Drug Dosage Calculator
This calculator is for educational and reference purposes only. Always verify dosage calculations with a licensed healthcare provider, pharmacist, or prescribing clinician before administering any medication. One wrong decimal place can result in a ten-fold dosing error.
Enter the medication name for your records
Amount of drug per kilogram of body weight per dose
Enter as label notation (250mg/5mL) or as mg/mL value (50). Leave empty for mg-only results.
Enter values to calculate dosage
Fill in the patient information and medication details to see dosage calculations, safety checks, and visual breakdowns.
Important
This calculator is provided for general information only and is not medical advice, diagnosis, or treatment. Results are estimates based on population averages and cannot account for your individual circumstances. Speak to a qualified healthcare professional before acting on anything you read here.
Calculate accurate medication doses with weight-based, BSA, IV drip, and dilution modes
Welcome to our free Drug Dosage Calculator, a comprehensive clinical reference tool that helps patients, caregivers, nurses, and healthcare professionals perform accurate medication dose calculations. Whether you need to calculate a weight-based dose for a child, determine a body surface area (BSA) dose for chemotherapy, figure out an IV drip rate for gravity infusion, or compute a drug dilution concentration, this calculator covers all major dosing scenarios in one unified interface. Accurate drug dosage calculation is one of the most critical skills in healthcare. Medication errors are among the leading causes of preventable harm in hospitals and outpatient settings alike. Studies published in the Journal of Patient Safety estimate that medication errors affect millions of patients annually, with dosing errors being the single most common type. A misplaced decimal point can turn a therapeutic dose into a toxic overdose, and an under-dose can leave a patient without the treatment they need. Our calculator supports four distinct calculation modes. The Weight-Based mode is the most common approach, used for the vast majority of pediatric medications and many adult drugs. You enter the patient's weight, the prescribed dosage in milligrams per kilogram (mg/kg), the dosing frequency, and optionally the medication concentration to get the liquid volume. The calculator instantly shows the single dose in milligrams, the volume per dose in milliliters, the total daily dose, and the total daily volume. The BSA-Based mode is essential for oncology and certain specialty medications where dosing is calculated per square meter of body surface area. You enter the patient's height and weight, select either the Mosteller or DuBois formula for BSA calculation, and provide the prescribed dose in mg/m². The calculator computes the BSA and derives the exact dose. The IV Drip Rate mode helps nurses and clinicians calculate gravity infusion rates. Enter the total volume to infuse, the infusion time, and the drop factor of your IV tubing set. The calculator returns both drops per minute (gtt/min) for gravity drip and milliliters per hour (mL/hr) for pump-driven infusion. A warning appears if the calculated drip rate exceeds 100 drops per minute, which is generally considered too fast for safe gravity administration. The Drug Dilution mode calculates the resulting concentration when a known amount of drug is dissolved in a given volume of solution. This is essential for reconstituting powdered medications, preparing IV admixtures, and compounding solutions. A key differentiating feature is our concentration label parser. Instead of manually converting medication label strengths like 250mg/5mL into mg/mL, you can enter the label notation directly (such as 250/5 or 250mg/5mL) and the calculator automatically extracts the correct concentration. We also provide presets for the five most common pediatric liquid medications: children's acetaminophen (160mg/5mL), children's ibuprofen (100mg/5mL), amoxicillin suspension in both standard (250mg/5mL) and high-dose (400mg/5mL) formulations, and azithromycin suspension (200mg/5mL). For safety, the calculator includes optional maximum dose ceiling input with visual warning alerts, a safe dose range checker that compares your calculated dose against min/max mg/kg parameters, and a BulletChart visualization showing exactly where your dose falls relative to the safe range. The dose timing timeline provides a visual 24-hour overview of when each dose should be administered based on the selected frequency. All calculations run entirely in your browser. No patient data is transmitted to any server. Results can be copied to clipboard, shared via the Web Share API, printed in a clean format, or exported to CSV for documentation purposes. This tool is designed to supplement, never replace, professional clinical judgment.
