This weight-based drug dose calculator computes the total dose required from a patient's weight and a prescribed mg/kg dose, and the volume to administer if a drug concentration is given. This calculator is for educational/informational purposes only and is not a substitute for professional medical judgment.
How to Use the Drug Calculator
- Enter the patient's weight in kilograms.
- Enter the prescribed dose in milligrams per kilogram (mg/kg).
- Optionally, enter the drug's concentration in milligrams per milliliter (mg/mL) to also get the volume to administer.
- Click "Calculate" to see the total dose (and volume, if provided).
Many medicines, especially for children, are prescribed per kilogram of body weight so that the dose scales with the size of the patient. The calculator applies two steps:
- Dose: Total dose (mg) = Weight (kg) × Prescribed dose (mg/kg).
- Volume: Volume (mL) = Total dose (mg) ÷ Concentration (mg/mL). This is the amount of liquid to draw up or measure.
Results are shown to a maximum of three decimal places. In practice the volume is then rounded to something that can actually be measured with the syringe or cup being used, as directed by the prescriber or pharmacist.
Worked Example
A child weighing 18 kg is prescribed a medicine at 15 mg/kg per dose. The syrup label reads 250 mg/5 mL. Total dose = 18 × 15 = 270 mg. The concentration is 250 ÷ 5 = 50 mg/mL, so the volume = 270 ÷ 50 = 5.4 mL per dose. Entering 18, 15 and 50 in the calculator gives exactly these figures.
If the order is written per day instead, for example 30 mg/kg/day for a 12 kg child divided into three doses, the calculator gives the daily total of 360 mg. Divide that by 3 to get 120 mg for each dose.
Converting Label Strengths to mg/mL
The concentration field needs mg per mL, but labels in India and elsewhere express strength in several ways. Convert first, then enter the mg/mL figure:
| Label says | How to convert | mg/mL to enter |
|---|
| 125 mg/5 mL | Divide mg by 5 | 25 |
| 250 mg/5 mL | Divide mg by 5 | 50 |
| 100 mg/mL (drops) | Already in mg/mL | 100 |
| 500 mg/10 mL | Divide mg by 10 | 50 |
| 1% w/v | 1% means 1 g in 100 mL, which is 10 mg/mL | 10 |
| 1:1000 | 1 g in 1000 mL, which is 1 mg/mL | 1 |
Common Dosing Errors to Avoid
- Weight in pounds instead of kilograms. Entering pounds more than doubles the dose. Divide pounds by 2.2046 first; 44 lb is about 20 kg.
- Mixing up per-dose and per-day orders. A mg/kg/day order entered as if it were per dose gives two to four times the intended amount at each administration.
- Confusing mg with mcg (micrograms). One milligram is 1000 micrograms, so this mistake causes a thousandfold error. This calculator works only in mg.
- Decimal point slips. Write 0.5 mL, never .5 mL, and avoid trailing zeros such as 5.0 mL, which can be misread as 50.
- Ignoring the maximum dose. For larger children and adults, many weight-based doses are capped at a maximum single or daily dose, and some drugs are dosed on ideal rather than actual body weight. The calculator does not apply any cap.
- Using kitchen spoons. Household teaspoons vary widely in size; measure liquid medicines with the oral syringe or measuring cup supplied with the product.
Any dose calculated here must be checked against the actual prescription and the product label, and verified by a doctor, pharmacist or nurse before it is given. Never use this tool to decide a dose yourself.
Frequently Asked Questions
Total Dose (mg) = Patient Weight (kg) x Prescribed Dose (mg/kg). For example, a 70 kg patient at 5 mg/kg needs a 350 mg total dose.
Volume (mL) = Total Dose (mg) / Concentration (mg/mL). This is only shown when you enter a drug concentration.
No. This calculator is for educational and informational purposes only and is not a substitute for professional medical judgment. Always verify dosage calculations with a licensed healthcare provider or pharmacist before clinical use.
Enter the mg/kg/day figure as the dose. The result is the total for 24 hours. Divide it by the number of doses per day written on the prescription (for example, every 8 hours means 3 doses) to get the amount for each dose, and divide the volume the same way.
Divide the milligrams by the millilitres: 250 ÷ 5 = 50, so enter 50 mg/mL. For 125 mg/5 mL enter 25. Always read the strength from the bottle you are actually using, because the same medicine is often sold in more than one strength.