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.

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.