How do I know my deficit is real?

Track the weekly average of morning body weight for three or four weeks while the food log stays consistent. A slow downward drift means the gap is open. A flat line means intake and expenditure are matched, whatever the formula predicted.

Should I eat back calories burned in exercise?

If your intake target already assumes you train, eating those calories back closes the gap. If you calculated a sedentary maintenance and then started training, adding some food on long training days is reasonable. Watch the weekly average rather than the watch's burn number.

Can I take a higher day once a week?

A single higher day does not erase a week that is under maintenance. It can make the plan easier to keep. If the higher day becomes three higher days, you no longer have the weekly gap.

Some lean mass loss is possible in a deficit. Resistance training and a protein intake in the range the International Society of Sports Nutrition describes for exercisers reduce how much. The protein article walks through those numbers.

The Complete Guide to Calorie Deficit for Fat Loss

A calorie deficit means you eat less energy than you use. This guide shows how to estimate that gap with the Mifflin-St Jeor equation, why the old 3,500-calorie rule misleads, and how to keep the cut moderate enough to live with.

A calorie deficit is the gap between the energy you eat and the energy you use. Fat loss, when it happens, happens in that gap. Kevin Hall and colleagues showed in The Lancet in 2011 that the size of the gap does not translate into weight change in a straight line, because expenditure itself shifts as body weight changes.

What the deficit actually is

Your body spends energy staying alive, digesting food, moving through the day, and doing deliberate exercise. Researchers group the living-cost piece as resting energy expenditure. Add the rest and you have total daily energy expenditure. If intake sits below that total for long enough, stored fuel covers the difference. Much of that stored fuel is body fat. Some can be carbohydrate stored with water, and some can be lean tissue. The mix depends on how large the gap is, how you train, and how much protein you eat.

You might be thinking the scale should drop the morning after a careful day. It often does not. A single day is smaller than the noise from salt, a late meal, or a hard workout. Hall's model put the half-time of the body-weight response to a lasting change in intake on the order of a year. That is a modeling result, not a promise about your next month, and it is a useful correction to the idea that fat loss is a weekend project.

The calorie tools on this site are a place to sketch a target. They are estimates. Treat the first two weeks as a test of the estimate, not as a verdict on your discipline.

Estimating resting needs with Mifflin-St Jeor

In 1990, Mifflin, St Jeor, and colleagues published a predictive equation for resting energy expenditure in healthy people in the American Journal of Clinical Nutrition. It is still the equation many dietitians start with, because it was derived from measured resting metabolism rather than from a guess. The published formulas are:

Women: (10 × weight in kilograms) + (6.25 × height in centimeters) − (5 × age in years) − 161
Men: (10 × weight in kilograms) + (6.25 × height in centimeters) − (5 × age in years) + 5

Here is a worked example, not a case study and not a result anyone logged. Take a 35-year-old woman who is 165 cm tall and weighs 70 kg. Resting expenditure is about (10 × 70) + (6.25 × 165) − (5 × 35) − 161 = 700 + 1031.25 − 175 − 161 = 1,395 calories a day, before activity. A man of the same age, height, and weight lands about 166 calories higher because the constant at the end is +5 instead of −161.

Activity multipliers are a second layer of guesswork. A desk-based day with little planned exercise is often multiplied by about 1.2. A job or training schedule that is on your feet most days needs a higher factor. The point of the arithmetic is a starting band, not a number you defend to the calorie. If your average weight is stable for two weeks, you are near maintenance whatever the formula said.

Why "3,500 calories equals a pound" is a poor map

A pound of body fat stores roughly 3,500 calories, and people have used that figure for decades as if a 500-calorie daily gap must produce a pound of loss each week, forever. Hall's 2011 analysis is the check on that shortcut. As weight falls, resting expenditure falls with it, and the same intake gap shrinks. A cut that felt moderate in week two can become a stall in month four without any change in what you eat. The practical response is to re-estimate, not to slash another large chunk of food out of frustration.

There is a second measurement problem. Lichtman and colleagues reported in the New England Journal of Medicine in 1992 that people in their study under-reported intake and over-reported exercise compared with measured values. You do not need a percentage from that paper to use the lesson: a food log written from memory is a sketch. Weighing a few high-calorie foods for a fortnight — oils, nut butters, restaurant portions — usually teaches more than buying a new app.

How large a gap to open

A modest gap is easier to repeat. In practice that often means a few hundred calories under your current maintenance estimate, enough that the weekly average weight trends down slowly, and small enough that training and mood survive. Very large cuts raise the chance that you lose more lean tissue, feel cold and irritable, and quit. This is education about energy balance, not a prescription. If you are pregnant, underweight, ill, or have a history of disordered eating, do not open a deficit because an article suggested one. A registered dietitian or physician is the right person for that decision.

Pair the gap with two things that have their own evidence. Protein intake during a cut is covered in the protein guide. Sleep changes how a reduced-calorie diet partitions weight loss, which is the subject of the sleep article. The U.S. Physical Activity Guidelines for Americans (2nd edition, 2018) recommend at least 150 minutes a week of moderate-intensity aerobic activity for adults, plus muscle-strengthening work on two or more days. That activity supports the expenditure side and, more importantly, gives you a reason to keep muscle while the scale moves.

A four-week way to run the experiment

Week one is measurement, not heroics. Eat as you normally do and write it down with portions you actually weighed or measured. Weigh yourself several mornings, after the bathroom, before breakfast, and average the week. One morning is not a trend.

Week two, set intake about 300 to 500 calories under the maintenance level your log and your weight suggested. Keep protein in the range discussed in the protein article. Keep training if you already train. If you do not, two short full-body strength sessions and daily walking are enough to start.

Week three, look at the weekly average weight, your strength in two or three lifts or bodyweight moves, and your hunger in the evening. If the average weight is unchanged and the log is honest, the gap is too small or the log is still missing calories. If strength is falling fast and you are ravenous, the gap is too large. Change one thing.

Week four, hold the adjusted intake. The glossary entry for a core blueprint is a useful reminder that food, training, and recovery belong in one plan. A deficit with no sleep and no strength work is just a smaller plate.

Foods that make the same calories easier

Energy density is the practical trick. A large bowl of vegetables, fruit, potatoes, oats, beans, and lean protein occupies the stomach for fewer calories than the same volume of oils, sweets, cheese, and fried food. You do not have to remove any food group. You do have to notice the foods that are easy to eat past fullness. Liquid calories — juice, alcohol, sweetened coffee — are the usual leak, because they do not fill you the way a meal does.

Restaurant meals are not a moral failure. They are hard to count. If you eat out often, pick one default order you have looked up once, and keep it as the known quantity. The unknown meals are what blow the weekly average.

What stalls people who are "doing everything right"

The pushback I hear in comments is that the math must be wrong because the scale paused. Often the math is fine and the week is not what the memory says. A spoon of oil is about 120 calories. A "small" handful of nuts can be two servings. A fitness watch can credit exercise calories the lab would not. None of that is a character flaw. It is why a two-week average beats a single virtuous Monday.

Another stall is compensation. A hard evening session followed by a large unplanned snack can erase the session. If that happens most nights, shorten the session or eat a planned meal before you are empty and irritable. The deficit lives in the week, not in the workout.