"Just eat less than you burn" is technically correct and almost useless as actual advice. The size of the deficit, how you get there, and how long you can sustain it all matter far more than the basic principle, which most people already understand anyway.
Start from your maintenance number
Before setting a deficit, you need a baseline: your Total Daily Energy Expenditure (TDEE), the calories you burn in an average day including activity. Our calorie calculator estimates this from your age, height, weight, sex, and activity level using the Mifflin-St Jeor formula. Every deficit is a percentage or fixed amount below that number, not below some generic guess.
How big should the deficit be
A moderate deficit of 500 calories per day below maintenance is a common, well-supported starting point, since it produces roughly one pound of fat loss per week (a pound of fat is approximately 3,500 calories). That rate is slow enough to preserve muscle mass reasonably well and fast enough to stay motivating.
Larger deficits, 750-1000 calories or more below maintenance, produce faster results but come with real trade-offs: more muscle loss alongside fat loss, greater hunger and fatigue, and a much higher chance of the plan collapsing within a few weeks. Smaller deficits of 250 calories are gentler and easier to sustain, useful for someone who isn't in a rush or who has struggled with restrictive diets before.
Protein protects muscle during a deficit
Whatever the deficit size, protein intake matters more during weight loss than during maintenance. Adequate protein, generally cited around 1.6-2.2 grams per kilogram of body weight for people who are resistance training, helps preserve lean muscle mass while in a deficit, so more of the weight lost comes from fat rather than muscle. Our macro calculator can split your calorie target into protein, carb, and fat grams based on the diet style you choose.
Why the scale doesn't move in a straight line
Body weight fluctuates day to day from water retention, sodium intake, hormonal changes, and digestion, sometimes by a kilogram or more, even when fat loss is happening consistently underneath the noise. Looking at a weekly average, rather than any single day's number, gives a far more honest picture of whether the deficit is actually working.
Adjusting as you go
TDEE estimates are a starting point, not a fixed truth. If weight isn't trending down after two to three consistent weeks at a given calorie target, the estimate was probably a bit high for your actual metabolism, and the target needs a modest downward adjustment. This kind of iteration, rather than getting the number perfect on the first try, is how a calorie deficit actually gets dialed in over time.