Why is the per-kilogram field empty?

Because that number has to come from your prescription or product labelling. Pre-filling it would amount to suggesting a dose, which this tool does not do for any substance.

Should I use actual or ideal body weight?

That depends entirely on the instruction you are following, and the two can differ substantially. Whoever wrote the per-kilogram figure should also have specified which weight it applies to.

Weight-Based Dose Calculator

Multiply a per-kilogram figure from your own prescription by body weight, in either pounds or kilograms.

Draft — awaiting review. Not indexed.

Total— mg

Total: — mcg
Body weight: — kg

Formula: Total = body weight in kg × the per-kilogram amount you enter. Pounds convert at 0.45359237 kg per lb.

This tool performs mathematical conversions only. It is not medical advice and does not recommend, endorse, or evaluate the safety, efficacy, or legal status of any substance. Consult a licensed healthcare provider before using any medication or peptide, and verify the legal status of any substance in your jurisdiction before use.

What this calculates

Some dosing instructions are written per kilogram of body weight rather than as a flat amount. This multiplies the two together, converting pounds to kilograms first if that is how you know your weight. It is deliberately the emptiest calculator in this category: the per-kilogram field starts at zero and has no suggested value, because that figure belongs to a prescription or a product label and there is no version of it this page could supply without making a claim it has no business making.

How it works

Body weight is converted to kilograms — the pound is defined as exactly 0.45359237 kg, so that conversion is exact rather than approximate. The per-kilogram amount is converted to milligrams, then the two are multiplied. The result is shown in both milligrams and micrograms, since per-kilogram figures are commonly written in micrograms while vials are commonly labelled in milligrams, and the mismatch between the two is a routine source of error.

Worked example

Using an arbitrary per-kilogram figure of 0.5 mg/kg, purely to show the arithmetic: a person weighing 70 kg gives 70 × 0.5 = 35 mg. If the same person knows their weight as 154 lb, that converts to 154 × 0.45359237 = 69.85 kg, and 69.85 × 0.5 = 34.93 mg — close enough that the difference is a rounding artefact of how the weight was stated rather than a real difference. Change the per-kilogram figure and the answer scales directly with it.

Common mistakes

Entering a total dose in the per-kilogram field. The two are different quantities and swapping them silently multiplies the result by body weight, which for an adult is a seventy-fold error rather than an obvious one. Second, unit mismatch: a per-kilogram figure written in micrograms entered while the selector reads milligrams overstates the total a thousandfold. Third, using an outdated body weight for a dose that is meant to track it — if the instruction is per kilogram, the weight it is applied to is meant to be current.

Common questions

Why is the per-kilogram field empty?: Because that number has to come from your prescription or product labelling. Pre-filling it would amount to suggesting a dose, which this tool does not do for any substance.
Should I use actual or ideal body weight?: That depends entirely on the instruction you are following, and the two can differ substantially. Whoever wrote the per-kilogram figure should also have specified which weight it applies to.