What Is a Calorie Deficit?

A calorie deficit means eating fewer calories than your body uses in a day. Your body covers the gap from stored energy, and that is how weight comes down. The 2013 AHA, ACC and TOS guideline says adults in weight-loss programs are typically prescribed a gap of at least 500 kcal a day. That is a starting size. The arithmetic behind it holds best in the first weeks and drifts after that.

What does a calorie deficit mean?

The gap between the calories you eat and drink and the calories your body uses is your calorie deficit. The half of the ledger people forget is what the body spends. It uses energy for everything: breathing, digesting, walking, sitting still. The daily total is your maintenance calories, also called total daily energy expenditure (TDEE), and every deficit is measured against it. Eat that much and your weight stays about level. Eat less and you’re in a deficit. Eat more and you’re in a surplus.

According to the NHS, you’re likely to lose weight if you eat and drink fewer calories than you use. Your body then uses its stored fat for energy. The same page gives 2,500 kcal a day for an average man and 2,000 kcal for an average woman. Both are averages. Your own figure depends on age, weight, height and activity.

On a label, a calorie and a kilocalorie are the same thing, and 1 kcal = 4.184 kJ according to the FAO. A 550 kcal gap is 2,301 kJ, which matters if your labels are in kilojoules.

Maintenance is estimated from body measurements. Your food diary plays no part in it. The 2023 report from the National Academies (NASEM) built its equations from doubly labeled water measurements of energy expenditure. You enter age, sex, height, weight and an activity category, and the equation returns a daily estimate in kcal. The category you pick matters a lot, as the next sections show.

If you would rather skip the sums, the calorie calculator runs your age, height, weight and activity through the NASEM 2023 equations. Its targets stop at 1,200 kcal for women and 1,500 kcal for men.

Eating less, moving more or both opens the gap. I find the food side easier to control, because I can weigh what goes in, while the energy I burn is always an estimate.

How does a calorie deficit lead to weight loss?

The body makes up the shortfall from its stores, and the NHS names stored fat. Per kilogram, fat tissue holds 39.5 MJ of energy and lean tissue 7.6 MJ, as Hall and colleagues reported in The Lancet in 2011. That is 9,441 kcal and 1,816 kcal. Weight you lose is a mix of fat and lean tissue, so the real cost of a kilogram falls somewhere between those two figures.

Most people use a shortcut, the 3,500 kcal per pound rule. Multiply by 2.2046 and you land near 7,700 kcal per kg. Divide a weekly deficit by that figure and you get an expected loss. Here are four paces:

Weekly loss	Daily deficit by the rule	Weekly deficit
0.25 kg (0.55 lb)	275 kcal	1,925 kcal
0.5 kg (1.1 lb)	550 kcal	3,850 kcal
0.75 kg (1.65 lb)	825 kcal	5,775 kcal
1 kg (2.2 lb)	1,100 kcal	7,700 kcal

Those weekly figures are the upper bound of what a gap delivers. Hall’s paper calls the rule ubiquitous and says it ignores how the body adapts: a lighter body burns less. Thomas and colleagues compared the rule with seven weight-loss experiments where intake was supervised or measured, and found it grossly overestimates the loss. In their simulation, a 103 kg (227 lb) man on a 25% calorie restriction reaches a plateau after 1.4 years. The rule goes on predicting loss.

Hall’s model slows the pace in the same way. Weight change has a half-time of about one year, so half of the eventual change takes about 12 months. The first weeks match the table best, and later months fall behind it.

The guideline’s review of diet trials found that average weight loss peaks at 6 months, with smaller losses held for up to 2 years. So a deficit runs for a stretch, and then you move to maintenance. That maintenance is lower than before because you weigh less. Decide that step before you start.

How big should a calorie deficit be?

Moderate. The AHA, ACC and TOS guideline lists a 500 kcal or 750 kcal daily deficit as options. The NHS aims for 1 to 2 lb (0.5 to 1 kg) a week. The CDC says people who lose at a gradual, steady pace of about 1 to 2 pounds a week are more likely to keep the weight off.

I add a second limit based on body weight. Helms and colleagues wrote a review for lifters preparing for contests. They set intake to lose about 0.5 to 1% of body weight a week, to keep muscle. At 70 kg (154 lb) that is 0.35 to 0.7 kg. Their advice is for athletes, so I read it as a ceiling and not a target for everyone. Whether you can build muscle in a calorie deficit is its own question.

The floor matters as much as the pace. The guideline’s prescribed ranges start at 1,200 kcal a day for women and 1,500 kcal for men, and I treat those as floors. It reserves very-low-calorie diets, under 800 kcal a day, for medical settings with trained practitioners.

Your largest possible gap is therefore maintenance minus the floor. A woman who maintains at 2,000 kcal has at most 800 kcal to work with. Few should use all of it. That’s why a 1,000-calorie deficit is too big for most women who maintain anywhere near that number. The other calorie deficit articles share them.

What does a calorie deficit look like in real life?

You can make one by eating less, by moving more, or both. They aren’t equal in how well you can measure them. The NASEM report sorts adults into four activity categories and gives an equation for each. I ran the NASEM equations for a woman aged 35, 165 cm (5 ft 5 in) and 68 kg (150 lb). Her maintenance is 2,080 kcal a day if she is inactive, 2,245 kcal if low active and 2,383 kcal if active. The jump from inactive to low active is 165 kcal a day.

