Tracking calories accurately is a balance between good data and time investment.

Absolutely true, and getting good data is really, really hard. In fact, the nutritional content on food labels (in the USA) is allowed to be over by as much as 20% by the FDA.

In addition to that the processes and formulas used to calculate those numbers are also far from precise .

The most accurate method to measure energy and calories in food is called a bomb calorimeter, Li said. The process works on the theory that when one thing releases heat — in this case, food — it’s absorbed by another, in this case whoever is digesting that food, as energy.

To make this calculation, food scientists seal a piece of food in a pressurized, oxygen-filled steel container — the “bomb.” That container is encased in an insulated box filled with water.

The scientists light the food on fire and let it burn completely. Then, they take the temperature of the water. They use an equation to tell them how much energy — and therefore calories — are in that piece of food, based on how much the burning piece of food raised the temperature of the water surrounding it.

However, this is only an estimate because not everything that burns and releases energy to heat the water can be digested by a person eating it. Fiber, for example, burns in the calorimeter but will just pass through your digestive system without giving up its energy.

Many food companies don't even test food like this. They simply estimate the calorie content based on the number of grams of fat, carbs, and proteins in the food. Still, unless and until someone comes up with a better way to determine the calories in a given measure of food, it's the best system we've got.