Dosage Calculator - Free Online Tool

Calculate medication doses by weight, nursing D/H/Q method, pediatric rules, or IV drip rate — with step-by-step breakdowns

Welcome to our free Dosage Calculator, a comprehensive medication calculation tool designed for healthcare students, nurses, pharmacists, caregivers, and anyone who needs to verify drug dosing with confidence. This calculator covers four essential clinical calculation methods in one interface: weight-based dosing (the standard mg/kg approach), the Desired/Have/Quantity nursing formula, pediatric dosing rules including Clark's Rule, Young's Rule, Fried's Rule, and the BSA method, plus IV drip rate calculation for gravity infusions. All calculations run entirely in your browser — no data is ever sent to any server. Weight-based dosing is the most common method used in both pediatric and adult clinical settings. A prescriber orders a drug at a specific number of milligrams per kilogram of body weight (mg/kg), and the dose is then individualized to the patient's actual weight. For example, if amoxicillin is prescribed at 25 mg/kg/day divided into three doses, and the patient weighs 20 kg, the daily dose is 500 mg and each single dose is approximately 167 mg. This calculator handles the full chain: weight times dose per kg equals single dose, single dose times daily frequency equals daily total, and if a liquid concentration is entered, the volume per dose is calculated automatically. Tablet counts work the same way when tablet strength is entered. The Desired/Have/Quantity method — also known as D/H/Q or the fraction method — is a fundamental nursing calculation technique taught in every nursing school and used at the bedside every day. The formula is simple: the amount to administer equals the desired dose divided by the dose on hand, multiplied by the quantity on hand. For example, if the order is for 500 mg and the available tablets are 250 mg each, the patient receives two tablets. For liquid medications, if the order is 150 mg and the available concentration is 125 mg per 5 mL, the patient receives 6 mL. This method works for tablets, capsules, and liquid medications alike. Pediatric dosing from adult reference doses uses approximation rules that are especially useful when weight-based prescribing data is unavailable or when a reference dose for adults is known. Clark's Rule is weight-based and the most accurate of the three approximation methods: child dose equals adult dose multiplied by child weight in pounds divided by 150 (the assumed average adult weight in pounds). In metric, the divisor is 68 kg. Young's Rule uses the child's age in years and is appropriate for children between two and twelve years old: child dose equals adult dose times age divided by age plus twelve. Fried's Rule is used for infants under two years: child dose equals adult dose times age in months divided by 150. The BSA method using the Mosteller formula is the most precise approach and is required for chemotherapy and other high-stakes medications — it calculates body surface area from height and weight, then scales the adult dose proportionally using a 1.73 square meter reference for an average adult. IV drip rate calculation is essential in settings where electronic infusion pumps are not available — including resource-limited clinics, home infusion settings, and emergency situations. The formula is: drops per minute equals volume in milliliters times drop factor in drops per milliliter, divided by infusion time in minutes. Drop factors vary by IV tubing type: microdrip tubing designed for pediatric or critical care delivers 60 drops per mL, while standard macrodrip adult tubing comes in 10, 15, or 20 drops per mL variants. The correct drop factor is printed on the IV tubing packaging. Safety is built into every aspect of this calculator. Tablet fraction warnings alert you when a calculated tablet count is not a whole number or a clean half-tablet, prompting a check with the pharmacist before administering. For pediatric rules, the calculator includes educational notes about the limitations of each method — Clark's, Young's, and Fried's rules are pharmacological approximations, not precise clinical standards. The weight-based mg/kg method is the current clinical standard for pediatric dosing. BSA-based dosing is the gold standard for oncology and certain antibiotics. Quick-fill medication presets allow you to auto-fill common OTC drug concentrations with a single click, covering acetaminophen, ibuprofen, amoxicillin, and diphenhydramine in both pediatric liquid and adult tablet forms. These presets are provided as reference values only and do not constitute dosing recommendations. All results include a step-by-step calculation breakdown showing the formula, intermediate values, and final answer — making this tool ideal for nursing students practicing dimensional analysis, pharmacy students checking their work, and healthcare professionals who want to double-verify a calculation before administration. A prominent medical disclaimer is displayed with every result: this calculator is for educational and reference purposes only. Always verify dosage calculations with a licensed pharmacist or physician before administering any medication to any patient.

Understanding Medication Dosage Calculation

Accurate drug dosing is one of the most critical skills in healthcare. Whether calculating a weight-based pediatric dose, using the nursing D/H/Q formula, or setting an IV drip rate, understanding the underlying math prevents medication errors.

Weight-Based Dosing: The Clinical Standard

Weight-based dosing (mg/kg) is the most accurate and widely used method for individualizing drug doses. It accounts for differences in body mass that affect drug distribution and clearance. The prescriber orders a dose in mg/kg/day or mg/kg/dose, and you multiply by the patient's weight in kilograms to get the actual dose. For liquid medications, dividing the calculated dose in mg by the concentration in mg/mL gives the volume to administer in mL. This method is standard practice in pediatrics and for many adult medications where therapeutic ranges are narrow.

The D/H/Q Nursing Formula

The Desired/Have/Quantity formula — sometimes called the fraction method — is a systematic approach to calculating how much of an available medication to give when the dose on hand differs from the ordered dose. Desired is the prescribed dose, Have is the drug strength available, and Quantity is the amount (volume or count) of the available strength. The formula Always to Administer equals Desired divided by Have, multiplied by Quantity. This formula works for tablets, capsules, and liquid medications. Double-checking by cross-multiplication or dimensional analysis is recommended for safety.

