Dosage Calculator
Calculate the correct medication dose by body weight using the mg/kg formula.
💊 What is a Dosage Calculator?
Weight-based dosing is the standard method used by pharmacists and clinicians to calculate how much medication a patient should receive. Rather than prescribing a fixed quantity for everyone, a mg/kg dose rate is multiplied by the patient's body weight to give a dose that is proportional to their physiology. This approach is especially important in pediatrics, where drug metabolism, organ function, and drug distribution change dramatically with age and size.
The core formula is straightforward: Dose (mg) = Weight (kg) x Dose rate (mg/kg). A prescriber determines the correct mg/kg rate from the drug monograph based on the indication, severity, and patient characteristics. The calculator then converts that rate into the actual milligrams to administer, and for liquid formulations, further converts milligrams into millilitres based on the concentration of the available product. Clinicians use weight-based dosing for antibiotics, analgesics, anticoagulants, chemotherapy agents, sedatives, and many other drug classes.
A common point of confusion is distinguishing dose from dosage. The dose is the amount given per administration; the dosage is the complete regimen including frequency and duration. Both must be correct for treatment to be effective and safe. This calculator covers the dose calculation and shows how the total daily dosage changes with different administration frequencies from every 4 hours to once daily.
This tool is intended for educational reference and medication verification. It does not enforce drug-specific maximum doses or age-related restrictions. Always cross-reference the calculated dose with the prescribing information or a qualified pharmacist before administering any medication, particularly to children or vulnerable patients.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1: Amoxicillin for a child with an ear infection
Child weighs 44 lbs. Prescription: amoxicillin 25 mg/kg/day in divided doses every 8 hours.
Example 2: Ibuprofen for fever in a child
Child weighs 55 lbs. Ibuprofen dose: 10 mg/kg, every 6-8 hours as needed.
Example 3: Adult antibiotic by weight
Adult weighs 80 kg. Antibiotic prescribed at 15 mg/kg once daily.
❓ Frequently Asked Questions
🔗 Related Calculators
How do you calculate medication dose by weight in mg/kg?
The weight-based dosing formula is: Dose (mg) = Patient weight (kg) x Dose rate (mg/kg). For example, a 20 kg child prescribed 10 mg/kg of amoxicillin receives 200 mg per dose. If the weight is in pounds, divide by 2.205 to convert to kilograms first. Then multiply by the mg/kg rate specified on the prescription or package insert.
What is mg/kg dosing and why is it used?
mg/kg dosing (milligrams per kilogram of body weight) ensures that each patient receives a dose proportional to their size. Children have different body composition and drug metabolism than adults, making weight-based dosing essential for safety. A dose that is safe for a 70 kg adult could be toxic to a 15 kg child, and a dose sized for a child would be ineffective for an adult.
How do I convert a mg dose to mL of liquid suspension?
Use the formula: Volume (mL) = Dose (mg) / Concentration (mg/mL). The concentration is found on the label. For example, amoxicillin 250 mg/5 mL has a concentration of 50 mg/mL. A 250 mg dose requires 250 / 50 = 5 mL. This calculator shows volumes for five common concentrations simultaneously: 1, 5, 10, 20, and 25 mg/mL.
How many doses per day for every 8 hours vs every 6 hours?
Every 6 hours (q6h) = 4 doses per day. Every 8 hours (q8h) = 3 doses per day. Every 12 hours (q12h) = 2 doses per day. Every 24 hours (q24h) = 1 dose per day. The frequency determines the total daily dose: daily dose = single dose x doses per day. Frequency is prescribed by the clinician based on the drug's half-life and indication.
What is the difference between dose and dosage?
In pharmacology, the dose is the amount of medication given at one time (e.g. 250 mg). Dosage refers to the complete regimen: dose amount plus frequency and duration (e.g. 250 mg every 8 hours for 10 days). Both concepts are critical: giving the right dose at the wrong frequency can mean under-treatment (too infrequent) or toxicity (too frequent).
How do I calculate a pediatric dose from an adult dose?
The safest method is weight-based dosing (mg/kg) from the prescriber or drug monograph. Avoid calculating a child's dose as a fraction of the adult dose unless specifically instructed, because children are not simply small adults, and their renal clearance, liver enzyme maturity, and body composition differ significantly. Always use the pediatric-specific mg/kg rate.
What are common mg/kg dosing rates for frequently prescribed medications?
Common rates include: amoxicillin 25-45 mg/kg/day divided every 8-12 hours; ibuprofen 5-10 mg/kg every 6-8 hours (max 40 mg/kg/day); acetaminophen 10-15 mg/kg every 4-6 hours (max 75 mg/kg/day); azithromycin 10 mg/kg on day 1 then 5 mg/kg on days 2-5. These are illustrative ranges only; always follow the specific prescription.
Is there a maximum safe dose per kg?
Yes, every medication has a maximum dose per kg and an absolute maximum daily dose, whichever is lower. Exceeding these limits can cause toxicity. For example, ibuprofen is capped at 40 mg/kg/day and 2400 mg/day total (prescription); acetaminophen at 75 mg/kg/day and 4000 mg/day for adults. This calculator shows the mg/kg result but does not enforce drug-specific caps. Always cross-check with the prescribing information.
What concentration is amoxicillin 250 mg/5 mL suspension?
Amoxicillin 250 mg/5 mL has a concentration of 50 mg/mL. A 500 mg dose requires 10 mL of this suspension. Other common amoxicillin formulations include 125 mg/5 mL (25 mg/mL) and 400 mg/5 mL (80 mg/mL). Always read the label and confirm the concentration before measuring. The suspension must be shaken well before each dose.
How do I dose medications in pounds without converting to kg?
Select the lb unit in this calculator and it automatically converts to kilograms using the factor 1 lb = 0.4536 kg before applying the mg/kg formula. If doing this manually: divide weight in pounds by 2.205 to get kg, then multiply by the mg/kg rate. For example, a 44 lb child: 44 / 2.205 = 20 kg; at 10 mg/kg = 200 mg per dose.
Can I use this calculator for adult dosing?
Yes. Weight-based dosing applies to adults as well as children, particularly for antibiotics, anticoagulants, chemotherapy, and pain medications. Many adult drug protocols specify mg/kg or mcg/kg/min rates. Enter the adult weight and the prescribed mg/kg rate to get the dose. The suspension volume rows are more relevant for pediatric liquid formulations.
What should I do if the calculated dose does not match a standard tablet strength?
If the calculated dose falls between available tablet sizes, ask your pharmacist which tablet strength and splitting instructions apply. Never crush or split extended-release, enteric-coated, or sublingual tablets without first confirming with the pharmacist. For children who cannot swallow tablets, a liquid formulation or compound may be available.