Dosage Calculator
From the prescriber order (e.g. 25 mg/kg/day)
Enter the mg/mL of the liquid medication to calculate volume per dose
Enter mg per tablet to calculate tablet count per dose
Enter maximum allowed single dose to check for overdose risk
Click to auto-fill concentration field
Enter Dosage Information
Fill in the required fields to calculate the correct medication dose with a full step-by-step breakdown.
How to Use the Dosage Calculator
Select Your Calculation Mode
Choose from four calculation modes using the tabs at the top: Weight-Based for mg/kg prescriptions, D/H/Q for the nursing Desired/Have/Quantity formula, Pediatric Rules for calculating a child's dose from an adult reference dose, or IV Drip Rate for gravity infusion calculations. Each mode shows only the fields relevant to that calculation method.
Enter the Required Inputs
Fill in the required fields for your chosen mode. For Weight-Based mode, enter the patient's weight, the prescribed dose per kg, and the frequency. Optionally enter the liquid concentration in mg/mL or tablet strength in mg to get volume or tablet count outputs. Use the Quick-Fill preset buttons to auto-fill common medication concentrations for acetaminophen, ibuprofen, amoxicillin, and diphenhydramine.
Review the Step-by-Step Calculation
Results appear instantly as you type. The step-by-step breakdown shows the formula used, every intermediate calculation value, and the final result. For weight-based dosing, you will see single dose, daily total, volume per dose, and tablet count where applicable. For pediatric rules, the child-to-adult dose ratio is shown visually as a comparison bar. Check any warnings that appear — such as tablet fraction alerts or max dose exceeded flags.
Export or Print Your Results
Use the Export CSV button to download a record of your calculation for documentation purposes. Use the Print Results button to generate a clean, print-optimized view suitable for bedside reference. All calculations run locally in your browser — no data is stored or transmitted. Always verify results with a licensed pharmacist or physician before administration.
Frequently Asked Questions
What is the D/H/Q formula and when do I use it?
The Desired/Have/Quantity formula — also written as D/H × Q — is the standard nursing calculation method for determining how much of an available medication to give when the dose on hand is different from the ordered dose. D is the desired dose (what was ordered), H is the have dose (the strength of the medication you have), and Q is the quantity on hand (the volume in mL or the tablet count for that H dose). The formula calculates the amount to administer. For example: order is 750 mg, you have 500 mg per 5 mL, so you give (750 ÷ 500) × 5 = 7.5 mL. Use this method any time the available drug strength does not exactly match the ordered dose, which is the majority of real-world clinical situations.
What is the difference between Clark's Rule, Young's Rule, and Fried's Rule?
These three formulas are historical methods for estimating a child's medication dose from a known adult reference dose. Clark's Rule uses the child's weight in pounds divided by 150 (the assumed average adult weight), making it the most pharmacologically sound of the three since drug distribution correlates better with weight than age. Young's Rule uses the child's age in years divided by age plus twelve, and is appropriate for children between two and twelve years old. Fried's Rule uses the child's age in months divided by 150 and is intended for infants under two years of age. All three are approximations with significant uncertainty margins. In modern clinical practice, weight-based mg/kg dosing derived from pediatric pharmacokinetic studies is the preferred and more accurate approach. These rules are primarily taught as foundational concepts and used only when weight-based data is unavailable.
How do I calculate IV drip rate for a gravity infusion?
For a gravity IV infusion without a pump, the drip rate in drops per minute equals the volume to infuse in milliliters multiplied by the drop factor in drops per mL, divided by the infusion time in minutes. The drop factor is printed on the IV tubing packaging and varies by tubing type: 10, 15, or 20 drops per mL for standard macrodrip adult tubing, and 60 drops per mL for microdrip tubing used in pediatric and critical care settings. For example: infusing 500 mL over 60 minutes with 20 gtts/mL tubing gives 500 × 20 ÷ 60 = 167 drops per minute, which is then rounded to 167 or adjusted for the nearest whole drop count. IV drip rate calculations should always be verified by qualified nursing staff before starting the infusion.
What does the BSA method calculate and why is it more accurate for children?
Body Surface Area dosing uses the patient's height and weight to calculate body surface area in square meters, then scales the adult dose proportionally. The Mosteller formula used here is BSA in square meters equals the square root of height in centimeters times weight in kilograms divided by 3600. A standard adult BSA of 1.73 square meters serves as the reference. The child's dose equals the adult dose times the child's BSA divided by 1.73. BSA correlates more closely with many pharmacokinetic parameters — including drug clearance, volume of distribution, and cardiac output — than weight alone. This makes BSA dosing especially important for chemotherapy agents, where therapeutic windows are narrow and overdose risks are severe. For most routine pediatric medications, weight-based mg/kg dosing is sufficient and simpler to apply.
What does the tablet fraction warning mean?
When the calculated number of tablets per dose is not a whole number or a clean half-tablet (0.5), this calculator displays a warning. Most solid oral medications can only be split in half if they are scored tablets, and even scored tablets vary in how evenly they split. If a calculated dose requires 1.33 tablets, for example, achieving an accurate dose from solid tablets is not reliably possible. In these situations, the clinician or pharmacist should consider whether a liquid formulation is available, whether the nearest whole or half-tablet dose is therapeutically acceptable, or whether the prescriber should adjust the order. The warning is intended to prevent silent rounding errors that can lead to underdosing or overdosing. Always confirm non-standard tablet requirements with a licensed pharmacist.
Can I use this calculator for pediatric dosing in clinical settings?
This calculator is designed as an educational and reference tool to support — not replace — clinical judgment and institution-specific protocols. While the formulas implemented (weight-based mg/kg, Clark's, Young's, Fried's, BSA Mosteller) are standard and widely published, medication dosing in clinical settings requires verification against current drug references such as the Harriet Lane Handbook, Pediatric Dosage Handbook, or current institutional formulary, double-check by a second licensed professional, consideration of the specific patient's renal and hepatic function, allergy status, current medications, and clinical condition. This calculator performs the arithmetic correctly but cannot account for patient-specific clinical factors. It is appropriate for educational practice, checking your manual calculations, and as a bedside reference to verify arithmetic — always with a licensed pharmacist or physician as the final authority.