Medication Dosage Calculator

Calculating the right medication dose for a child—or any patient where weight matters—can feel stressful. You're staring at a prescription that says "25 mg/kg three times daily," the medication bottle shows "250 mg/5mL," and you need to figure out exactly how many milliliters to put in that syringe. This calculator takes the guesswork out of that moment.

Whether you're a nurse double-checking your math before administering a dose, a parent measuring your toddler's antibiotic at 2 AM, or a pharmacy student learning dosage calculations, this tool gives you accurate results in seconds. Just enter the patient's weight, the prescribed dose per kilogram, and the medication concentration—the calculator handles both the milligram calculation and the liquid volume conversion automatically.

Here's something that might surprise you: the same 500 mg dose of a medication that works perfectly for a 70 kg adult could be dangerously high for a 15 kg child—or completely ineffective for a 120 kg adult. Our bodies process drugs based on our size, which is why weight-based dosing exists.

When your pediatrician prescribes amoxicillin at "45 mg/kg/day," they're giving you a formula that scales the medication to your child specifically. This approach:

• Prevents underdosing — An ineffective dose means the infection doesn't clear, and you're back at the doctor's office in a week
• Prevents overdosing — Too much medication stresses developing organs and can cause serious side effects
• Works across all ages — The same mg/kg guideline can apply to a 6-month-old and a 12-year-old, with appropriate doses for each

Most antibiotics, many pain relievers, and numerous prescription medications use weight-based dosing. If you've ever wondered why children's medication instructions include weight ranges rather than just age, this is why—two 5-year-olds can easily differ by 10 pounds or more.

The math itself is simple once you understand it:

Dose (mg) = Patient Weight (kg) × Prescribed Dosage (mg/kg)

Let's say your child weighs 18 kg and the prescription says 15 mg/kg:

• 18 kg × 15 mg/kg = 270 mg

But wait—your medication comes as a liquid, and you need to know how many milliliters that 270 mg actually is. That's where the second formula comes in:

Volume (mL) = Dose (mg) ÷ Concentration (mg/mL)

If your bottle says "125 mg/5mL" (which equals 25 mg per mL):

• 270 mg ÷ 25 mg/mL = 10.8 mL

The calculator runs both calculations instantly, so you don't have to worry about making an error when you're exhausted or rushed.

One of the trickiest parts of liquid medication dosing is that the same drug comes in different concentrations. Here's a quick reference for medications you'll encounter most often:

Always check your specific bottle—this table is a general reference, but concentrations can vary by brand and formulation.

1. Enter the Patient's Weight

Type in the weight and select kilograms or pounds. If you only know the weight in pounds, the calculator converts automatically (1 kg = 2.205 lbs).

For the most accurate dose, use a recent weight. Children grow quickly—a weight from three months ago might be significantly different. When in doubt, step on the scale together and subtract your weight.

2. Enter the Prescribed Dosage Per Weight

This is the number from your prescription or medication guide, usually written as mg/kg. For example:

• "Amoxicillin 25 mg/kg" means you enter 25
• "Ibuprofen 10 mg/kg" means you enter 10

If your prescription says mg/kg/day (total daily dose), you'll need to divide by the number of doses per day to get the per-dose amount.

3. Select the Dosing Frequency

Choose how often the medication is given. This affects how the calculator interprets your dosage input and displays the results.

4. Enter the Medication Concentration

Look at your bottle for wording like "250 mg/5mL" or "100 mg per 5 mL." Enter the concentration so the calculator can determine the liquid volume.

5. Read Your Results

You'll see both:

• Dose in milligrams — the amount of actual medication
• Volume in milliliters — how much liquid to measure

Situation: Your 2-year-old (12 kg) has an ear infection. The pediatrician prescribes amoxicillin at 40 mg/kg/day, divided into two doses. The pharmacy dispenses 400 mg/5mL suspension.

