How to Calculate Your RER and Understand Its Significance

January 1, 2026 •

4 min read

According to sports science, the respiratory exchange ratio (RER) is a crucial physiological measurement that reveals your body's metabolic efficiency. This guide explains how to calculate your RER, offering a window into how your body uses carbohydrates and fats for energy.

Quick Summary

The respiratory exchange ratio (RER) is a scientific indicator derived from the ratio of carbon dioxide produced to oxygen consumed. Measured non-invasively, it reveals the proportion of fat and carbohydrates your body uses for energy. The value varies with exercise intensity, providing critical insights for optimizing training and nutrition strategies.

Key Points

RER Formula: Calculate RER by dividing the volume of carbon dioxide produced by the volume of oxygen consumed ($V{CO2}/V{O2}$).
Fuel Utilization: An RER value indicates your primary fuel source: ~0.70 for fats, ~1.0 for carbohydrates, and a mixed fuel source for values in between.
Measurement Tool: The RER is accurately measured using a metabolic cart via indirect calorimetry during rest or exercise tests.
Exercise Intensity: Your RER increases as exercise intensity rises, showing a shift from fat to carbohydrate metabolism.
Dietary Influence: The macronutrient composition of your diet significantly affects your RER, with higher carbohydrate intake resulting in a higher RER.
Performance Indicator: Athletes use RER to identify training zones and assess metabolic flexibility, which is the body's ability to efficiently switch fuel sources.

The respiratory exchange ratio (RER) is a powerful physiological measurement used to determine which fuels—carbohydrates or fats—your body is primarily using for energy. An RER of 0.70 signifies that fats are the main fuel source, while an RER of 1.0 indicates a reliance on carbohydrates. A value between these numbers represents a mix of both. Measuring your RER is crucial for tailoring nutrition, assessing metabolic health, and optimizing athletic performance.

The Fundamental Formula for Calculating RER

At its core, the RER calculation is straightforward, though it requires specialized equipment to collect the necessary data. The formula is the ratio of carbon dioxide produced to oxygen consumed.

RER Formula: $RER = V{CO2} / V{O2}$

$V_{CO2}$ is the volume of carbon dioxide expired.
$V_{O2}$ is the volume of oxygen consumed.

To obtain these values, a process called indirect calorimetry is used. This involves wearing a mask or using a mouthpiece connected to a metabolic cart, which analyzes the gases you breathe in and out. The cart measures the volume and concentration of oxygen and carbon dioxide, from which the RER is calculated at regular intervals.

Practical Application: A Step-by-Step Scenario

Resting Measurement: Before exercise, you sit or lie quietly in a rested, fasted state. A metabolic cart collects your expired gases for several minutes until a stable reading is achieved.
Moderate-Intensity Measurement: You begin exercising at a low to moderate intensity, such as walking or jogging on a treadmill. The cart continues to collect data, and the RER will increase as your body shifts its fuel source.
High-Intensity Measurement: As you increase your effort towards maximal exertion, the RER will climb further, potentially exceeding 1.0, which is a key indicator of reaching your maximum aerobic capacity.

This method is a non-invasive way to get a real-time snapshot of your metabolic activity. For a more direct measure of metabolism at the cellular level, the respiratory quotient (RQ) is sometimes discussed, though it requires invasive testing and is not typically used outside of advanced clinical settings.

Interpreting Your RER: What the Numbers Mean

The RER value provides a window into your body's metabolic efficiency and preferred fuel source at a given moment. Here’s a quick guide to what different values indicate:

RER ≈ 0.70: Indicates the body is primarily burning fat for fuel. This is typical during rest, prolonged low-intensity exercise, or in a fasted state.
RER ≈ 0.85: Suggests a balanced mix of fat and carbohydrate utilization for energy. This is often seen during moderate-intensity aerobic exercise.
RER ≈ 1.00: Indicates that carbohydrates are the body's predominant fuel source. This occurs during high-intensity exercise when the body needs a fast energy supply.
RER > 1.00: Occurs during maximal or supra-maximal intensity exercise. The body's anaerobic energy systems kick in, and extra carbon dioxide is produced as a byproduct of lactic acid buffering, causing the RER to exceed the value of 1.0 that is associated with carbohydrate metabolism alone.