Understanding Drug Dosage Calculations
Drug dosage calculation is the process of determining the correct amount of medication to administer to a patient based on various clinical parameters. Accurate dosing ensures therapeutic efficacy while minimizing the risk of adverse effects.
Weight-Based Dosing Fundamentals
Weight-based dosing is the standard method for most medications, especially in pediatrics. The prescribed dosage is expressed as milligrams per kilogram of body weight (mg/kg), and the actual dose is calculated by multiplying the patient's weight in kilograms by this rate. For example, if a child weighing 20 kg is prescribed ibuprofen at 10 mg/kg, the single dose is 200 mg. When the medication is in liquid form, the dose in milligrams is divided by the concentration in mg/mL to determine the volume to administer. Weight must always be converted to kilograms first if measured in pounds (divide by 2.2046). The dosing frequency determines the total daily dose: BID (twice daily) means the total daily dose is double the single dose, TID (three times daily) triples it, and so on.
Body Surface Area (BSA) Dosing
BSA-based dosing is used primarily for chemotherapy drugs, certain antibiotics, and medications with narrow therapeutic indices. BSA more accurately reflects metabolic mass than weight alone, making it the preferred method for drugs where precise dosing is critical. The Mosteller formula calculates BSA as the square root of (height in cm multiplied by weight in kg divided by 3600). The DuBois formula uses the equation 0.007184 times height raised to the power of 0.725 times weight raised to the power of 0.425. Both formulas produce similar results, but Mosteller is simpler and more commonly used in clinical practice. Once BSA is calculated in square meters, the dose is simply BSA multiplied by the prescribed mg/m² value.
IV Drip Rate Calculations
IV drip rate calculation is essential for nurses administering intravenous fluids and medications. For gravity-driven infusions, the rate is measured in drops per minute (gtt/min) and depends on three factors: the total volume to infuse, the infusion time, and the drop factor of the IV tubing. Drop factors vary by tubing type: standard macrodrip sets come in 10, 15, or 20 drops per mL, while microdrip (pediatric) sets deliver 60 drops per mL. The formula is (Volume in mL times Drop Factor) divided by (Time in minutes). For electronic infusion pumps, the rate is expressed as mL/hr, calculated simply as volume divided by time in hours. Drip rates above 100 gtt/min are generally considered too fast for safe gravity administration and may require a pump.
Drug Dilution and Concentration
Drug dilution calculations determine the concentration of a medication solution when a known amount of drug is dissolved in a specified volume of diluent. The basic formula is Concentration (mg/mL) equals Drug Amount (mg) divided by Solution Volume (mL). This calculation is critical when reconstituting powdered medications, preparing IV admixtures, or compounding custom solutions. Unit conversions are frequently needed: grams to milligrams (multiply by 1000), micrograms to milligrams (divide by 1000), and liters to milliliters (multiply by 1000). The resulting concentration is then used to calculate the volume needed to deliver a specific dose. Always verify the final concentration against established references before administration.
Key Dosage Formulas
Weight-Based Dose
Dose (mg) = Weight (kg) × Dosage (mg/kg)
The fundamental dosing equation. Multiply patient weight in kilograms by the prescribed dosage rate in milligrams per kilogram.
Liquid Volume
Volume (mL) = Dose (mg) ÷ Concentration (mg/mL)
Converts a milligram dose to the liquid volume to administer, using the medication's concentration.
BSA (Mosteller)
BSA (m²) = √(Height (cm) × Weight (kg) ÷ 3600)
The Mosteller formula for body surface area, widely used in oncology dosing.
IV Drip Rate
gtt/min = (Volume (mL) × Drop Factor) ÷ Time (min)
Calculates the gravity infusion drip rate in drops per minute for a given IV tubing set.
