What Is a Calorie Deficit? How Big It Should Be

What is a calorie deficit?

A calorie deficit means you take in less energy from food and drink than your body uses. The NHS explains it simply in its guide to calorie counting: to lose weight, you need to use more energy than you take in, and when you are in a calorie deficit your body relies on its fat stores for fuel. The reverse is also true. Eat more than you use for long enough and the extra energy is stored, mostly as fat.

The energy your body uses in a day is called your total daily energy expenditure, or TDEE. It includes the energy needed to keep you alive at rest, the energy used to digest food, and everything you do with your muscles, from training to walking to the shops. If those terms are new, our explainer on BMR vs TDEE breaks each one down. For this article, the key idea is that your deficit is always measured against your TDEE, also called your maintenance calories: the intake at which your weight would stay roughly the same.

How do you work out your calorie deficit?

You cannot set a sensible deficit without a reasonable estimate of maintenance. The process looks like this:

1. Estimate your maintenance calories. Use an equation such as Mifflin-St Jeor multiplied by an activity factor. As a very rough guide, the NHS page on understanding calories says an average man needs about 2,500 kcal a day and an average woman about 2,000 kcal, but your own figure depends on age, size and activity.
2. Check that estimate against reality. Track your intake and weight for two to three weeks. Our guide to calculating maintenance calories correctly walks through this step by step.
3. Choose a deficit size. Subtract your chosen deficit (see the next section) from maintenance to get your daily calorie target.
4. Set your macros inside that target. Protein first, then fat and carbohydrate. Our guide on how to calculate macros for weight loss covers this with worked examples.
5. Review every two to four weeks. Compare your average weight trend with what you expected and adjust if needed.

How big should a calorie deficit be?

For most adults with weight to lose, a moderate deficit is the practical sweet spot. The NHS calorie counting guide suggests aiming to reduce your intake by around 600 kcal a day, describing this as a safe and sustainable way to lose weight. Its tips for losing weight recommend aiming to lose 0.5 to 1 kg (1 to 2 lb) a week.

The table below shows what different deficits work out to on paper. It uses the common rule of thumb that about 7,700 kcal equals 1 kg of body weight (3,500 kcal per lb). As explained further down, that rule ignores the way your energy use changes as you lose weight, so treat these figures as rough first-month estimates, not promises. The last column shows each deficit as a share of an example TDEE of 2,500 kcal.

Daily deficit	Weekly deficit	Estimated loss per week	Share of a 2,500 kcal TDEE
250 kcal	1,750 kcal	About 0.23 kg (0.5 lb)	10%
500 kcal	3,500 kcal	About 0.45 kg (1 lb)	20%
600 kcal (NHS suggestion)	4,200 kcal	About 0.55 kg (1.2 lb)	24%
750 kcal	5,250 kcal	About 0.68 kg (1.5 lb)	30%
1,000 kcal	7,000 kcal	About 0.91 kg (2 lb)	40%

The same number of calories is a much bigger cut for a smaller person. A 600 kcal deficit is 24% of a 2,500 kcal TDEE but 30% of a 2,000 kcal TDEE. That is why it helps to think about the rate of loss as well as the raw calories.

What if you are already fairly lean?

Leaner people have less fat to lose and more to protect in terms of muscle. A review of contest preparation research for natural bodybuilders by Helms and colleagues recommended setting calories so that body weight falls by roughly 0.5 to 1% per week to maximize muscle retention. For a 70 kg (154 lb) person, that is about 0.35 to 0.7 kg (0.8 to 1.5 lb) a week. That research was in athletes, but the principle that slower is gentler on muscle is a reasonable one for anyone who is close to their goal.

How fast will you lose weight in a calorie deficit?

Expect the first week to look better than the rest. When you cut calories, especially carbohydrate and salt, body water shifts quickly. Hall and Chow note in their research on estimating energy intake from body weight that changes in dietary sodium and carbohydrate can cause rapid changes in body fluid. That early drop is real weight, but it is not all fat, and the pace usually slows afterwards.

Over months, a fixed calorie target also produces a smaller and smaller deficit. As you get lighter, your body uses less energy, so the gap between intake and expenditure narrows. In a 2011 analysis in The Lancet, Hall and colleagues modeled a 100 kg (220 lb) sedentary man who cut his intake by about 480 kcal a day and kept it there. The model predicted his weight would level off at about 75 kg (165 lb), taking roughly a year to reach half of that loss and about three years to reach 95% of it. The authors describe the 3,500 kcal per pound rule as ignoring these physiological adaptations.

The practical lesson: recalculate your maintenance as your weight changes, rather than expecting the same weekly loss forever. If progress stalls, our guide to why a calorie deficit stops working covers the usual causes.

Worked example: setting a deficit step by step

Take a person whose validated maintenance intake is 2,500 kcal a day and who weighs 80 kg (176 lb).

