Dosage Calculator
Enter the prescribed dosage in mg/kg, your weight, and how often you take the medication. The calculator works out the single dose in mg, the liquid volume in mL if your medicine is a suspension or syrup, the number of tablets, and the total daily amount. Switch between metric and imperial weight, and use the full schedule panel to see every dose across a 24-hour day.
What is a dosage calculator?
A dosage calculator helps you find the right amount of medication to take based on your body weight and the dose prescribed by your doctor. Many drugs, especially antibiotics, antivirals, and pediatric medicines, are prescribed as a number of milligrams per kilogram of body weight (mg/kg) rather than a flat number of milligrams. This approach ensures smaller patients get less and heavier patients get enough for the drug to be effective. Once you know the dose in milligrams, the calculator can convert it into the practical measurement you need, whether that is a liquid volume in millilitres for a suspension or syrup, or a number of tablets or capsules.
How the calculation works
For weight-based dosing, the formula is straightforward: Dose (mg) = Prescribed rate (mg/kg) x Body weight (kg). For example, if a doctor prescribes amoxicillin at 25 mg/kg and the patient weighs 20 kg, the single dose is 25 x 20 = 500 mg. For a liquid suspension with a concentration of 250 mg/5 mL (50 mg/mL), the volume per dose is 500 / 50 = 10 mL. The daily total multiplies the single dose by the number of administrations, and the total course dose multiplies the daily total by the number of treatment days. For fixed dosing, the prescribed milligram amount is used directly without any weight adjustment.
Weight-based vs. fixed dosing
Weight-based dosing (mg/kg) is the standard approach when the therapeutic margin of a drug is narrow or when patient size varies widely, as with children. By tying the dose to body mass, clinicians keep drug concentrations in the blood within a safe and effective range regardless of patient size. Fixed dosing is used when the drug has a wide safety margin and population differences in body size do not meaningfully change its effects, as with standard over-the-counter ibuprofen or paracetamol tablets for adults. Many prescriptions for older children and adults also transition from mg/kg guidance to a flat milligram cap once the weight-based dose would exceed that ceiling, so always check the prescriber's notes.
Converting to liquid volumes and tablet counts
Liquid medicines list their concentration on the label, typically in mg/mL or mg per 5 mL teaspoon. Dividing the required dose in mg by the concentration in mg/mL gives the volume in mL. Always use a calibrated oral syringe rather than a household spoon, since kitchen teaspoons vary by 20-30 percent from the nominal 5 mL. For tablets, divide the required dose by the tablet strength. If the result is not a whole or half number, ask your pharmacist whether a different tablet strength is available or whether the tablet can be safely split. Some coated or extended-release tablets must not be split.
Common dosing frequency abbreviations
| Abbreviation | Meaning | Doses per day | Interval |
|---|---|---|---|
| QD / OD | Once daily | 1 | 24 hours |
| BID | Twice daily | 2 | 12 hours |
| TID | Three times daily | 3 | 8 hours |
| QID | Four times daily | 4 | 6 hours |
| q4h | Every 4 hours | 6 | 4 hours |
| q6h | Every 6 hours | 4 | 6 hours |
| q8h | Every 8 hours | 3 | 8 hours |
| q12h | Every 12 hours | 2 | 12 hours |
Standard prescription abbreviations and their equivalent daily dose count.
Frequently asked questions
What does mg/kg mean on a prescription?
It means milligrams of drug per kilogram of your body weight per dose (or per day, depending on the prescription). To find your actual dose in milligrams, multiply the mg/kg number by your weight in kilograms. A prescription of 10 mg/kg for a 70 kg adult gives a dose of 700 mg.
How do I measure liquid medicine accurately?
Use a calibrated oral syringe or a medicine cup that shows millilitre markings, not a household teaspoon or tablespoon. Kitchen spoons are not standardised and can be 20-30% off from the intended volume, which matters most for narrow-margin drugs. Pharmacies give out oral syringes for free and they snap directly onto most medicine bottles.
What is the difference between a dose and a dosage?
A dose is the amount of medicine taken at a single point in time, for example 500 mg. Dosage refers to the full regimen: the amount, the timing, and the duration combined, for example 500 mg three times daily for seven days. This calculator shows you both the single dose and the full dosage schedule.
What if my tablet count comes out to a fraction like 1.5 tablets?
Many scored tablets can be safely split in half. However, coated tablets, extended-release tablets, and capsules must never be crushed or split. Ask your pharmacist before splitting any tablet. They can also check whether a different strength is available that would give you a whole tablet dose.
How do I calculate a child's dose from an adult dose?
The safest method is to use the weight-based dose (mg/kg) prescribed by the child's doctor and apply it to the child's actual weight. Scaling down an adult dose by age or fraction is much less reliable and is not recommended. Always consult a paediatrician or pharmacist for children's dosing, especially for infants under two years old.
Can I use this calculator for intravenous (IV) medications?
This calculator covers oral dosing in mg and liquid mL. Intravenous drugs involve additional factors such as infusion rate (mL/hour), dilution volumes, and compatibility with IV fluids. IV dosing should always be calculated and verified by a licensed healthcare professional using dedicated clinical tools.