When monitoring your health and fitness, the Apple Watch and iPhone both have ways of setting your preference for either Calories or kilojoules (kJ) burned.
We typically uses Calories (with a capital "C") to denote the energy the body has burned. Or the available energy in food items. But before we proceed to a discussion about health apps and burning calories, we need some background.
A standard calorie (with a small "c") is the energy it takes to raise the temperature of one gram water one degree Celsius (at sea level). Because that's such a small amount of energy, the health world uses a unit 1,000 times greater, the kilocalorie, abbreviated kcal, or as mentioned above the food Calorie (with the big "C"). That is, 1 Calorie = 1 kcal.
The Calorie is closely related to the SI metric system unit of energy. There, the unit of energy is called the joule (abbreviated "J"). Recall from Einstein's famous energy equation, E = mc2, the energy units are kg-m2/s2 (joules). The SI (metric) unit of food energy is the kJ, kilojoule.
Conveniently (or not so conveniently, depending on your point of view):
1 Calorie (kcal) = 4.184 kilojoules (kJ)
As always, there are many deeper technical nuances that are beyond the scope of this Quick Tip. In essence, what's presented in iOS is a choice between Calories (kcal) and kJ. You'll see that factor of roughly 4.2 when you switch back and forth.
Select on Apple Watch
- Launch the Workout app. (Running figure on green background.)
- Select a workout. Say, Outdoor Walk.
- Swipe left or right as needed to reveal the workout goal in energy.
- Force touch the display to bring up your choice and tap it.
On Apple Watch.
Select on iPhone
- Launch the Health app on your iPhone.
- Select Health Data tab at the bottom.
- Select Fitness.
- Select Active Energy (or Resting Energy).
- Tap Unit > Calories or kJ.
If you make a choice on one device, the units are immediately updated and reflected on the other device. It's that easy! Except for all that metric stuff.