"Eat about 500 fewer calories a day" is common weight-loss advice, but the actual number depends on your maintenance calories and how fast you want to change — not a fixed 500.
Calorie deficit calculator, where the math comes from
A kilogram of body fat is roughly 7,700 calories. Spread a 0.5 kg/week target over 7 days, and that's about a 550 calorie/day deficit — not the commonly cited round 500, though close. This calculator computes the exact figure for whatever rate you choose, rather than a generic rule of thumb.
Calories to lose weight calculator, a full example
Starting from a 2,759 calorie TDEE (moderately active, 80 kg), a 0.5 kg/week loss goal gives a 2,209 calorie/day target. The same rate in reverse — gaining 0.5 kg/week — would add rather than subtract that 550 calories.
How many calories to gain weight, and the speed limit
Faster rates than about 1 kg/week (in either direction) are generally not recommended without medical supervision — they're harder to sustain and can affect muscle mass and metabolic rate more than slower, steadier changes.
