# Respiratory Exchange Ratio (RER = V̇CO₂/V̇O₂)

Calculate RER = V̇CO₂/V̇O₂ from measured gas exchange. Resting interpretation: 0.70 fat, 0.80 mixed diet, 1.00 carbohydrate; exercise interpretation: RER ≥ 1.10 supports maximal effort. Not medical advice.

> Canonical page: https://elysiatools.com/en/tools/respiratory-exchange-ratio

- **Category:** Health

- **Keywords:** respiratory exchange ratio, rer, respiratory quotient, rq, vco2, vo2, substrate utilization, fat oxidation, carbohydrate oxidation, vo2max, exercise testing, indirect calorimetry, metabolic cart

## Overview

The Respiratory Exchange Ratio calculator computes RER (V̇CO₂ / V̇O₂) from indirect calorimetry measurements to evaluate fuel substrate oxidation at rest or assess maximal exertion during cardiopulmonary exercise testing.

## Inputs

- **V̇CO₂ (mL/min)** (number): Carbon dioxide production (mL/min, STPD) from indirect calorimetry.
- **V̇O₂ (mL/min)** (number): Oxygen uptake (mL/min, STPD) from indirect calorimetry.
- **Interpretation Context** (select): Whether the gas volumes were collected at rest or during an exercise test.
- **Decimal Places** (number)

## When to use

- Analyzing resting metabolic rate (RMR) assessments to determine whether fat or carbohydrate is the predominant fuel source.
- Evaluating cardiopulmonary exercise test (CPET) stages to check if a subject reached maximal effort (RER ≥ 1.10).
- Translating metabolic cart gas exchange volumes (V̇CO₂ and V̇O₂) into substrate utilization data for nutritional and athletic monitoring.

## How it works

- Enter the measured carbon dioxide output (V̇CO₂) and oxygen uptake (V̇O₂) in milliliters per minute (mL/min).
- Select the interpretation context, choosing between steady-state rest or graded exercise testing.
- The tool divides V̇CO₂ by V̇O₂ to calculate the RER value rounded to your chosen decimal precision.
- The calculator outputs the numeric ratio along with context-specific physiological interpretation based on established ACSM and clinical calorimetry guidelines.

## Use cases

- Sports physiology labs verifying whether an athlete achieved valid maximal oxygen uptake criteria during a treadmill ramp protocol.
- Clinical nutritionists determining patient lipid versus carbohydrate utilization fractions from resting metabolic cart readouts.
- Exercise physiologists identifying metabolic cross-over points and fuel shift dynamics during submaximal endurance testing.

## Frequently asked questions

### What is the difference between RER and RQ?

Respiratory Quotient (RQ) measures gas exchange at the cellular level, whereas Respiratory Exchange Ratio (RER) measures gas exchange at the mouth via indirect calorimetry.

### What does a resting RER value indicate about substrate use?

An RER of ~0.70 indicates pure fat oxidation, ~0.80 represents a standard mixed diet, and 1.00 indicates pure carbohydrate oxidation.

### Why does RER exceed 1.0 during intense exercise?

High-intensity exercise produces excess CO₂ when bicarbonate buffers accumulating lactic acid, elevating expired CO₂ beyond metabolic substrate oxidation.

### What exercise RER value indicates maximal effort during a VO₂max test?

An RER of 1.10 or higher is widely used as a standard secondary criterion confirming maximal physical exertion during graded exercise tests.

### Can RER fall below 0.70?

Values below 0.70 rarely occur biologically and typically indicate metabolic cart calibration errors, prolonged ketosis, or hypoventilation.

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