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
Dose (mg) = Weight (kg) × Dose Rate (mg/kg)
Weight = patient body weight in kg (divide lbs by 2.205 to convert)
Dose Rate = milligrams per kilogram as specified in the prescription (mg/kg)
Example: 20 kg child at 10 mg/kg = 200 mg per dose
Volume (mL) = Dose (mg) ÷ Concentration (mg/mL)
Concentration = mg of drug per mL of liquid, printed on the label
Example: 200 mg dose in a 50 mg/mL suspension = 200 / 50 = 4 mL
Daily Dose (mg) = Single Dose × Doses per Day
Every 6 hours = 4 doses/day. Every 8 hours = 3 doses/day.
Example: 200 mg every 8 hours = 200 x 3 = 600 mg/day
📖 How to Use This Calculator
Steps
1
Enter the patient's weight: type the weight in the number box or drag the slider. Toggle between lbs and kg using the unit buttons above the slider.
2
Enter the prescribed mg/kg dose rate: find this on the prescription, package insert, or clinical guideline. Common rates range from 5 mg/kg to 30 mg/kg depending on the medication.
3
Select dosing frequency: choose how often the medication is given. This determines the number of daily doses and the total daily dose.
4
Click Calculate: the single dose in mg, total daily dose, and suspension volumes for 5 concentrations appear in the results panel.
5
Verify before administering: always confirm the result with the drug's package insert and a licensed pharmacist or prescriber, especially for children.
💡 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.
1
Convert weight: 44 lbs / 2.205 = 19.95 kg (approximately 20 kg).
2
Daily dose: 20 kg x 25 mg/kg = 500 mg/day. Divided every 8 hours = 3 doses, so 500 / 3 = 166.7 mg per dose.
3
Volume for amoxicillin 125 mg/5 mL (25 mg/mL suspension): 166.7 / 25 = 6.67 mL per dose.
Single dose: 167 mg (6.7 mL of 25 mg/mL suspension) every 8 hours
Try this example →Example 2: Ibuprofen for fever in a child
Child weighs 55 lbs. Ibuprofen dose: 10 mg/kg, every 6-8 hours as needed.
1
Weight: 55 lbs / 2.205 = 24.9 kg.
2
Single dose: 24.9 x 10 = 249 mg. Round to 250 mg (nearest 50 mg increment is clinically acceptable).
3
Volume for ibuprofen 100 mg/5 mL (20 mg/mL): 249 / 20 = 12.45 mL. Maximum daily dose: 40 mg/kg = 996 mg (up to 4 doses per day).
Single dose: 249 mg (12.5 mL of 20 mg/mL) every 6-8 hours
Try this example →Example 3: Adult antibiotic by weight
Adult weighs 80 kg. Antibiotic prescribed at 15 mg/kg once daily.
1
Weight already in kg: 80 kg. Select kg unit in the calculator.
2
Single dose: 80 x 15 = 1,200 mg. Frequency: once daily (every 24 hours).
3
Total daily dose: 1,200 mg. Volume for a 10 mg/mL IV solution: 1,200 / 10 = 120 mL.
Single dose: 1,200 mg (120 mL of 10 mg/mL solution) once daily
Try this example →❓ Frequently Asked Questions
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 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 must be correct for treatment to be effective and safe. This calculator covers the dose calculation and daily totals based on different frequencies.
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. 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; 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 and the drug's package insert.
Is there a maximum safe dose per kg?
Yes, every medication has a maximum dose per kg and an absolute maximum daily dose. Exceeding these limits can cause toxicity. For example, ibuprofen is capped at 40 mg/kg/day and 2,400 mg/day total (prescription); acetaminophen at 75 mg/kg/day and 4,000 mg/day for adults. This calculator shows the mg/kg result but does not enforce drug-specific caps. Always cross-check with 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 confirming with the pharmacist first. For children who cannot swallow tablets, a liquid formulation or compound may be available. The pharmacist can also advise on rounding the dose to the nearest practical increment.