In medicine, a MET (metabolic equivalent of task) is a unit that measures how much energy your body uses during any activity compared to sitting still. One MET equals the energy you burn at complete rest, so an activity rated at 5 METs means your body is working five times harder than it does while sitting quietly. Doctors use METs to assess your fitness level, evaluate your heart health, and determine whether you’re safe for surgery.
How One MET Is Defined
One MET corresponds to an oxygen consumption rate of 3.5 milliliters per kilogram of body weight per minute. In calorie terms, that works out to roughly 1 kilocalorie per kilogram per hour. This baseline was originally derived from the resting oxygen consumption of a single reference subject: a 70-kilogram, 40-year-old man. Every other activity gets its MET value by comparing how much oxygen it demands relative to that resting rate.
So if you weigh about 70 kg (154 lbs), sitting in a chair burns approximately 70 calories per hour. An activity rated at 3 METs would burn roughly 210 calories per hour for that same person. The higher the MET value, the harder your body is working.
MET Ranges for Light, Moderate, and Vigorous Activity
Physical activities fall into three broad intensity categories based on their MET values:
- Light activity (under 3 METs): slow walking, cooking, light housework, casual stretching
- Moderate activity (3 to 6 METs): brisk walking at 3 to 4 mph, vacuuming, recreational cycling, water aerobics
- Vigorous activity (over 6 METs): jogging, swimming laps, singles tennis, heavy yard work like shoveling
These categories matter because most public health guidelines recommend at least 150 minutes per week of moderate-intensity activity (3 to 6 METs) or 75 minutes of vigorous activity (above 6 METs). Knowing where an activity falls on the MET scale tells you whether it counts toward those targets.
Why Doctors Care About Your MET Level
Your MET capacity, sometimes called exercise capacity, is one of the strongest predictors of how long you’ll live. A study published in the American Heart Association’s journal Circulation found that for every 1-MET increase in exercise capacity, the risk of death dropped by 12%. That relationship held even after adjusting for age and other health conditions. In practical terms, improving from a 6-MET capacity to an 8-MET capacity represents a meaningful reduction in mortality risk.
This is why fitness testing in a medical setting almost always reports results in METs. It gives your doctor a single number that captures your cardiovascular fitness and helps track changes over time.
METs in Cardiac Stress Testing
The most common place you’ll encounter METs is during a treadmill stress test, often using the Bruce protocol. The treadmill starts slow and gets progressively steeper and faster in three-minute stages:
- Stage 1: 1.7 mph at a 10% incline, equivalent to about 5 METs
- Stage 2: 2.5 mph at a 12% incline, roughly 7 METs
- Stage 3: 3.4 mph at a 14% incline, roughly 9 METs
The test continues until you reach your target heart rate, develop symptoms, or can’t continue. The MET level you achieve at peak effort tells your cardiologist a great deal. Someone who can reach 10 or more METs generally has good cardiovascular fitness. Someone who stalls at 4 or 5 METs may need further evaluation.
The 4-MET Threshold Before Surgery
One of the most clinically important uses of METs is in preoperative assessment. The 2024 guidelines from the American Heart Association and American College of Cardiology identify 4 METs as the key threshold for functional capacity before noncardiac surgery. Patients who can perform activities at 4 METs or above are considered at relatively low risk for heart complications during and after surgery, and they can typically proceed without additional cardiac testing.
Four METs is roughly equivalent to climbing one to two flights of stairs without stopping, or walking on a flat surface at 3 mph or faster. If you can do those things without chest pain, unusual shortness of breath, or dizziness, your functional capacity likely meets that benchmark. Patients who fall below 4 METs face increased risk for perioperative cardiac events regardless of the reason for their limited capacity, whether it’s heart disease, lung disease, or simply being deconditioned.
Limitations of the Standard MET Value
The 3.5 milliliters-per-kilogram-per-minute definition has a well-known limitation: it was based on a single person’s resting metabolism and doesn’t account for individual variation. Your actual resting metabolic rate depends on your age, sex, body composition, and fitness level. An older adult at rest may consume significantly less oxygen per kilogram than the standard assumes, which means a given activity could be relatively harder for them than its MET rating suggests.
Exercise capacity also declines naturally with age. A 30-year-old reaching 12 METs on a stress test and a 70-year-old reaching 8 METs might both have excellent fitness relative to their peers, even though their absolute numbers differ. Researchers have noted that age-specific cutoffs would be more useful for risk assessment than a single universal threshold, though the standard MET scale remains the default in clinical practice. Despite these imperfections, METs remain the most widely used and practical way to quantify physical effort in medicine.