What is a MET and how exercise calories work

A MET is a multiple, not a unit of energy

MET stands for metabolic equivalent. What it measures is not energy but a ratio: how many times more oxygen an activity consumes than sitting still does. A walk rated at 4 METs means spending four times what that person spends in a chair.

The denominator of the ratio is fixed by convention. One MET is defined as 3.5 millilitres of oxygen per kilogram of body weight per minute, and that figure sits underneath every table ever published. Each row is a measurement divided by that denominator.

Working in ratios is a practical choice: it lets one line describe the same activity on a 55-kilogram body and a 110-kilogram one without anybody having to measure their own metabolism. The price is that everybody shares the same denominator.

Turning the ratio into calories comes afterwards, when oxygen becomes energy and energy meets your duration and body weight; that step is what the calories burned calculator does. What follows here is the assumption sitting in front of that step.

Where the 3.5 came from

The denominator was not measured on you. It was chosen: 3.5 millilitres per kilogram per minute was set as the resting oxygen uptake of a 70-kilogram, 40-year-old man, and it has stayed the same for everyone ever since.

Real measurements usually come in below it. Resting oxygen uptake in adults tends to land somewhere between 2.6 and 3.0 millilitres per kilogram per minute, and when the denominator is set too high, the multiple that comes out of it is too high as well.

The size of that is not trivial. For someone whose true resting value is 3.0, the table is about 17% generous; at 2.6 it is 35% generous. Every tool reading that table inherits the bias before it does any arithmetic of its own.

The bias is not spread evenly either. Fat tissue consumes far less oxygen at rest than muscle does, so average uptake per kilogram falls as body fat rises, which means the table overstates heavier and older bodies most, while the group it fits best is the one the reference was taken from.

The multiple is not multiplying your metabolism

A MET table cannot tell two people apart. Two bodies of the same weight walking at the same speed get the same multiple, even though their measured resting metabolisms can differ by hundreds of calories a day.

The direction of the error is not fixed either. Someone carrying a lot of muscle and training regularly burns more oxygen per kilogram at rest than the average, so the table understates them, while the same table overstates a sedentary body with a high fat percentage.

For the multiple to belong to you, the denominator would have to belong to you as well. In practice the denominator is frozen and the only input that moves is body weight. The difference between BMR and total daily expenditure is exactly what that denominator is about.

Which makes the table a population average rather than a personal estimate. Good enough to compare one session against another. Not good enough to know a single session to the nearest calorie.

One row hides every intensity

MET tables do separate activities by speed: walking runs from about 3.0 to 5.0 across several rows, running from 8 up to 16. In an interface, though, you usually see a single “running” option and no clue which row is behind it.

The difference is not something you can wave away. Between 8 km/h and 14 km/h the MET value more than doubles, so for the same duration, which row gets used decides whether the session is worth one number or two of them.

Weight-bearing matters too. In walking and running, the energy cost is driven mostly by the mass being carried, so an experienced runner and a beginner spend almost the same at the same speed; cycling and swimming reverse this, because there the loss to water and air is a matter of technique, and a beginner swimmer can spend twice what an efficient one spends over the same distance.

The table does not see any of it. There is one line for swimming. There is one for cycling, and wind, gradient and riding position never enter it.

The costliest mistake: counting the same energy twice

If your daily calorie target was built with an activity multiplier, that multiplier already contains your training. Choosing 1.375 because you train three times a week means that 1.375 already stands for those three sessions.

Adding the 400 calories your watch reports at the end of a session on top of that counts the same energy twice. For anyone trying to lose weight this is quiet and expensive: the deficit exists on paper and not on the plate.

The rule is simple. Either the activity multiplier or the session log, never both. If you want to log sessions individually, pull the multiplier down to the sedentary end and add each session as it happens; if you would rather leave the multiplier where it is, keep session calories out of the plan entirely.

Not writing the number down at all is also an option. Keeping training outside the calorie ledger and watching only the target removes the double-counting risk at the root.

MET-minutes: where the number genuinely earns its keep

Multiplying a MET value by minutes instead of converting it to calories gives you a far sturdier measure. MET-minutes fold intensity and duration into one figure and are not disturbed by the uncertainty in the denominator.

An example: an 80-kilogram person doing a 4.3 MET brisk walk for 45 minutes banks 194 MET-minutes. Repeat it three times a week and it comes to 581.

That number has a reference to be read against. The World Health Organization’s recommendation of 150 minutes of moderate or 75 minutes of vigorous activity a week works out at roughly 500 to 1,000 MET-minutes. Those three walks meet the lower end on their own. It also explains why vigorous minutes count double in the recommendation: MET-minutes already weight intensity, so half an hour at 8 METs gives the same total as an hour at 4.

Body weight never appears in a MET-minute, which is precisely why the figure can be compared between people.

What to do with the number

The soundest reading treats the calories from a MET table as a ceiling. Real expenditure almost always sits a little below it and never above.

The second sound use is comparison. Because the table applies the same assumption to both sessions, you can see reliably which workout was the heavier one; the absolute figure can be wrong while the ratio between them stays right.

Using it as food credit is the weakest use of all. The walking-side version of the same problem is set out in how many calories 10,000 steps burn.

The calories burned calculator runs the MET table against your own weight and duration. To build the daily target itself there is the calorie calculator, and to track walking separately, the steps to calories calculator.