Medication Dosage by Weight Calculator

How Medication Dosage Calculations Work

Medication dosage calculations determine how much of a drug to administer based on prescribed dose, patient weight, and available concentration. These calculations are critical for patient safety, since errors are a leading cause of preventable harm.

Basic dose calculation:

Volume to administer = Prescribed dose ÷ Available concentration

Worked example, liquid antibiotic:

• Prescribed: amoxicillin 375 mg
• Available: 250 mg/5 mL suspension

Volume = 375 ÷ (250/5) = 375 ÷ 50 = 7.5 mL

Weight-based dose calculation (most common in pediatrics and IV medications):

Total dose = Weight (kg) × Dose per kg

Volume = Total dose ÷ Concentration

Example, IV gentamicin:

• Patient weight: 68 kg
• Order: 5 mg/kg IV once daily
• Available: 40 mg/mL

Total dose = 68 × 5 = 340 mg Volume = 340 ÷ 40 = 8.5 mL

The one thing to get right: per dose, or per day?

This is the trap in weight-based dosing, and it is worth slowing down for. “10 mg/kg” on its own is ambiguous, and the two readings differ by however many doses a day the patient takes.

mg/kg/dose means each individual dose is that size. Ibuprofen at 10 mg/kg/dose, three times daily, means a 20 kg child gets 200 mg three times, so 600 mg over the day.

mg/kg/day means that is the whole day’s allowance, divided up. Amoxicillin at 45 mg/kg/day divided every 8 hours means a 20 kg child gets 900 mg total, given as 300 mg three times.

Read the same number the wrong way and you are out by a factor of three. That is why this calculator asks which one you have rather than assuming, and why it shows the per-dose and per-day figures side by side so you can check both against the prescription.

Prescriptions for antibiotics are usually written per day and divided. Most over-the-counter analgesics are written per dose. When the label does not say, assume nothing and ask the pharmacist.

Related calculations this tool does not do

The following come up constantly in clinical practice but need inputs this calculator does not collect. They are here because people arrive looking for them.

Drip rate calculation for IV infusions:

Drip rate (mL/hr) = Volume (mL) ÷ Time (hr)

If 500 mL must infuse over 4 hours:

Drip rate = 500 ÷ 4 = 125 mL/hr

Drop rate (manual drip sets):

Drops per minute = (Volume mL × Drop factor) ÷ (Time in minutes)

Standard drop factor: 10, 15, or 20 drops/mL depending on the IV set.

The Five Rights of medication administration:

Right patient · Right drug · Right dose · Right route · Right time

Always double-check by having a second clinician independently verify calculations for high-alert medications (insulin, heparin, chemotherapy). A tenfold dosing error is more common than most people realize.