Dosage Calculator

Dosage Forms

Depending on the route of administration, dosage forms come in several different types. The most popular route of administration is oral; however, in some medical situations, this route is unavailable, ineffective, or associated with a high number of adverse reactions.

Patients may receive drugs through parenteral means (this includes subcutaneous, intramuscular, intraosseous, intravenous, etc.). Moreover, some drugs are only given topically — they're applied directly onto the skin or eye, into the rectum or vagina, or as an inhalation.

The route of administration of a drug dramatically influences its dosage and efficacy. Always check that you are administering the appropriate drug to the appropriate person through the appropriate route in the appropriate dose.

What Is an Appropriate Dose?

When treating a patient or just taking a medication by yourself, you always need to determine the exact dose of the chosen drug. Regarding drugs, there are several different kinds of dosing that depend on the specific drug and/or the patient:

1. One dosage — Drugs with one fixed dosage are the easiest to use; you just take it and forget about it (e.g., vaccines). No dosage calculation is needed!
2. Different, fixed dosages — The dosage is usually set by the doctor according to the patient's needs or age (e.g., antihypertensive drugs).
3. Different, variable dosages — The doctor suggests a sample dosage; the patient then modifies it according to their needs and the situation (e.g., insulin).
4. Blood concentration-dependent dosages — A relatively small group of drugs. The doses differ depending on the concentration of the drug's active compound in the serum (e.g., vitamin K antagonists, lithium).
5. Body weight-dependent dosages — The dose of the drug depends on the weight of the patient. Two patients with the same disease may receive a different amount of the same drug! Our dosage calculator helps you calculate this type.

Why Do We Need to Calculate Doses?

In the sixteenth century, the Swiss scientist Paracelsus said:

This sentence still holds today. Every substance, especially drugs, may be toxic if given in an inappropriate dose. An example of a well-known but potentially toxic substance is Tylenol (paracetamol/acetaminophen).

Dosage calculations should be performed carefully and accurately. This is especially critical when administering drugs to children, as a slight mistake in the dose may cause a significant change in the effect of its action.

Contemporary scientists are also looking at the issue of drug dosage. There are plenty of ongoing clinical trials in which different doses of drugs are being compared. Scientific data increasingly suggests that individually performed dosage calculations improve patient survival when compared with standard treatment of fixed doses.

For instance, in 2016 a group of scientists from France published an article in the Journal of Clinical Oncology in which they summarized the results of their clinical trial on patients with metastatic colorectal cancer. The group treated with an individually adjusted dose of fluorouracil had a better response to therapy, a higher survival rate, and a lower grade of toxicity than those treated with standard doses.

Calculating Medication Dosage by Weight

Medical dosage calculations that consider a patient's weight are very common in obese patients and on pediatric wards. The weight of an adolescent may be 40 times larger than that of a newborn baby, so the need to dose drugs accurately is paramount.

Moreover, some specific drugs for adults need to be dosed according to the patient's weight, including:

• Heparin (anticoagulant)
• Antibiotics (e.g., vancomycin, gentamicin)
• Muscle relaxants
• Chemotherapy agents

Dosage Calculation Formulas

If you want to calculate the dose of a medication, you need to use the following equation:

Where:

• Weight — Patient's weight, expressed in kg or lb. It is very important that you input an accurate result.
• Dosage — Prescribed amount of drug in mg per kg of body weight. You can usually find this number on the medication box or on the prescription.
• Dose — Total amount of medication you need to take (in mg).

The situation gets more complicated if your medicine is in liquid form. To find how many mL to take:

This is especially useful when giving fluid drugs (e.g., syrup) to children and in hospitals, where many drugs are given intravenously.

How to Calculate Drug Dosage — Step by Step

1. Determine the dosage of the medication. Let's say the appropriate dosage of the active substance is 2 mg/kg of body weight.
2. Weigh yourself. Let's assume you weigh 80 kg.
3. Multiply these two values to get the dose of medication in mg:
   2 × 80 = 160 mg
   You need to take 160 mg of active substance.
4. For liquid medication: Type the concentration into the proper box. Let's say the concentration is 2 mg/mL.
5. Divide the dose by the medicine concentration to obtain the liquid dose:
   160 ÷ 2 = 80 mL

Tablets Dispensing Calculation

If you need to figure out how many tablets to dispense, you can use a simple approach:

For example, if your required dose is 500 mg and each tablet contains 250 mg, then you need 500 ÷ 250 = 2 tablets.

Frequently Asked Questions