---
title: "Milligrams per Kilogram (mg/kg) — AP Enviro Definition"
description: "mg/kg measures how much of a chemical a body absorbs per unit of body weight, the standard unit for LD50 toxicity in AP Environmental Science Unit 8."
canonical: "https://fiveable.me/ap-enviro/key-terms/milligrams-per-kilogram-mgkg"
type: "key-term"
subject: "AP Environmental Science"
---

# Milligrams per Kilogram (mg/kg) — AP Enviro Definition

## Definition

Milligrams per kilogram (mg/kg) is a dose unit that expresses how many milligrams of a chemical an organism receives for every kilogram of its body weight. In AP Enviro, it's the standard way to report LD50, the dose lethal to 50% of a test population.

## What It Is

Milligrams per kilogram (mg/kg) tells you how much of a substance an organism takes in relative to its size. One milligram per kilogram means one milligram of [chemical](/ap-enviro/unit-8/lethal-dose-50-percent-ld50/study-guide/TAa4nnWGzeffK0Gvo6iO "fv-autolink") for every kilogram the organism weighs. Because it scales to body weight, the same mg/kg dose is roughly fair to compare across a tiny mouse and a large human.

This is the unit you'll almost always see attached to **LD50** (lethal dose 50%), the dose of a chemical that kills 50% of a test population of a species (8.12.A). When a [toxicity](/ap-enviro/key-terms/toxicity "fv-autolink") study reports an LD50 of, say, 5 mg/kg versus 5,000 mg/kg, the lower number means the chemical is far more toxic because it takes a much smaller dose per kilogram of body weight to be lethal.

## Why It Matters

mg/kg lives in [Unit 8](/ap-enviro/unit-8 "fv-autolink") (Aquatic and Terrestrial Pollution), specifically in topic 8.12, where the learning objective [AP Enviro](/ap-enviro "fv-autolink") 8.12.A asks you to define LD50. mg/kg is the unit that makes LD50 a real, comparable number. Without a dose-per-body-weight unit, you couldn't rank chemicals by how dangerous they are. On the exam, this connects to the bigger theme of pollution and human/ecosystem health: dose determines harm, and the unit is how you measure that dose.

## Connections

### LD50 (Lethal Dose 50%) (Unit 8)

mg/kg is the unit LD50 is reported in. They travel together: LD50 is the value, mg/kg is the scale. A lower mg/kg LD50 means a more toxic chemical because less of it per kilogram of body weight kills half the population.

### Parts per million (ppm) (Unit 8)

Both are concentration-style units, but they answer different questions. ppm measures how much of a substance is in an environment like [water](/ap-enviro/unit-6/hydrogen-fuel-cell/study-guide/VBHYpOxkIwXQuPkI6px8 "fv-autolink") or air, while mg/kg (for dosing) measures how much enters a body relative to its weight.

### Toxicity threshold (Unit 8)

A toxicity threshold is the dose where harmful effects begin to appear, often expressed in mg/kg. LD50 sits well above that threshold because it's the dose that kills half the population, not just where damage starts.

## On the AP Exam

Expect mg/kg to show up as the unit on a dose-response graph or in an LD50 data table. The skill being tested is reading and comparing values: if Chemical A has an LD50 of 10 mg/kg and Chemical B has 800 mg/kg, you should be able to say A is more toxic because a smaller dose per kilogram of body weight is lethal. You may also need to scale a dose to body weight, multiplying the mg/kg value by an organism's mass in kilograms to get the total milligrams. Watch the direction of the logic. Lower mg/kg LD50 equals higher toxicity, which trips up a lot of people.

## Milligrams per kilogram (mg/kg) vs Parts per million (ppm)

mg/kg as a dose is about how much chemical enters an organism per kilogram of its body weight. ppm is about how concentrated a substance is in a medium like water, soil, or air. One describes the dose hitting a body, the other describes the contamination level in the environment. Note that mg/kg and ppm can be numerically equal as mass ratios, but in AP Enviro you'll see mg/kg used for dosing and LD50.

## Key Takeaways

- Milligrams per kilogram (mg/kg) expresses a dose as milligrams of chemical per kilogram of body weight.
- mg/kg is the standard unit for reporting LD50, the dose lethal to 50% of a test population (8.12.A).
- A lower mg/kg LD50 means a more toxic chemical because less of it per kilogram is needed to kill.
- Because mg/kg scales to body weight, it lets you compare toxicity fairly across species of different sizes.
- To find a total dose, multiply the mg/kg value by the organism's body mass in kilograms.

## FAQs

### What does mg/kg mean in AP Environmental Science?

It means milligrams of a chemical per kilogram of body weight, the unit used to express dose. In Unit 8 it's how LD50 values are reported so you can compare how toxic different chemicals are.

### Does a higher mg/kg number mean a chemical is more dangerous?

No, it's the opposite for LD50. A higher mg/kg LD50 means it takes a larger dose per kilogram of body weight to kill, so the chemical is less toxic. The lower the mg/kg, the more toxic it is.

### How is mg/kg different from ppm?

mg/kg as a dose describes how much chemical enters a body relative to its weight, while ppm describes how concentrated a substance is in an environment like water or air. In AP Enviro, mg/kg goes with LD50 and dosing, ppm goes with environmental concentration.

### How do I calculate a total dose from mg/kg?

Multiply the mg/kg value by the organism's body mass in kilograms. For example, a 5 mg/kg dose for a 70 kg person is 5 times 70, or 350 milligrams total.

### Is mg/kg the same thing as LD50?

No. LD50 is the value, the dose that's lethal to 50% of a population, and mg/kg is the unit that value is measured in. You'll usually see them together, like an LD50 of 25 mg/kg.

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