Pediatric Dosing Rules: Approximations and Their Limits

Clark's Rule, Young's Rule, and Fried's Rule are historical methods for estimating a child's dose from an adult reference dose when weight-based prescribing information is not available. Clark's Rule (weight in lbs divided by 150) is the most accurate because weight correlates best with pharmacokinetic parameters. Young's Rule (age divided by age plus 12) is useful when weight is unknown. Fried's Rule (age in months divided by 150) applies to infants under two years. All three are approximations with significant uncertainty — modern clinical practice strongly prefers weight-based mg/kg dosing for children.

IV Drip Rates and Drop Factors

Gravity IV infusions require calculating the drip rate — how many drops per minute fall from the IV bag into the drip chamber. The formula is: drip rate in drops per minute equals volume times drop factor divided by time in minutes. The drop factor depends on the IV tubing: 60 drops per mL for microdrip (pediatric/critical care), and 10, 15, or 20 drops per mL for standard macrodrip tubing. Always verify the drop factor printed on the specific tubing being used. In pump-driven settings, the equivalent calculation is mL per hour, which equals total volume divided by total infusion time in hours.

Dosage Calculation Formulas

Weight-Based Dose

Dose (mg) = Patient Weight (kg) × Dose per kg (mg/kg)

The standard clinical method for individualizing drug doses. Multiply the patient's weight in kilograms by the prescribed mg/kg dose to get the actual dose in milligrams.

D/H/Q Nursing Formula

Amount to Administer = (Desired Dose / Have Dose) × Quantity on Hand

The fundamental nursing calculation for determining how much of an available medication to give when the dose on hand differs from the ordered dose. Works for tablets, capsules, and liquid medications.

IV Drip Rate

Drip Rate (gtts/min) = (Volume (mL) × Drop Factor (gtts/mL)) / Time (minutes)

Calculates the drops per minute for gravity IV infusions. The drop factor depends on tubing type: 10, 15, or 20 gtts/mL for macrodrip, 60 gtts/mL for microdrip.

BSA Dose (Mosteller)

BSA (m²) = √(Height(cm) × Weight(kg) / 3600); Child Dose = Adult Dose × (BSA / 1.73)

Uses body surface area calculated via the Mosteller formula to scale an adult dose proportionally. The gold standard for chemotherapy and narrow therapeutic index medications.

Dosing Reference Data

Common Pediatric Medication Dosages

Standard weight-based dosing for frequently prescribed pediatric medications. Always verify against current formulary and adjust for renal/hepatic function.

Medication	Dose (mg/kg/dose)	Frequency	Max Single Dose
Acetaminophen	10–15 mg/kg	q4–6h	1,000 mg
Ibuprofen	5–10 mg/kg	q6–8h	400 mg
Amoxicillin (standard)	25 mg/kg	q8h (TID)	500 mg
Amoxicillin (high-dose)	45 mg/kg	q12h (BID)	1,000 mg
Azithromycin	10 mg/kg day 1, then 5 mg/kg	qD × 5 days	500 mg day 1
Diphenhydramine	1–1.25 mg/kg	q6h	50 mg
Prednisolone	1–2 mg/kg	qD or BID	60 mg

Pediatric Dosing Rules Comparison

Historical methods for estimating child doses from adult reference doses. Weight-based mg/kg dosing is the modern standard and should be used whenever available.

Rule	Formula	Age Range	Accuracy
Clark's Rule	Child Dose = Adult Dose × (Weight lbs / 150)	All ages	Most accurate (weight-based)
Young's Rule	Child Dose = Adult Dose × Age / (Age + 12)	2–12 years	Moderate (age-based)
Fried's Rule	Child Dose = Adult Dose × Age in months / 150	< 2 years	Least accurate (infant only)
BSA Method	Child Dose = Adult Dose × BSA / 1.73	All ages	Gold standard for chemo

Worked Examples

Amoxicillin Dose for a 20kg Child at 25 mg/kg/dose

A 6-year-old child weighing 20 kg is prescribed amoxicillin at 25 mg/kg/dose TID (three times daily). Available suspension is 250 mg/5 mL.

Calculate single dose: 20 kg × 25 mg/kg = 500 mg per dose

Calculate daily total: 500 mg × 3 doses = 1,500 mg/day

Calculate volume per dose: 500 mg / (250 mg/5 mL) = 500 / 50 = 10 mL per dose

Check max dose: 500 mg is at the max single dose limit — verify with prescriber if appropriate

Give 10 mL of amoxicillin 250 mg/5 mL suspension three times daily (every 8 hours) for a total of 1,500 mg/day. Each dose delivers 500 mg.

IV Drip Rate for 1,000 mL Over 8 Hours

A patient is ordered 1,000 mL of normal saline to be infused over 8 hours via gravity IV. The macrodrip tubing has a drop factor of 20 gtts/mL.

Convert time to minutes: 8 hours × 60 = 480 minutes

Apply the drip rate formula: (1,000 mL × 20 gtts/mL) / 480 min

Calculate: 20,000 / 480 = 41.7 gtts/min

Round to nearest whole drop: 42 gtts/min

Also express as flow rate: 1,000 mL / 8 hours = 125 mL/hr

Set the IV drip rate to approximately 42 drops per minute (125 mL/hr). Count drops in the drip chamber for 15 seconds and multiply by 4 to verify the rate. Recheck every 1–2 hours.

D/H/Q Calculation for Liquid Medication

Order: 750 mg of medication. Available: 500 mg per 5 mL liquid suspension.

Desired (D) = 750 mg

Have (H) = 500 mg

Quantity (Q) = 5 mL

Apply formula: Amount = (750 / 500) × 5 = 1.5 × 5 = 7.5 mL

Administer 7.5 mL of the 500 mg/5 mL suspension. This delivers exactly 750 mg. Use a calibrated oral syringe for accurate measurement.

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.