MET

MET stands for “metabolic equivalent” and tells you how much more energy your body uses during an activity than at rest. 1 MET corresponds to your resting energy expenditure while sitting quietly – roughly 1 kilocalorie per kilogram of body weight per hour (more precisely: 3.5 millilitres of oxygen per kilogram per minute). An activity rated at 4 METs therefore uses four times as much energy as sitting.

The values come from the Compendium of Physical Activities (Ainsworth et al., 2011 edition), a table listing more than 800 activities. A few examples: strolling slowly 2.8 METs, brisk walking 4.3 METs, vacuuming 3.3 METs, leisurely cycling 4.0 METs, recreational swimming 6.0 METs, jogging at 8 km/h 8.3 METs, casual football 7.0 METs.

With METs, working out your energy expenditure is easy: METs × body weight in kilograms × hours. Someone weighing 70 kilograms who walks briskly for half an hour burns about 4.3 × 70 × 0.5 ≈ 150 kilocalories. That's the total including resting expenditure; the extra amount burned is a little lower.

The WHO also uses METs for its classification: light activity is below 3 METs, moderate between 3 and 5.9 METs, and vigorous from 6 METs upwards. For the recommended 150 minutes of activity a week, everything from 3 METs counts – so brisk walking is very much “real” exercise.

For children, the adult tables only apply to a limited extent, because their resting energy expenditure per kilogram is higher. There are separate compendiums for children, but for everyday family life a rule of thumb is enough: anything that leaves you out of breath is moderate to vigorous.