A quick, human definition
A MET (Metabolic Equivalent of Task) is a way to describe how hard an activity is relative to rest. Think of it as a rate of energy use, not a total score.
By convention, 1 MET is defined as the energy cost of sitting quietly at rest, standardized to 3.5 mL of oxygen per kilogram of body weight per minute (often approximated as ~1 kcal per kg per hour).
That definition is intentionally simple — and also intentionally imperfect. Real humans don’t all rest at the same metabolic rate. METs exist because perfect accuracy is impossible at scale, but useful comparison is not.
A helpful mental model:
- METs are like speed (how fast energy is being used)
- MET‑minutes or MET‑hours are like distance (how much total energy accumulates over time)
You don’t “complete METs” any more than you complete miles‑per‑hour. You accumulate MET‑minutes by sustaining an intensity.
Why METs exist at all (and why that matters)
Long before fitness trackers and calorie apps, physiologists were trying to answer a basic question:
How much energy does this activity cost compared to doing nothing?
In the early 1900s, researchers studying oxygen consumption realized something powerful: oxygen uptake scaled reasonably well with energy expenditure. That insight eventually led to indirect calorimetry, exercise testing, and — much later — practical shortcuts like METs.
By the mid‑20th century, exercise physiology needed a portable abstraction. Labs could measure VO₂ precisely, but public health, epidemiology, and coaching needed something simpler. The MET was born as a compromise: standardized, imperfect, but extremely useful.
This history matters because it explains why METs look the way they do. They were never meant to be individualized precision tools. They were meant to let researchers and practitioners speak a common language about activity intensity.
The Compendium of Physical Activities (where the numbers come from)
If you’ve ever seen a list saying things like:
- Walking at 3 mph = ~3.3 METs
- Running at 6 mph = ~9.3 METs
those numbers almost certainly come from the Compendium of Physical Activities.
First published in 1993 by Barbara Ainsworth and colleagues, the Compendium attempted something ambitious: catalog everyday human activities and assign each a representative MET value.
The 2024 Adult Compendium update now includes:
- 1,114 specific activity codes across 22 categories
- ~82% of MET values based on direct measurement, with the rest carefully estimated
Today, the Compendium is maintained by a collaborative research group and underpins everything from public‑health guidelines to consumer fitness apps.
The key takeaway: MET values are averages of measured reality, not guesses — but they are still averages.
What METs quietly assume (and why that causes confusion)
When you look up a MET value, a lot is being assumed without being stated:
- A standardized resting metabolic rate
- Average body composition
- Typical movement efficiency and technique
- Relatively steady‑state effort
- No added load, incline, wind, or terrain challenges
None of those assumptions are crazy — but none of them are universally true either.
This is where people start to feel that METs are “wrong.” They’re not wrong — they’re just agnostic to individuality.
Turning METs into calories (and what the math really means)
The most common formula you’ll see is:
Calories ≈ MET × body weight (kg) × time (hours)
This estimates gross energy expenditure, meaning it includes the calories you would have burned anyway while resting.
Example:
Running at 6 mph (~9.3 METs):
- 150 lb (68 kg) for 10 minutes:
- 9.3 × 68 × (10/60) ≈ 105 kcal
- 200 lb (91 kg) for 10 minutes:
- 9.3 × 91 × (10/60) ≈ 140 kcal
If you care about net exercise calories, a common adjustment is:
Net calories ≈ (MET − 1) × body weight (kg) × time (hours)
Neither version is “right” in all contexts — they answer slightly different questions.
Body size, fitness, and the uncomfortable nuance
This is where things get interesting — and where oversimplified explanations fall apart.
At rest, people with more lean mass generally burn more calories. Muscle tissue is metabolically active, so two people of the same body weight can have meaningfully different resting expenditures.
During activity, energy cost is driven less by “how muscular you are” and more by:
- External work performed (speed, load, grade)
- Movement efficiency and biomechanics
- Relative intensity (how hard the task is for you)
This leads to counterintuitive but important outcomes:
- Heavier individuals often burn more calories during weight‑bearing activities simply because more mass is being moved.
- Well‑trained athletes may burn fewer calories at the same pace due to superior efficiency — yet can sustain much higher absolute workloads when pushed.
So statements like “athletes burn less” or “obese individuals burn more” are both incomplete. Context matters. METs can’t see that context — humans can.
Heart rate, METs, and why wearables feel smarter
MET tables assume a fixed intensity. Real exercise rarely behaves that way.
Modern wearables improve on this by combining:
- Activity classification (what you’re doing)
- Motion and GPS (external work)
- Heart rate (internal physiological response)
- Personal calibration over time
Heart rate isn’t a calorie meter, but it does reflect how hard your body is working relative to its capacity. That’s why two people doing the same workout can see very different calorie numbers — and why those numbers often feel more “right” than a static MET lookup.
The watch isn’t replacing METs so much as contextualizing them.
EPOC: real, but not magical
After harder exercise, oxygen consumption remains elevated — a phenomenon known as Excess Post‑Exercise Oxygen Consumption (EPOC).
This reflects the cost of restoring homeostasis: replenishing energy stores, clearing metabolites, and repairing tissue.
EPOC is real, measurable, and relevant — but it’s often overstated. For short or moderate efforts, it’s modest. For long or intense sessions, it can meaningfully contribute to total daily energy burn, but it’s not a free calorie loophole.
Where METs shine — and where they struggle
METs work well for:
- Steady‑state aerobic activities
- Comparing relative intensity across tasks
- Population‑level planning and estimation
They struggle with:
- Resistance training
- HIIT and interval work
- Sports with frequent starts, stops, and bursts
In those cases, a single MET value simply can’t capture what’s happening minute to minute.
The real way to use METs
METs aren’t broken. They’re just often misunderstood.
They’re best used as:
- A thinking framework
- A comparison tool
- A starting point, not a verdict
Think of METs as a map, not a GPS. They won’t tell you exactly where you are — but they’ll help you understand the terrain well enough to make better decisions.