The math:

• Daily dose: 12 kg × 40 mg/kg = 480 mg/day
• Per dose: 480 mg ÷ 2 = 240 mg every 12 hours
• Volume: 240 mg ÷ 80 mg/mL = 3 mL twice daily

That's a manageable amount of a medication that doesn't taste too bad—your toddler might actually cooperate.

Situation: Your 8-year-old (27 kg) wakes up with a fever at 3 AM. You reach for the children's ibuprofen (100 mg/5mL). The dosing is 10 mg/kg.

The math:

• Dose: 27 kg × 10 mg/kg = 270 mg
• Volume: 270 mg ÷ 20 mg/mL = 13.5 mL

You can round to 13 or 14 mL—either is fine for an over-the-counter fever reducer.

Situation: Your 4-month-old (6.5 kg) needs azithromycin at 10 mg/kg for the first dose. The concentration is 100 mg/5mL.

The math:

• Dose: 6.5 kg × 10 mg/kg = 65 mg
• Volume: 65 mg ÷ 20 mg/mL = 3.25 mL

With an infant, you'd likely round to 3 mL and use the oral syringe provided. Precision matters more with smaller patients, so don't round too aggressively.

Situation: You only know your child weighs 55 pounds and need to give acetaminophen at 15 mg/kg.

The conversion:

• 55 lbs ÷ 2.205 = 24.9 kg (call it 25 kg)
• Dose: 25 kg × 15 mg/kg = 375 mg

The calculator handles this conversion automatically when you select "pounds."

Prescriptions use specific abbreviations that tell you how to space doses throughout the day:

The tricky part: When a prescription says "90 mg/kg/day divided TID," you need to calculate the daily total first, then divide by 3 for each dose. For a 15 kg child:

• Daily total: 15 kg × 90 mg/kg = 1,350 mg
• Each dose: 1,350 mg ÷ 3 = 450 mg three times daily

Get an accurate weight. This is the foundation of everything. Weigh young children without clothes or a wet diaper. For infants, a pediatric scale is ideal—ask if you can use one at your pediatrician's office or local pharmacy.

Use the right measuring tool. The dosing syringe or cup that comes with the medication is calibrated for that product. Kitchen spoons are notoriously inaccurate—a "teaspoon" can vary by 25% or more between spoons in your drawer.

Check the concentration every time. You might have two bottles of the same medication with different concentrations in your cabinet. A dose of 5 mL from a 100 mg/5mL bottle is very different from 5 mL of a 250 mg/5mL bottle.

Know your maximum doses. Weight-based calculations can sometimes exceed safe limits, especially for larger children or adults. Common maximums:

• Acetaminophen: 75 mg/kg/day or 4,000 mg/day (whichever is less)
• Ibuprofen: 40 mg/kg/day or 2,400 mg/day (whichever is less)

Keep a dosing log. When you're giving medication multiple times daily, especially with a sick child who has multiple caregivers, write down what you gave and when. A simple note on your phone prevents accidental double-dosing.

Shake liquid medications. Many suspensions settle. Without shaking, the first doses from the bottle might be weaker and the last doses stronger—or vice versa.

Recalculate if something seems off. If your calculated dose seems unusually large or small compared to what you've given before, run the numbers again. Trust your instincts.

Call your pharmacist if:

• The calculated dose doesn't match the prescription label
• You're unsure which concentration you have
• The dose seems very different from what you expected
• You need to give multiple medications and want to verify interactions

Contact your healthcare provider if:

• Your child spits out a dose and you're unsure whether to re-dose
• You accidentally gave too much medication
• Symptoms aren't improving despite completing the prescribed course
• You're seeing unexpected side effects

Pharmacists are medication experts, and most are happy to double-check your dosing math—especially for pediatric patients. Don't hesitate to call.

This calculator provides dosing calculations for educational and reference purposes. Always follow the specific instructions from your healthcare provider or pharmacist. When in doubt about any medication dose—especially for infants and children—ask a professional before administering.