The general calculation in the UK & EU is as follows: total carbs - polyols = net carbs.
If you are in the US (or have an American labelled product) the calculation is: total carbs - polyols - fibres = net carbs.
We will explain US labelling in a separate article but here we will breakdown the Net Carb calculation from a European perspective.
Total Carbs: the total carbs include all the different types of carbs in food, for example including starches and sugars.
Polyols
Generally, polyols are sugar alcohols that are non-calorific and have no impact on your
body, for example erythritol.
Erythritol is a sugar alcohol that has a glycemic index of zero- which means it has no effect on blood glucose level. However, there are calorific polyols out there that should NOT be taken off the total carbs, including: maltitol, sorbitol and xylitol!
For example, Maltitol has a glycemic index of around 50, which is close to that of sugar (GI = 60)- so should not be taken off the carb count.
Net Carbs
We are then left with the Net Carbs- the carbohydrates in your food that actually have an impact on your body. 
Why 5-10% Net Carbs (/Total Cals) is perfect for Keto
In general, if you’re a very active person who exercises 4 to 5 times a week, you’re more likely to be able to consume more carbohydrates and stay in ketosis. But if you live a sedentary lifestyle and are looking to lose weight, it is encouraged that you to keep carb intake on the lower end to ensure you achieve a state of ketosis.