Medication Dosage Calculator

The dosage calculator converts a weight-based dose, expressed in milligrams per kilogram, into a total dose for a specific body weight, and can convert that total dose into a volume using a medication's concentration. It is a general-reference arithmetic tool. It does not know the identity of a drug, its approved dose range, or a person's medical history, and it must never be the sole basis for administering a medication to yourself, a child, or anyone else.

The formula

Total dose (mg) = weight (kg) x dose rate (mg/kg). Volume to give (mL) = desired dose (mg) / concentration (mg/mL)

• weight = body weight in kilograms
• dose rate = the prescribed or labeled dose per kilogram of body weight
• concentration = the amount of drug per millilitre of the liquid preparation being used

Weight-based dosing and the desired-over-have method, a standard pharmacology calculation taught in nursing and pharmacy training

How the calculation works

Weight-based dosing multiplies a per-kilogram dose rate, set by a prescriber or a drug's approved labeling, by a person's actual body weight to get a total dose in milligrams. This step is especially relevant to pediatric medicine, where body weight varies enormously between patients of the same age.

The desired-over-have method then converts that total dose into a practical volume to administer, when the medication is a liquid. It divides the desired dose by the concentration of the preparation on hand, expressed as milligrams per millilitre, to get the number of millilitres to give.

Both steps depend entirely on accurate inputs. An error in the weight used, the dose rate, or the labeled concentration carries directly through to the final volume, with no built-in check against a safe or approved range.

How to read your result

The numbers below illustrate the arithmetic only. They are not dosing recommendations for any specific drug, and the mg/kg rate for a real medication must always come from its approved labeling or a prescriber, never from a table like this.

Why pediatric dosing is weight-based

Adult medication doses are often given as a single fixed amount because most adults fall within a comparatively narrow weight range for the purposes of many drugs. Children range from roughly 3 kilograms at birth to well over 60 kilograms in adolescence, a variation of twenty-fold or more, so a fixed adult-style dose would be either ineffective or dangerous across that range.

Weight-based dosing scales the amount of drug to the individual child, which is why accurate, current body weight, not an estimate and not a weight recorded at a previous visit, matters so much in pediatric and neonatal care.

Even with weight-based dosing, many drugs have a maximum total dose that should not be exceeded regardless of how the per-kilogram calculation scales, particularly once a child's weight approaches adult size.

Dose versus dose rate, and why divided doses cause errors

A dose rate, such as 30 mg/kg/day, describes how much drug is given per kilogram over a defined period, usually a day. A dose is the actual amount given at one administration. These are not the same number whenever a medication is given more than once a day, and mixing them up is a well-documented source of both underdosing and overdosing, particularly outside a hospital setting where a second person does not automatically check the arithmetic.

When a labeled rate is expressed per day but the medication is given in divided doses, for example three times daily, the daily total must be divided by the number of doses before it is administered. Treating the labeled per-day rate as if it were the per-dose amount multiplies every single dose by the number of times it is given each day, a threefold error for a three-times-daily schedule, which is large enough to cause serious harm for many drugs.

The reverse mistake also occurs: applying a per-dose rate as if it applied to each of several doses in a day. Because 'dose' and 'dose rate' are often used loosely even though they mean different things on a label, reading the labeled unit carefully matters as much as getting the weight and concentration right.

• mg/kg/day describes a 24-hour total; mg/kg/dose describes a single administration. Confirm which one appears on the label before doing any arithmetic.
• mcg and mg differ by a factor of 1,000; misreading one for the other produces an error of the same magnitude, one of the most common serious dosing mistakes.
• A dose calculated correctly for the labeled rate can still be wrong if it is then given more or fewer times per day than the schedule specifies.

Limitations

• This calculator performs arithmetic only. It does not know the identity of a drug, its approved dose range, its maximum single or daily dose, or whether the calculated amount is safe for a specific person. Using it to determine an actual dose to give is a misuse of the tool.
• Dose rates and concentrations must come from a medication's official labeling, a pharmacist, or a prescriber, never from memory or an unverified source; a decimal point or unit error at this step, such as confusing mg with mcg or mg/kg with mg/kg/day, can cause a tenfold dosing error or worse.
• It does not adjust for renal or hepatic impairment, prematurity, obesity, or other conditions that change how a drug should be dosed even when the weight-based arithmetic is followed correctly.
• It does not check the result against a maximum safe dose, a therapeutic range, or drug interactions. A mathematically correct answer can still be an unsafe dose.
• Every calculated dose intended for actual administration should be independently verified by a second qualified person, such as a nurse, pharmacist, or prescriber, before it is given, particularly in pediatric and neonatal care where errors are both more common and more consequential.
• If there is any doubt about a dose, a medication, or a concentration, stop and contact a pharmacist, prescriber, or, in the case of a suspected overdose, emergency services or a poison control center immediately. Do not rely on this calculator to resolve that doubt.

Frequently asked questions

Can I use this calculator to figure out how much medicine to give my child?

No. This tool only performs the arithmetic of weight-based dosing. The correct dose rate for any specific medication must come from the product labeling, a pharmacist, or your child's prescriber. Always confirm the actual dose with a qualified professional before giving any medication.

What is the desired-over-have method?

It is a standard way of converting a prescribed dose into a volume to administer: divide the desired dose in milligrams by the concentration of the medication on hand in milligrams per millilitre. It is taught in nursing and pharmacy training as a routine dosage calculation.

Why do dosing errors happen even when the maths involved is simple?

Most dosing errors trace back to wrong inputs, not wrong arithmetic: an outdated weight, a misread concentration, or confusing mg with mcg. Because these calculations are usually straightforward, they are sometimes checked less carefully, which is exactly why independent double-checking matters.

Is weight-based dosing used for adults too?

Yes, for some drugs, especially those with a narrow safety margin or that are used in critical care, such as certain anticoagulants and chemotherapy agents, though many other adult medications use a fixed standard dose instead.

What is the difference between a dose and a dose rate?

A dose rate, such as mg/kg/day, describes the total amount over a period, usually a full day. A dose is the amount given at one administration. When a medication is given more than once a day, the daily total from the dose rate must be divided across the number of doses; treating the daily rate as a single dose multiplies the amount given at each administration.

References

1. U.S. Food and Drug Administration. Drugs.
2. CDC. Medication Safety.
3. National Kidney Foundation.