Calculate the historical Clark’s-rule pediatric dose estimate from child weight and adult dose, or solve for adult dose or weight in metric or imperial units.

Historical formula for education only. Do not use these estimates to dose a child or choose medication amounts or volumes. Follow medication-specific labeling and clinician or pharmacist instructions.

Separate BSA comparison

Blank omits the comparison. This does not validate either estimate for a medication.

Important: Educational arithmetic only. Do not use this tool to choose or administer medication to a child. Follow drug-specific labeling and clinician or pharmacist instructions, including maximum limits. If overdose is suspected, seek urgent medical help or contact Poison Control. This calculator is not medically reviewed.

Clark’s Rule Formula

The following formula is used to estimate a child dose using Clark’s Rule.

Dc = (W × Da) / 150

Variables (classic form shown in lb and mg; this calculator converts selected units internally):

  • Dc is the child’s dosage (typically shown in mg or g)
  • W is the child’s weight (classic formula uses pounds)
  • Da is the adult dosage amount (typically shown in mg or g)

To estimate the child’s dosage using Clark’s Rule, multiply the child’s weight by the adult dosage amount. Then, divide the result by 150. The result is the dose estimate produced by this formula.

What is Clark’s Rule?

Clark’s Rule is a historical proportional model: adult amount × child weight in pounds ÷ 150 lb. The 150 lb reference is an assumption, not a measured adult weight. This calculator converts that reference exactly to 68.0388555 kg; rounded 68 kg or 70 kg conventions are different approximations. The optional separate BSA comparison uses Mosteller BSA = √(height in cm × weight in kg ÷ 3600), then adult amount × BSA ÷ an assumed adult reference of 1.73 m². Neither relationship establishes a safe or appropriate pediatric dose.

How to Calculate Clark’s Rule?

The following steps outline how to calculate Clark’s Rule.

  1. First, determine the child’s weight (W) in pounds.
  2. Next, determine the adult dosage (Da) in milligrams.
  3. Next, use the formula Dc = (W * Da) / 150 to calculate the child’s dosage (Dc).
  4. Finally, calculate Clark’s Rule.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Child’s weight (W) = 40 lbs

Adult dosage (Da) = 200 mg. For this arithmetic example, Dc = 40 × 200 / 150 = approximately 53.333333 mg. This is a historical formula result, not an instruction to administer medicine.