Every activity has a value called a "MET value" which calculates the energy required for that activity.

Multiplying MET value by weight in kilograms tells you calories burned per hour.

You can look up research-backed MET values on the Compendium of Physical Activities website.

Throughout the day, everything we do burns calories.

Some things — like sitting — keep us at our resting rate. Vigorous activity can burn more than ten times as much energy.

And while calorie-counting isn't necessarily the best way to lose weight, it can be useful or just plain fun to know whether that post-work soccer game is enough to burn off the donuts your co-worker brought in this morning.

Fortunately, there's a science-backed way to calculate how many calories you burn doing almost anything. Sure, there are apps out there that will help you calculate how many calories you burn on your run or your bike ride, but this goes deeper than that.

Want to know how many calories you burn backpacking, milking a cow (manually), cleaning a church, or engaging in an hour of vigorous sex? There's data that will help you calculate that — along with calories burned while engaging in all kinds of different sports.

Researchers have assessed the amount of energy required to engage in all kinds of activities over the years. In order to make it easier for other scientists to conduct large scale studies, researchers from the National Institutes of Health (NIH) and Arizona State University have compiled updated versions of that data on a website, the Compendium of Physical Activities. And anyone can go to that website, look up an activity, and calculate how many calories they'll burn doing something. It just takes some simple math.

Here's how it works:

This calculation relies on a key value known as a MET, which stands for metabolic equivalent. One "MET" is "roughly equivalent to the energy cost of sitting quietly," according to the Compendium, and can be considered 1 kcal/kg/hour. Since sitting quietly is one MET, a 70 kg person would burn 70 calories (kcal) if they sat quietly for an hour.

If an activity's MET value was two, that same person would burn 140 calories in an hour.

On the Compendium's website, you can look up a huge number of activities. We've included calorie counts for some of the most popular activities in another article, but if you want to make the calculation for yourself, here's how it works.

First, calculate your weight in kilograms — 1 kg is 2.2 lbs, but you can always type "X pounds to kg" into Google, with X being your weight, to get a number.

Second, look up your activity on the Compendium. There's a dropdown menu on the site labeled "Activity Categories." Under that menu, you'll see a long list of categories, starting with bicycling and finishing with volunteer activities. If you open up a category, you can see the activities that fall under it.

If you open up sports (category 15) you can then select an activity. There are many listings for some activities — there's a difference between boxing in a ring and boxing by hitting a punching bag, for example. Look for the MET value from the 2011 Compendium, as it's the most up to date. If the MET value is blue, there are published studies supporting that value. If it's red, it's an estimate.

Here's your equation: MET value multiplied by weight in kilograms tells you calories burned per hour (MET*weight in kg=calories/hour). If you only want to know how many calories you burned in a half hour, divide that number by two. If you want to know about 15 minutes, divide that number by four.

So if a 175-pound person like myself were to play competitive soccer (MET value of 10) for one hour, the equation for calories burned would be: 79.38 kg*10=793.8 calories/hour.

There are a few caveats. Everyone's resting metabolic rate differs slightly — some people of the same weight naturally burn more or fewer calories, depending on a number of factors, and these differences can be significant. As the Compendium website explains, this sort of calculation doesn't take into account differences caused by body mass, body fat, age, sex, efficiency of movement, and conditions like high altitude that may have an impact on the energy required for an activity. Also, these calculations are calculated based only on time spent in movement — so if half of my "competitive soccer" game was really just standing around, I'd have to divide that number in half and then add in the amount of calories I burned standing around to know how much energy I actually used in that hour.

That said, this is the easiest way to get a science-backed estimate of calories burned in an activity. And when you look through activities, there are all kinds of fun things that make the list — it's worth taking some time to explore.