One MET is the rate at which a body uses energy sitting still. An activity rated at 8 METs uses roughly eight times that rate. It is a multiplier, not a quantity, which is what makes it portable between people of different sizes.
Turning it into calories takes weight and time: a 75 kilogram person doing 45 minutes at 8 METs burns about 473 calories, roughly 10.5 calories a minute.
Why weight is in the formula
Moving a heavier body the same distance takes more energy, so the same activity at the same MET value burns more calories for a heavier person. This is arithmetic, not a judgement, and it cuts both ways: as weight falls, the calories burned by the identical session fall with it.
That is one reason weight loss slows over time even when nothing about the routine has changed. The same walk costs less than it used to.
What MET values do not know
A MET value is a population average for an activity described in words. "Cycling" covers a gentle flat commute and a climb into a headwind. Fitness matters too: a trained person is more efficient, and often burns slightly less doing the same measurable work.
Terrain, technique, temperature and how much rest is built into the session all move the real figure. A MET table cannot see any of it, so a MET based estimate is a reasonable central guess with meaningful spread around it.
Machine displays and watches
Gym equipment usually assumes a default weight unless you enter your own, so an unconfigured display is calculating for a person who is not you. Wrist devices infer effort from heart rate, which brings its own error, since heart rate responds to heat, caffeine, stress and sleep as well as to exercise.
Estimates from any of these sources are best used for comparison against themselves rather than as absolute totals. Whether today's session was harder than last week's is a question they answer far better than how many calories it burned.
The scale of the number
473 calories for 45 vigorous minutes is a useful sanity check against the common assumption that exercise creates enormous room to eat. It is a meaningful contribution, and it is smaller than most people estimate.
The MET calculator takes the MET value, weight and duration directly. The calories burned calculator covers common activities, and the heart rate based calculator uses the other common estimation route.