Enter the child’s weight and an adult dose amount into the calculator to estimate a child dose using the selected formula. This calculator can also evaluate any of the variables when the others are known.

Clark’s Rule Calculator (Educational)

Clark’s Rule
Young’s Rule
Fried’s Rule
BSA (Mosteller)
mg/kg Dose

Enter any 2 values to calculate the missing variable

Important: Educational use only (not medical advice). Do not use this tool to dose a child or to choose a medication amount/volume. Always follow the product’s Drug Facts/label directions and maximum limits, or use dosing instructions from a clinician/pharmacist. If overdose is suspected, seek urgent medical help or contact Poison Control. Results are rounded for typical measurement; use a proper dosing device marked in mL (not kitchen spoons). Not medically reviewed. Last updated: 2025-12-31.

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 a Clark's Rule?

Clark's Rule is a medical formula used to calculate the dosage of medicine for children. It is based on the child's weight in pounds. The rule states that the child's dose is equal to the adult dose multiplied by the child's weight in pounds, divided by 150 (the average adult weight). This rule is commonly used in pharmacology, however, it's important to note that it's not applicable for all medications and situations.

How to Calculate Clark's Rule?

The following steps outline how to calculate the 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 the 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