How many calories should I eat per day to lose weight?

Weight loss is governed by energy balance: eat below what you burn and the body's energy stores shrink. The hard part isn't the principle but the number, because needs vary widely between people. A systematic review of the most common equations for resting metabolic rate found that the best of them (Mifflin–St Jeor) lands within 10 percent of the measured value for most people – but for a large share it is off by more than that. An app that shows "2,000 kcal" with three-digit precision is hiding a range that, for you, could just as easily be 1,800 or 2,200.

So a formula is a starting guess, not a verdict. What makes the estimate reliable is the feedback loop: weigh yourself regularly, watch how the average moves over a few weeks, and adjust intake if it stalls or drops too fast.

Macronutrient split matters less than many assume. In a two-year trial of 811 people, four diets with different amounts of fat, protein and carbohydrate were compared. All produced similar weight loss – it was the calorie deficit, not which nutrient was emphasized, that drove the result. A moderate deficit of 300–500 kcal per day is sustainable for most people; larger deficits are faster but harder to hold and cost more muscle.

The myth of the exact number survives because of how it's served. A calorie calculator that spits out "1,847 kcal" feels more credible than a range, even though the range is the honest answer – our brains read precision as certainty. There's also a clear incentive: apps and diet programs sell the feeling of control, and a neat, exact daily figure is easier to market than "experiment for a few weeks." Add that the first number you're given tends to become an anchor you cling to, even when the scale says otherwise, and it's clear why the figure feels more important than it is.

Work out a rough maintenance need, subtract 300–500 kcal, and treat the number as a hypothesis you test. Weigh yourself a few times a week and look at the average over two to three weeks, not at single days. If the weight stalls, lower your intake a little; if it drops fast, raise it. Which foods those calories come from is best guided by what keeps you full and what you can sustain.