Interspecies Dose Conversion in Preclinical Studies: The…

For laboratory and research use only. Not for human consumption.

Published preclinical papers report doses in milligrams per kilogram of body weight (mg/kg), but a mg/kg figure from a mouse study and a mg/kg figure from a dog study are not directly comparable on their face, because smaller animals metabolize compounds faster relative to their body mass than larger animals do. Allometric, body-surface-area-based scaling is the standard method the pharmacology and toxicology literature uses to normalize doses across species when comparing published animal studies. This article explains that math, strictly as a tool for reading and cross-referencing published animal literature.

This article does not provide, and should not be used to derive, human dosing information of any kind. It covers only animal-to-animal dose interpretation as reported in preclinical research papers.

Key Facts

Raw mg/kg doses are not directly comparable across animal species because metabolic rate per unit body mass varies with body size.
Allometric scaling uses body-surface-area normalization, applied through species-specific Km factors, to compare doses across animal species in published literature.
Km factors are standardized constants assigned to specific animal species (for example, mouse, rat, and dog) used in scaling formulas.
This scaling method is described in toxicology and pharmacology methodology literature and is used strictly for interpreting animal-to-animal study comparisons in this article.
Dose figures always come from a specific published study's methods section and should be read alongside that study's species, strain, and body-weight data.

Why Raw mg/kg Doesn't Scale Linearly Across Species

A dose expressed as mg/kg — total milligrams of compound divided by the animal's body weight in kilograms — looks like a simple, portable unit. In practice, it isn't directly portable between species, because metabolic rate does not scale in direct proportion to body weight. Smaller animals have a proportionally larger body surface area relative to their mass, and metabolic processes correlate more closely with body surface area than with raw mass. This is why a mouse study and a rat study reporting the same numerical mg/kg figure are not necessarily describing an equivalent physiological exposure — and why published preclinical papers rely on standardized scaling methods rather than direct mg/kg comparison when drawing cross-species conclusions.

Body-Surface-Area Normalization and the Km Factor

The most widely referenced method for cross-species dose normalization in the toxicology literature is body-surface-area (BSA) normalization, implemented through a species-specific constant called the Km factor. Each animal species commonly used in preclinical research has an established Km value, derived from that species' average body weight and estimated body surface area. The general form of the conversion, as described in pharmacology methodology literature, is:

Dose normalized to body surface area (mg/m²) = Dose (mg/kg) × Km

Because Km differs by species, converting two different species' mg/kg doses into a common body-surface-area-normalized unit allows a more meaningful comparison between, for example, a published mouse study and a published rat study evaluating the same compound class.

Representative Animal Km Factors Referenced in the Literature

Published toxicology methodology references list approximate Km values for common laboratory species, including mouse, rat, hamster, guinea pig, rabbit, dog, and non-human primates, each derived from that species' typical body weight and surface area. These values vary meaningfully — a mouse's Km is substantially different from a dog's — which is precisely why a raw mg/kg figure cannot be assumed equivalent across species without this normalization step. When reading a paper that references Km-based scaling, the specific values used should always be cited from that paper's own methods or from a recognized toxicology methods reference, rather than assumed.

Worked Example: Comparing Two Published Animal Doses

Suppose Published Study A reports a dose in mice at a given mg/kg figure, and Published Study B reports a dose in rats at a different mg/kg figure, and a researcher wants to know whether these two studies are describing a comparable body-surface-area-normalized exposure. The process, as described in the literature, is to multiply each study's mg/kg figure by that species' respective Km factor, producing two mg/m² values that can be compared on the same normalized scale. If the resulting mg/m² values are similar, the two studies can be read as testing a roughly comparable surface-area-normalized exposure level, despite reporting different raw mg/kg numbers. If they diverge substantially, that divergence itself is a meaningful finding to note when synthesizing the literature.

Reading This Alongside Other Concentration Math

Interspecies scaling is one of several calculation skills relevant to interpreting preclinical literature accurately. Once a dose figure from a paper is understood in normalized terms, researchers often need to translate that into a working solution concentration for their own bench records — see our guide on molarity versus mass concentration calculations and our walkthrough of building a serial dilution series for preparing a concentration range from a stock solution. Our reconstitution calculator handles the mass-to-volume math for a given vial once a working concentration has been determined from the literature.

Reading Study Endpoints Alongside Dose Data

Dose figures are only meaningful in context with the study's reported endpoints and safety thresholds. Terms like NOAEL (no-observed-adverse-effect level), LD50, and MTD (maximum tolerated dose) appear throughout the toxicology literature alongside dose data, and understanding what each term measures is necessary to interpret a paper's dose-response findings correctly. Our preclinical safety and toxicology glossary defines these terms as they're used in published animal research.

Scope and Limits of This Method

Allometric and body-surface-area scaling is a tool for comparing published animal data to other published animal data. It is a methodology described in toxicology and pharmacology literature for interpreting and cross-referencing preclinical papers — nothing in this article extends that math to deriving a dose for a person, and no such extension is provided, implied, or intended here.

Frequently Asked Questions

Does this article explain how to convert an animal study dose into a dose for a person?

No. This article is strictly about interpreting and comparing doses between animal species reported in published preclinical literature. It does not provide, and should not be used to derive, human dosing information of any kind.

Why can't researchers just compare raw mg/kg figures between species?

Because smaller animals have a higher metabolic rate relative to body mass, a raw mg/kg dose that is well tolerated in a mouse does not correspond to the same mg/kg figure in a larger animal. Allometric scaling using body-surface-area normalization and Km factors accounts for this, which is why published papers use it rather than direct mg/kg comparison.

What is a Km factor?

A Km factor is a standardized body-surface-area-to-body-weight conversion constant assigned to a given animal species, used in allometric scaling formulas to normalize doses reported in mg/kg across species of different sizes.

Where can I find published mg/kg values to practice this math on?

Peer-reviewed preclinical papers typically report dose in the methods section, along with the animal species, strain, and body weight range used. Toxicology sections of published studies are a common source of the mg/kg figures referenced in scaling calculations.