Effort
The MET is the common currency that lets you compare a hike to a bike ride. It is also a population average applied to an individual, and knowing where it breaks makes it more useful, not less.
One metabolic equivalent is defined as an oxygen consumption of 3.5 millilitres per kilogram of body mass per minute — the approximate resting metabolic rate of a reference adult. An activity rated at 6 METs demands roughly six times resting oxygen consumption.
Because oxygen consumption maps closely to energy expenditure, MET values allow direct comparison between activities that share nothing else. This is the mechanism behind any statement that forty-five minutes of hiking is equivalent to some other duration of cycling.
The Compendium of Physical Activities, first published by Ainsworth and colleagues in 1993 and revised several times since, catalogues MET values for hundreds of activities. Values derive from indirect calorimetry — measuring oxygen consumption and carbon dioxide production during the activity — averaged across measured subjects, supplemented by estimates where direct measurement was impractical.
| Activity | MET |
|---|---|
| Walking, 2.5 mph, level | 3.0 |
| Walking, 3.5 mph, brisk | 4.3 |
| Hiking, cross-country | 6.0 |
| Cycling, 12–14 mph | 8.0 |
| Running, 6 mph (10:00 pace) | 9.8 |
| Cycling, 16–19 mph | 12.0 |
The standard conversion:
kcal = MET × 3.5 × body mass in kg ÷ 200 × minutes
A 75 kg person hiking for 90 minutes at 6 METs: 6 × 3.5 × 75 ÷ 200 × 90, which comes to about 709 kcal.
Note that this is gross expenditure. Roughly one MET-hour of that would have been spent at rest anyway, so net expenditure attributable to the activity is lower — in this example by about 92 kcal. Whether gross or net is the right figure depends on what you are doing with it.
The resting rate is not universal. The 3.5 ml/kg/min definition is an average that overestimates resting metabolic rate for many people, particularly those with higher body mass or higher body fat percentage. Measured resting values commonly fall between 2.6 and 3.5. For a heavier individual, MET-derived expenditure may overstate reality by fifteen percent or more.
Efficiency varies between individuals. Two people covering identical ground at identical pace can differ measurably in oxygen cost through gait mechanics, training history and neuromuscular efficiency. Trained individuals are typically more economical, meaning they burn fewer calories for the same work — the opposite of intuition.
Terrain and conditions are not captured. A single MET value for hiking cannot represent the difference between a graded path and steep scree, or between still air and a headwind, or between 15°C and 35°C. Grade in particular has an enormous effect: uphill walking at a steep grade can exceed running on the flat in metabolic cost.
Load is usually excluded. Most values assume no carried weight. A loaded pack raises cost substantially and non-linearly, with the effect growing as pack mass approaches a significant fraction of body mass.
The honest use of a MET value is comparative rather than absolute. The claim that ninety minutes of hiking at 6 METs equals roughly fifty-five minutes of cycling at 10 METs is defensible, because the systematic errors largely cancel when both sides use the same assumptions.
The claim that you burned exactly 709 calories is not defensible to that precision, and building a nutrition plan on it compounds the error.
Trust the ratio between activities. Treat the calorie total as an order-of-magnitude figure with a wide band around it.
Public health guidance is expressed in MET-minutes for exactly this reason. The commonly cited target of 150 minutes of moderate activity weekly translates to roughly 500 to 1,000 MET-minutes, and MET-minutes allow substitution across activity types — which is the whole point when the alternative is prescribing one activity to everyone.
It also permits sensible planning across a week that mixes activities. If a long hike, a bike commute and two walks are all denominated in MET-hours, the week can be balanced without pretending they are the same thing.
The activity equivalence calculator converts any activity and duration into MET-hours and returns matched durations across other activities, which is the comparison the MET system is genuinely good at.