NASEM’s own example for the low active category is 60 to 80 minutes of walking at 3 to 4 mph on top of daily activities. The active category adds 30 to 50 minutes of walking, 45 minutes of moderate cycling and 40 minutes of doubles tennis. For the same woman that is worth 303 kcal a day. So an hour of walking a day closes about a third of a 500 kcal gap.

Now put the limits from the last section on her numbers. One percent of 68 kg is 0.68 kg a week, which is a daily gap of 748 kcal at 1,100 kcal per kg. Her floor of 1,200 kcal leaves room for 1,045 kcal (2,245 minus 1,200) if she is low active. The smaller limit wins, so her ceiling is a gap of 748 kcal. A gap of 275 to 550 kcal sits well inside it, and it leaves her eating 1,695 to 1,970 kcal.

I’d keep movement in anyway. For the gap itself, though, the plate is the easier lever. A change of 165 kcal is about 127 g of cooked rice, which takes seconds to weigh instead of an hour to walk. Most people will do some of each, and that’s fine.

On the food side, a gap is smaller than people expect. I looked up seven foods in USDA FoodData Central and worked out how many grams carry 100 kcal. Where a record lists specific Atwater factors, the energy figure rests on them.

Food (state)	Grams per 100 kcal	One portion	Energy per portion
Rice, white, medium-grain, cooked	76.9 g	1 cup (186 g)	242 kcal
Bread, white	37.0 g	1 slice (27.3 g)	74 kcal
Cheddar cheese	24.5 g	1 slice (17 g)	69 kcal
Croissant, butter	24.6 g	1 medium (57 g)	231 kcal
Doughnut, cake-type, glazed	23.5 g	1 medium (45 g)	192 kcal
Beer, regular	232.6 g	1 can (356 g)	153 kcal
Banana, ripe	113.6 g	1 banana (115 g)	101 kcal

Two cups of cooked rice (372 g) come to 484 kcal. A medium croissant, a medium doughnut and one slice of cheddar add up to 492 kcal. So a 500 kcal gap is about two cups of rice or three small items, and it’s not one big thing you’d notice missing.

A gap can leak without anyone noticing. Say you cut 500 kcal Monday to Friday, which is 2,500 kcal, then eat at maintenance on the weekend. The week’s deficit is 2,500 kcal, or 0.32 kg by the 7,700 kcal rule, not the 0.45 kg you planned. Add 300 kcal extra on each weekend day and it falls to 0.25 kg.

Weigh the two or three foods you eat most often for one week and log the grams. That shows you where a gap can come from. You may find the gap hides in oil and drinks, so nothing you love has to go. When it doesn’t, calorie deficit snacks are an easy place to make room.

How do I know I’m in a calorie deficit?

You compare weights, since how you feel on a given day tells you little. A calculator can’t tell you, because it only estimates your maintenance. The NASEM equations from 2023 have a typical error of 246 kcal a day for women and 339 kcal for men. A planned 500 kcal gap for a woman could really be anywhere from about 250 to 750 kcal.

The scale check goes like this. Weigh yourself on at least three mornings a week, on the same scale. Average each week. After two weeks, take the drop in kilograms and divide by the days. Then multiply by 7,700. Here is an example:

• Week 1 average 80.4 kg, week 3 average 79.8 kg.
• Drop of 0.6 kg over 14 days is 0.043 kg a day.
• 0.043 × 7,700 is about 330 kcal a day.

Say you planned a 500 kcal gap and the math shows 330. Either your logging misses about 170 kcal, often oil, drinks and bites, or the calculator guessed your maintenance too high. Check the log first, then move your target in steps of 100 to 200 kcal. This is my own method built on the 3,500 kcal rule, not a published equation, and two weeks is short. Daily weights swing with water, so judge the average and not the morning.

The NIDDK Body Weight Planner is a second way to estimate. It starts from your weight, sex, age, height and activity level. It’s an estimate too.

Who shouldn’t use a calorie deficit?

A deficit is for adults who have a reason to lose weight. Under 18, leave it alone. In pregnancy or while breastfeeding, your midwife or doctor sets the numbers, and the same goes for anyone with a health condition or a medication, since a calculator can’t see what they see. If your weight is already in the usual range for your height, a deficit has no job to do, and eating at maintenance is a fine plan.

Not sure whether any of this applies to you? Ask your doctor before you start. They can say which intake floor suits your health. The equations I used in this guide cover adults aged 19 and over, so they say nothing useful about teenagers.

Past eating disorders change the picture. If tracking food has ever turned into something you couldn’t stop, talk to a professional before you pick any target.

Pick one gap between 275 and 550 kcal, hold it for two weeks, and compare your weekly averages. When the two disagree, I fix the log before I cut deeper.

If counting calories has started to feel hard to stop, or eating feels out of control, talk to someone: ANAD in the US (1-888-375-7767), Beat in England (0808 801 0677), NEDIC in Canada (1-866-633-4220) or the Butterfly Foundation in Australia (1800 33 4673). If you’re in danger right now, call your local emergency number. Other UK numbers and crisis lines are on our crisis support page.