How do you know how many calories to consume a day if you want ot loose weight?

Wt Loss. It's not just counting calories. The most important is the source of those calories. One can consume more calories from healthy food and it varies on what goal one wants to reach.
Losing weight. As a general rule of thumb, it takes about 20 calories for each kilogram of weight to maintain current weight (2.2 pounds for each kilogram) so a person weighting 70 kg (154 pounds) needs 1400 calories per day to stay the same weight. To lose weight one has to decrease total calories. However, types of foods, how often you eat and your exercise level all play a role in weight loss.