Drug Dilution
Concentration (mg/mL) = Drug Amount (mg) ÷ Solution Volume (mL)
Determines the resulting concentration when a drug is dissolved in a solution.
Reference Tables
Common Pediatric Medication Concentrations
Standard liquid medication concentrations available in most pharmacies.
| Medication | Label Strength | Concentration (mg/mL) |
|---|---|---|
| Acetaminophen (children's) | 160 mg/5 mL | 32 |
| Ibuprofen (children's) | 100 mg/5 mL | 20 |
| Amoxicillin suspension | 250 mg/5 mL | 50 |
| Amoxicillin suspension (high) | 400 mg/5 mL | 80 |
| Azithromycin suspension | 200 mg/5 mL | 40 |
Dosing Frequency Reference
Standard medical abbreviations for dosing intervals.
| Abbreviation | Meaning | Doses/Day | Interval |
|---|---|---|---|
| QD | Once daily | 1 | 24h |
| BID | Twice daily | 2 | 12h |
| TID | Three times daily | 3 | 8h |
| QID | Four times daily | 4 | 6h |
| q4h | Every 4 hours | 6 | 4h |
| q3h | Every 3 hours | 8 | 3h |
| q2h | Every 2 hours | 12 | 2h |
IV Tubing Drop Factors
Common drop factors for different IV administration sets.
| Set Type | Drop Factor (gtt/mL) | Typical Use |
|---|---|---|
| Blood set | 10 | Blood products |
| Standard macrodrip | 15 | General IV fluids |
| Standard macrodrip | 20 | General IV fluids |
| Microdrip (pediatric) | 60 | Pediatric / precise dosing |
Worked Examples
Pediatric Amoxicillin Dosing
A 15 kg child is prescribed amoxicillin at 25 mg/kg/dose BID. The suspension concentration is 250mg/5mL (50 mg/mL).
Calculate single dose: 15 kg × 25 mg/kg = 375 mg
Convert to liquid volume: 375 mg ÷ 50 mg/mL = 7.5 mL
Total daily dose: 375 mg × 2 (BID) = 750 mg/day
Total daily volume: 7.5 mL × 2 = 15 mL/day
Administer 7.5 mL (375 mg) of amoxicillin 250mg/5mL suspension every 12 hours.
IV Drip Rate Calculation
A nurse needs to infuse 1000 mL of normal saline over 8 hours using a 20 gtt/mL macrodrip set.
Convert time to minutes: 8 hours × 60 = 480 minutes
Apply drip rate formula: (1000 mL × 20 gtt/mL) ÷ 480 min = 41.7 gtt/min
Calculate mL/hr: 1000 mL ÷ 8 hours = 125 mL/hr
Rate is under 100 gtt/min, safe for gravity infusion
Set the drip rate to approximately 42 drops per minute, or program the pump to 125 mL/hr.
BSA-Based Chemotherapy Dosing
A patient weighing 70 kg and 175 cm tall is prescribed 75 mg/m² of a chemotherapy agent. Use the Mosteller formula.
Calculate BSA: √(175 × 70 ÷ 3600) = √(3.403) = 1.845 m²
Calculate dose: 1.845 m² × 75 mg/m² = 138.4 mg
Verify BSA is within expected range (1.5–2.2 m² for adults)
Round dose per institutional protocol
The calculated dose is 138.4 mg based on a BSA of 1.845 m².
How to Use
Select Calculation Mode
Choose the appropriate mode for your calculation: Weight-Based for standard dosing by body weight, BSA-Based for body surface area dosing (commonly used in oncology), IV Drip Rate for infusion rate calculations, or Dilution for concentration calculations.
Enter Patient and Medication Details
Input the required values for your selected mode. For weight-based dosing, enter patient weight, prescribed dosage (mg/kg), dosing frequency, and optionally the medication concentration. You can select a medication preset or enter the label notation (e.g., 250mg/5mL) directly.
