mg, mg/kg and mg/m2 Are Not Interchangeable Instructions

Quick Medical Calculator · Reading label 9 September 2026 · 4 min read

Retrospective study sequence, not publication date

A dose-normalisation basis is part of the instruction, not an optional display preference. An amount in mg is already an absolute mass. An amount in mg/kg needs the prescribed dosing weight. An amount in mg/m2 needs the protocol-specified body surface area method. Choosing a basis after seeing which answer looks familiar reverses the checking process.

This guide addresses verification of an existing instruction. It does not provide chemotherapy doses, convert adult regimens into paediatric regimens or suggest that body size alone determines a suitable dose.

Three Independently Specified Fictional Instructions

For arithmetic teaching, imagine an adult with a measured height of 180 cm and actual body weight of 80 kg. In this fictional exercise, the surface-area instruction explicitly uses actual weight for Mosteller, while the separate mg/kg instruction independently specifies an 80 kg dosing weight. Their numerical agreement is a stated assumption, not a rule that adjusted or ideal dosing weight belongs in a BSA equation. Consider three unrelated, invented instructions, each referring to one administration:

• Fixed amount: 90 mg per administration remains 90 mg.
• Weight-based amount: 1.2 mg/kg per administration gives 1.2 x 80 = 96 mg.
• Surface-area amount: 45 mg/m2 per administration, using Mosteller, gives 90 mg because the estimated area is 2 m2.

These are separate hypothetical instructions, not equivalent regimens. The first and third happen to produce the same mass in this example. That coincidence does not make their normalising bases interchangeable, and the second result is not an error merely because it differs.

Identify the Surface-Area Method

The implemented Mosteller relationship is BSA = sqrt(height in cm x weight in kg / 3600). With the fictional measurements, sqrt(180 x 80 / 3600) = sqrt(4) = 2 m2. The Cancer Institute NSW eviQ calculator explicitly identifies both Mosteller and Du Bois methods, illustrating why “BSA calculated” is incomplete documentation without the method. eviQ body surface area calculator.

The local BSA dose-adjustment calculator uses Mosteller and a separately entered mg/m2 instruction. It does not divide a standard adult dose by a reference area of 1.73 m2. That latter reference belongs to other indexing conventions and must not be introduced simply because it is familiar from a renal report.

Record the weight convention for the BSA calculation separately from any mg/kg dosing-weight convention. Both must come from the applicable instruction. An estimated area is not a measurement of drug clearance, and the calculator does not choose actual, ideal or adjusted weight for a particular medicine.

A Cap Is Not the Same as a Maximum-Dose Warning

In a separate version of the fictional surface-area exercise, suppose the source instruction explicitly caps the area at 1.8 m2. The arithmetic becomes 45 x 1.8 = 81 mg. Without that explicit cap, it remains 90 mg. The example neither endorses this cap nor suggests that all protocols need one.

The local tool distinguishes the uncapped area from the area used. Its maximum-dose field is another check: an exceeded entered maximum produces a warning, not an automatic replacement prescription. A maximum is not permission to reduce the calculated dose silently. A cap applies only if the source instruction states one; it must not be inferred.

Body Size Is Not the Whole Pharmacokinetic Model

In a primary pharmacokinetic analysis of 69 adults with soft-tissue sarcoma receiving ecteinascidin 743, investigators found no significant association between absolute clearance and BSA. That single-drug study does not authorise abandoning a current regimen's dose basis. It demonstrates why a body-size calculation should not be described as an individual clearance measurement or a universal explanation of exposure. Puchalski and colleagues.

Organ function, interactions, toxicity and treatment-specific adjustments require the relevant current prescribing framework. None is settled by proving that height and weight were multiplied correctly. The original study's suggested dosing interpretation is not reproduced here as a clinical recommendation.

Original Basis-Verification Record

Retain the exact dose expression; administration or daily time basis; dosing-weight descriptor and measurement; named area equation if applicable; explicit cap policy; unrounded calculated mass; and the source of any maximum or other adjustment. Mark unresolved fields rather than filling them from habit.

A second checker should be able to explain why each factor appears in the equation. If the instruction is fixed mg, multiplying by weight creates a different instruction. If it is mg/m2, omitting area leaves an incomplete quantity. The most important verification is not agreement between two calculators, but agreement between the documented prescription and the calculation actually performed.

Retrospective educational reading plan: Day 5 of 50, labelled 2026-09-09. This is a study-sequence label, not a historical publication or clinical-review date. The publication date above records the actual release.