it is not needed for Young’s Rule. This is a common exam trap.

Step 1: Set up the formula

Child’s dose = 200 mg × [9 ÷ (9 + 12)]

Step 2: Complete the denominator

Child’s dose = 200 mg × (9 ÷ 21)

Child’s dose = 200 mg × 0.43

Child’s dose = 86 mg

Even though Clark’s rule and Young’s rule are comparatively straightforward calculations, it’s important to keep these three exam tips in mind:

• Do not use extra information just because it is provided. If you are asked to use Young’s rule, then ignore weight.
• Avoid mixing up the two rules. If the formula includes weight in pounds divided by 150, that’s Clark’s rule. If it includes age divided by (age + 12), that’s Young’s rule.
• Work step by step. Even if the math looks simple, performing one calculation at a time reduces errors, especially under exam pressure.

PTCB Sample Test Questions.

Below you can find five sample PTCB questions on these two formulae.

As usual, you can find the answer key to all five questions in the section below, along with all answer explanations. Before that, take the time to practice these questions for yourself.

To replicate exam-like conditions, you may refer to the equations above if you need to.

Question 1.

The adult dose of a medicine is 400 mg. Using Clark’s rule, what is the appropriate dose for a child who weighs 55 lbs?

a. 110 mg

b. 147 mg

c. 200 mg

d. 275 mg

Question 2.

The adult dose of a medicine is 300 mg. Using Young’s rule, what dose should be given to a 6-year-old child?

a. 75 mg

b. 90 mg

c. 100 mg

d. 120 mg

Question 3.

The usual adult dose of amoxicillin is 500 mg per dose. Using Clark’s rule, what is the appropriate dose for a child who weighs 44 lbs?

a. 110 mg

b. 147 mg

c. 220 mg

d. 330 mg

Question 4.

The adult dose of ibuprofen is 400 mg. Using Young’s rule, what dose should be given to an 8-year-old child?

a. 100 mg

b. 120 mg

c. 160 mg

d. 200 mg

Question 5.

In the pharmacy, you are provided a child’s weight in pounds and instructed to calculate the pediatric dose based on a fraction of the adult dose using weight. Which rule must be applied?

a. Young’s rule

b. Clark’s rule

c. both rules may be used

d. not enough information is supplied

Answer Explanations.

Answer to Question 1 – b. 147 mg

Child’s dose = Adult dose × (Weight in lbs ÷ 150)

= 400 mg × (55 ÷ 150)

= 400 mg × 0.37

= 147 mg

Therefore, the appropriate pediatric dose is 147 mg.

Answer to Question 2 – c. 100 mg

Using Young’s rule:

Child’s dose = Adult dose × [Age ÷ (Age + 12)]

= 300 mg × [6 ÷ (6 + 12)]

= 300 mg × (6 ÷ 18)

= 300 mg × 0.33

= 100 mg

Therefore, the appropriate dose for a 6-year-old child is 100 mg.

Answer to Question 3 – b. 147 mg

Using Clark’s rule:

Child’s dose = Adult dose × (Weight in lbs ÷ 150)

= 500 mg × (44 ÷ 150)

= 500 mg × 0.293

= 146.5 mg

Rounded up, the dose is 147 mg.

Answer to Question 4 – c. 160 mg

Using Young’s rule:

Child’s dose = Adult dose × [Age ÷ (Age + 12)]

= 400 mg × [8 ÷ (8 + 12)]

= 400 mg × (8 ÷ 20)

= 400 mg × 0.4

= 160 mg

Therefore, the appropriate dose for an 8-year-old child is 160 mg.

Answer to Question 5 – b. Clark’s rule

Clark’s rule is the pediatric dosing formula that uses weight in pounds to calculate a child’s dose as a fraction of the adult dose. Young’s rule uses age in years, not weight. Therefore, when a question specifies weight in pounds, Clark’s rule is the correct method.

PTCB Cheat Sheet – Clark & Young’s Rule.

In review, we learned:

Clark’s rule

• Purpose: to calculate pediatric dose based on weight (lbs).
• Formula:
  Child’s dose = Adult dose × (Weight in lbs ÷ 150)
• Units: weight must be in pounds; convert kg → lbs using: kg × 2.2 = lbs
• Target: best for children 2–17 years old.
• Example: adult dose = 500 mg, child weight = 55 lbs
  Child dose = 500 × (55 ÷ 150) = 500 × 0.37 = 185 mg
• Common exam trap: forgetting to convert kilograms to pounds.

Young’s rule

• Purpose: to calculate pediatric dose based on age (years).
• Formula:
  Child’s dose = Adult dose × [Age ÷ (Age + 12)]
• Key tip: only age is used; ignore weight even if provided in the question
• Example: adult dose = 300 mg, child age = 6 years
  Child dose = 300 × (6 ÷ (6 + 12)) = 300 × (6 ÷ 18) = 300 × 0.33 ≈ 100 mg
• Memory trick: “Young” → think age

PTCB exam tips

• Do not mix rules: weight → Clark’s rule; age → Young’s rule.
• Step-by-step approach: perform one step at a time to avoid errors.
• Unit consistency: always check if weight is in kg → convert to lbs for Clark’s Rule.
• Rounding: round doses appropriately according to pharmacy practice and PTCB guidelines (if it’s 0.5 or higher, then always round up).

Hope you found today’s guide helpful on Clark’s rule and Young’s rule. Master math on the PTCB exam by accessing our dedicated calculations module – everything you need to succeed at math. Check back to our blog soon for more exclusive content to help you study for your pharmacy technician test.