Review Results and Safety Checks
The calculator instantly displays the dose in milligrams, liquid volume in milliliters, and daily totals. If you entered safe dose range parameters, a visual chart shows where the dose falls relative to the recommended range. Maximum dose warnings appear automatically if exceeded.
Export or Share Your Calculation
Use the action buttons to copy results to clipboard, share via your device's share feature, print a clean summary, or export to CSV for documentation. Always verify calculations with the prescribing clinician before administering medication.
Frequently Asked Questions
How accurate is this drug dosage calculator?
This calculator uses the same standard pharmaceutical formulas taught in nursing and pharmacy programs worldwide. The weight-based formula (Dose = Weight x Dosage) and the IV drip rate formula (gtt/min = Volume x Drop Factor / Time) are universally accepted. The BSA formulas (Mosteller and DuBois) have been validated against direct measurements with accuracy within 5-10%. However, this calculator is a reference tool only. It does not account for patient-specific factors such as renal function, hepatic impairment, drug interactions, or clinical condition. All calculated doses must be verified by a licensed healthcare provider before administration. Never rely solely on any calculator for medication dosing decisions.
What is the difference between mg/kg and mg/kg/day dosing?
This is one of the most common sources of dosing errors. When a prescription reads mg/kg, it typically means milligrams per kilogram per dose — the amount to give at each individual administration. When it reads mg/kg/day, it means the total daily amount per kilogram, which must be divided by the number of doses per day. For example, amoxicillin 45 mg/kg/day divided TID means you divide 45 by 3 to get 15 mg/kg per dose, then multiply by weight. Our calculator uses mg/kg per dose as the input. If your prescription is written as mg/kg/day, divide that number by the dosing frequency before entering it.
How do I convert a medication label like 250mg/5mL to mg/mL?
Our calculator does this automatically with the concentration label parser. Simply enter the label notation as it appears on the bottle — for example, type 250mg/5mL or 250/5 in the concentration field. The calculator recognizes this format and converts it to 50 mg/mL automatically. If you prefer to do the math manually, divide the milligram amount by the milliliter volume: 250 divided by 5 equals 50 mg/mL. This conversion is essential for determining how many milliliters to administer for a given dose in milligrams.
When should I use BSA-based dosing instead of weight-based dosing?
BSA-based dosing is the standard for chemotherapy drugs, certain immunosuppressants, and medications with narrow therapeutic indices where precise dosing is critical. BSA correlates more closely with metabolic rate and organ function than weight alone, making it more accurate for drugs with significant toxicity potential. In pediatrics, BSA-based dosing may be preferred when a child's weight is at extremes (very underweight or obese), as weight-based dosing can over- or under-dose in these cases. Your prescriber will specify whether the dose is in mg/kg or mg/m². If the prescription uses mg/m², use the BSA-Based mode in this calculator.
What drop factor should I use for IV calculations?
The drop factor depends on the IV administration set (tubing) being used, and it is always printed on the packaging. Standard macrodrip sets come in 10, 15, or 20 drops per mL. The 10 gtt/mL set is typically used for blood products. The 15 and 20 gtt/mL sets are used for general IV fluid administration. Microdrip sets deliver 60 drops per mL and are used in pediatrics or when very precise flow control is needed. Using the wrong drop factor in your calculation will give an incorrect drip rate, so always check the tubing package. If you are using an electronic infusion pump, you only need the mL/hr rate, not the drip rate.
Is my patient data stored or transmitted anywhere?
No. All calculations are performed entirely in your web browser using JavaScript. No patient data, weights, drug names, or calculation results are ever sent to our servers or any third party. The calculator works completely offline once the page has loaded. When you use the export or share features, the data is generated locally on your device. This makes the tool safe for use in clinical settings where patient privacy and HIPAA compliance are concerns. However, always follow your institution's policies regarding the use of online tools for clinical calculations.