• Deficit: 600 kcal a day, following the NHS suggestion.
• Daily target: 2,500 minus 600 equals 1,900 kcal.
• Expected rate on paper: 600 times 7 is 4,200 kcal a week, or about 0.55 kg (1.2 lb). As a share of body weight, that is about 0.7% a week, inside both the NHS range and the 0.5 to 1% range above.
• Check after four weeks: compare the average weight of week four with the average of week one. Ignore the first few days of water loss when judging the trend.
• Adjust: if the weekly trend is well below what you expected, first check your logging accuracy before cutting further. If you have lost a meaningful amount of weight, re-estimate maintenance.

What is the lowest safe calorie intake?

There is no single number that is safe for everyone, which is why this article focuses on moderate deficits rather than minimum intakes. What official guidance does say is that very low calorie diets need support. The NHS page on obesity treatment notes that a very low calorie diet (under 800 or 1,200 calories) may be followed with support from a dietitian, that such diets are not suitable or safe for everyone, and that they are usually only recommended when weight is affecting other health conditions.

Talk to a doctor or registered dietitian before starting a calorie deficit if you are pregnant or breastfeeding, under 18, have diabetes or kidney disease, take medication that affects appetite or blood sugar, or have a history of disordered eating. A deficit is a tool for adults who want to lose weight, not something to push as hard as possible.

How do you keep muscle in a calorie deficit?

Some lean tissue is usually lost along with fat, but you can influence how much. A review in Advances in Nutrition, Preserving Healthy Muscle during Weight Loss, concluded that a high protein intake helps preserve lean body and muscle mass during weight loss, and that both endurance and resistance exercise help preserve muscle mass, with resistance exercise also improving strength. In practice that means:

• Prioritize protein at each meal. Our guide to how much protein you need per day covers targets by goal.
• Keep lifting weights or doing other resistance training while you diet.
• Avoid very aggressive deficits, especially when you are already lean.

Common calorie deficit mistakes

• Starting too aggressive. A huge cut is harder to stick to and leaves less room for protein and nutrient-dense foods.
• Judging progress by one weigh-in. Day-to-day weight moves with water, salt and food in your gut. Compare weekly averages instead.
• Underestimating intake. Cooking oils, drinks, sauces and weekend meals are easy to miss, and research consistently finds that people report eating less than they actually do.
• Never updating the target. Your maintenance calories fall as you lose weight, so the same intake becomes a smaller deficit over time.
• Treating a deficit as a test of willpower. A plan you can follow for months beats one you abandon after two weeks.

Accurate, low-effort logging is what makes all of this work. MyMacro AI, which is not yet released, is being built so you can log a meal by saying it in a normal sentence or snapping a photo, then see your calories and macros against your target. If that sounds useful, you can join the MyMacro waitlist.

Frequently asked questions

What is a calorie deficit in simple terms?

It means eating and drinking less energy than your body uses in a day. Your body makes up the difference from stored energy, mainly fat, so your weight goes down over time.

How many calories should I cut to lose weight?

NHS guidance suggests reducing intake by around 600 kcal a day for most adults, aiming to lose 0.5 to 1 kg (1 to 2 lb) a week. Smaller or leaner people may prefer a smaller cut.

Is a 1,000 kcal deficit too much?

On paper it equals about 0.9 kg (2 lb) a week, the top of the NHS range, and it is a large share of intake for smaller people. Larger deficits are harder to sustain and make it harder to keep muscle, so get professional advice if you are considering one.

Why did I lose a lot of weight in the first week?

Cutting calories, carbohydrate and salt quickly changes how much water your body holds. Part of the early drop is water, so the rate usually slows after the first week or two.

Can I be in a calorie deficit and not lose weight?

Over a few days, yes, because water shifts can hide fat loss. If your weekly average has not moved for several weeks, your intake may be higher or your maintenance lower than you think.

Do I need to count calories to be in a deficit?

No. Any approach that consistently reduces intake below expenditure creates a deficit. Counting simply makes the size of the deficit visible and easier to adjust.

Sources

1. Calories and weight loss, NHS Better Health (2026)
2. Tips to help you lose weight, NHS (2023)
3. Understanding calories, NHS (2023)
4. Overweight and obesity in adults: Treatment, NHS (2023)
5. Quantification of the effect of energy imbalance on bodyweight, The Lancet (Hall et al.) (2011)
6. Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation, Journal of the International Society of Sports Nutrition (Helms et al.) (2014)
7. Estimating changes in free-living energy intake and its confidence interval, American Journal of Clinical Nutrition (Hall and Chow) (2011)
8. Preserving Healthy Muscle during Weight Loss, Advances in Nutrition (Cava et al.) (2017)

Not medical advice. This article is general information for healthy adults and is not a substitute for advice from a doctor or registered dietitian. Talk to a qualified professional before changing your diet, exercise or supplements, especially if you are pregnant, under 18, have a medical condition such as diabetes or kidney disease, take medication, or have a history of disordered eating. See our health disclaimer.