Factors Influencing Your RER Value

Several factors can cause your RER to fluctuate, making it a dynamic rather than static metric:

Diet: The macronutrient composition of your recent meals has a significant impact. A high-carbohydrate meal will increase your RER, while a high-fat meal will lower it.
Exercise Intensity: As detailed above, increasing the intensity of your workout pushes your RER higher as you rely more on carbohydrates.
Duration of Exercise: During a long-duration, low-intensity exercise session, your RER will typically start higher and gradually decrease as your body shifts toward burning more fat for fuel.
Fitness Level: A trained endurance athlete may have a lower RER at a given intensity compared to an untrained individual, indicating greater fat-burning efficiency.
Hormonal and Nervous Controls: Hormonal imbalances and other physiological factors can affect your metabolic rate and, consequently, your RER.

RER vs. Resting Energy Expenditure (REE)

While often measured using the same equipment, RER and REE serve different purposes. RER tells you what fuel is being burned, while REE is a measure of the total number of calories your body burns at rest.


Feature	Respiratory Exchange Ratio (RER)	Resting Energy Expenditure (REE)
What it measures	The ratio of CO2 produced to O2 consumed, indicating fuel source.	The total number of calories burned at rest over a period (e.g., 24 hours).
Key insight	Informs on metabolic substrate utilization (carbs vs. fat).	Informs on total caloric need and overall metabolic rate.
Measurement method	Calculated from $V{CO2}$ and $V{O2}$ during indirect calorimetry.	Calculated from $V{O2}$ and $V{CO2}$ using equations like the Weir formula.
Interpretation	A value between 0.7 and 1.0 (or higher during intense exercise).	A value typically expressed in kcal/day.
Best for...	Optimizing training zones, nutritional planning, and assessing metabolic flexibility.	Weight management, calculating total daily energy needs.

Conclusion

Calculating your RER, typically done with specialized equipment like a metabolic cart, offers invaluable insight into your body's metabolic engine. It tells you directly what type of fuel—carbohydrate or fat—is predominantly being oxidized at any given exercise intensity. By monitoring your RER, you can fine-tune your training protocols to optimize performance and refine your nutritional strategy to match your body's specific energy demands. Understanding this metric is a key step toward achieving greater metabolic efficiency and overall fitness. For advanced physiological insights, consulting the comprehensive resource provided by the National Center for Biotechnology Information can be beneficial.

How to Calculate Your RER: A Simple Summary

Measure: Use indirect calorimetry to measure the volume of carbon dioxide expired ($V{CO2}$) and the volume of oxygen consumed ($V{O2}$).
Divide: Divide your measured $V{CO2}$ by your measured $V{O2}$ to get the RER value.
Interpret: Interpret the result to understand your body's primary fuel source: RER near 0.70 means fats, near 1.0 means carbs, and in between is a mix.
Factors: Consider exercise intensity, diet, and fitness level, as these factors significantly influence your RER value.
Application: Use your RER data to optimize your nutrition and training plan for specific goals, whether it's weight management or athletic performance.

Frequently Asked Questions

Respiratory Exchange Ratio (RER) is measured externally at the mouth from expired air, while Respiratory Quotient (RQ) is measured directly at the cellular level. At rest and during moderate exercise, they are considered equal, but RER can exceed RQ during intense exercise due to other physiological processes like lactic acid buffering.

An RER value can exceed 1.0 during very intense, anaerobic exercise. This happens because the body begins to rely more on anaerobic energy pathways. The resulting metabolic acidosis is buffered, producing extra CO2 that is expelled through the lungs, which artificially inflates the RER measurement.

No, you cannot accurately calculate RER without a metabolic cart, as it requires precise measurements of both oxygen consumption ($V{O2}$) and carbon dioxide production ($V{CO2}$) from exhaled air. Specialized sensors are necessary to analyze the gas exchange.

Your diet profoundly affects your RER. A meal rich in carbohydrates will lead to a higher RER, as your body is burning carbs for energy. Conversely, a diet high in fats or a fasted state will result in a lower RER.

By measuring your RER at different exercise intensities, you can identify your 'fat-burning zone.' This allows you to tailor your training to target specific metabolic pathways. For example, a lower RER at a given intensity indicates greater fat-burning efficiency, a desirable trait for endurance athletes.

There is no single 'good' RER value, as the ideal number depends on the context. A low RER (closer to 0.7) during low-intensity exercise or rest is considered good for metabolic health and indicates efficient fat metabolism. A high RER (closer to 1.0) during intense exercise is also normal and expected.

Yes, a higher level of cardiovascular fitness generally leads to a lower RER at a specific submaximal exercise intensity. This indicates that a more conditioned body is better at using fat as a fuel source, conserving its limited carbohydrate stores.

No. While both are related to metabolism and measured using indirect calorimetry, RER is a ratio that indicates your body's fuel source, whereas REE is the total energy (calories) your body burns at rest over a day.