Calorie Deficit: How Much to Cut and How It Really Works

A calorie deficit means you take in less energy from food and drink than your body uses, so it draws on stored energy, mostly body fat, to make up the difference. For most adults, a deficit of about 500 to 600 kcal a day is a safe and sustainable starting point; the NHS suggests cutting around 600 kcal a day. Expect steady rather than dramatic results, and expect progress to slow as your body gets smaller.

Here is how a calorie deficit works, how to calculate the right size for you, and what the old “3,500 calories per pound” rule gets wrong.

What is a calorie deficit?

Your body needs energy for everything it does: keeping your organs working, digesting food and moving. The total is your daily energy expenditure, often called TDEE (see BMR vs TDEE).

• Eat more than you use, and the extra is stored, largely as fat.
• Eat about the same, and your weight stays roughly stable.
• Eat less than you use, and you are in a calorie deficit. Your body fills the gap from its stores, and over time you lose weight.

As NHS Better Health puts it, to lose weight you need to use more energy than you take in; when you are in a calorie deficit, your body relies on its fat stores for fuel.

A deficit can come from eating less, moving more, or both. For most people, food makes the bigger difference, because it is much easier to skip 300 kcal of snacks than to burn 300 kcal on a treadmill. Activity still matters a lot for health, muscle and keeping weight off.

How big should your calorie deficit be?

Official guidance clusters around a moderate deficit:

Source	Suggested deficit	Expected pace
NHS	About 600 kcal a day	Safe, sustainable weight loss
NHLBI (BMI 27 to 35)	300 to 500 kcal a day	About 0.25 to 0.5 kg (½, 1 lb) a week
NHLBI (BMI 35 and above)	500 to 1,000 kcal a day	About 0.5 to 1 kg (1 to 2 lb) a week

The US NHLBI guidelines recommend aiming first for a loss of about 10% of your starting weight over roughly six months. That is a realistic first goal that brings real health benefits for people living with overweight.

Why bigger is not better

A very large deficit can look attractive on paper, but it comes with costs:

• Hunger and fatigue make it hard to stick to, and a plan you abandon stops working.
• Muscle loss. A review in Advances in Nutrition notes that diet-induced weight loss reduces muscle mass as well as fat.
• Nutrient gaps. The NHLBI guidelines do not recommend very-low-calorie diets (under 800 kcal a day) for routine weight loss, partly because nutritional inadequacies occur unless they are supplemented, and they note an increased risk of gallstones.

As a practical floor, do not go below about 1,200 kcal a day for women or 1,500 kcal for men without supervision from a doctor or registered dietitian.

How to calculate your calorie deficit

1. Estimate your maintenance calories (TDEE). Our guide on how many calories you should eat a day walks you through the Mifflin-St Jeor equation and activity factors.
2. Subtract your deficit. 500 kcal a day is a common default; the NHS uses about 600.
3. Check the floor. If the result is under 1,200 kcal (women) or 1,500 kcal (men), use the floor instead and accept a slower pace.
4. Test it for 2 to 3 weeks and adjust based on your average weight trend.

Worked example

Say your maintenance estimate is 2,200 kcal a day.

• Target with a 500 kcal deficit: 2,200 − 500 = 1,700 kcal a day
• Weekly deficit: 500 × 7 = 3,500 kcal

Using the popular rule that 1 kg of body weight equals about 7,700 kcal, that would predict 3,500 ÷ 7,700 = about 0.45 kg a week. That is a reasonable estimate for the first weeks. Over months, though, the real figure drifts lower. Here is why.

Why the “3,500 calories per pound” rule falls short

The old rule says every 3,500 kcal deficit (about 7,700 kcal per kg) takes off one pound (0.45 kg), forever, in a straight line. In a 2011 Lancet paper, researchers at the US National Institutes of Health showed why this overpromises.

• Your body gets smaller, so it burns less. As you lose weight, your energy use falls. A fixed intake that was a 500 kcal deficit at the start becomes a smaller deficit later.
• Weight lost is not pure fat. The paper gives about 39.5 MJ (roughly 9,400 kcal) per kg of body fat and 7.6 MJ (roughly 1,800 kcal) per kg of lean tissue. Real weight loss is a mix, so the energy per kilogram varies.

Their model of a 100 kg sedentary man cutting about 480 kcal a day showed that the classic rule predicts around 22 kg lost in the first year, about twice what the model predicts. Their simpler rule of thumb for an average adult with overweight:

• Every permanent change of 100 kJ (about 24 kcal) a day leads to an eventual weight change of about 1 kg.
• It takes about 1 year to reach half of that change and about 3 years to reach 95% of it.

So a steady 500 kcal (about 2,100 kJ) cut, without other changes, points to an eventual loss in the region of 20 kg, with roughly half of it in the first year. That is still a great result; it just arrives on a curve, not a straight line. If you want to model your own curve, try the free Body Weight Planner from NIDDK, the same NIH institute the researchers worked at.

How to create a calorie deficit without feeling deprived

NHS Better Health suggests splitting a 600 kcal reduction across the day rather than attacking one meal:

• About 100 kcal from breakfast (for example, measuring cereal and skipping sweet spreads)
• About 200 kcal from lunch (checking labels, choosing water or sugar-free drinks)
• About 200 kcal from dinner (fewer ready meals and takeaways, more home cooking)
• About 100 kcal from snacks (fruit or plain popcorn instead of crisps and chocolate)

Other changes that make the deficit easier to live with:

• Get enough protein at each meal. The Advances in Nutrition review found that higher protein intake helps preserve lean mass during weight loss, and recommends an adequate, but not excessive, amount.
• Add strength training. The same review found that both endurance and resistance exercise help preserve muscle during weight loss, and resistance training also improves strength.
• Fill up on volume. Vegetables, fruit, soups and pulses add bulk for relatively few calories.
• Watch the invisible extras. The NHS reminds you to count alcohol, fizzy drinks and juice, cooking oil and spreads, cheese and sauces.
• Keep weekends in the plan. A deficit Monday to Friday can disappear in two generous days.

Track, at least at the start

You cannot manage a deficit you cannot see. A systematic review of self-monitoring in weight loss consistently found an association between recording what you eat and losing weight, although the studies had limitations. Tracking for a few weeks teaches you where your calories come from; our guide on how to count calories shows you how.

An app can make this faster. With PixNutri you can photograph a meal for an AI estimate of calories and macros, scan barcodes on packaged food, or describe a meal by text or voice, then check the values and adjust the portion before saving. Treat photo results as estimates, especially when there is hidden oil or sauce.

When your deficit stops working

Almost everyone hits a slowdown. The NHLBI guidelines note that after about six months most people reach a new balance between intake and expenditure and need to adjust if they want to lose more. Before cutting further:

• Check your logging for a week. Portion creep and untracked drinks are common.
• Look at your 2 to 4 week weight trend, not single days.
• Recalculate your maintenance at your new weight.
• Consider moving more before eating less.

Our guide on how fast you can lose weight safely helps you set realistic expectations.

Talk to a doctor or registered dietitian before starting a calorie deficit if you are pregnant or breastfeeding, under 18, have diabetes or take medication that affects blood sugar, have kidney disease, or have a history of an eating disorder.

FAQ

How many calories should I cut to lose 0.5 kg a week?

Around 500 kcal a day is the usual starting point, though real results vary and slow over time. The NHS suggests about 600 kcal a day.

Is a 1,000 calorie deficit safe?

For some people with a higher body weight it can be, and the NHLBI mentions deficits of up to 1,000 kcal a day for people with a BMI of 35 or more. For most people it is too aggressive, and your intake should not fall below about 1,200 kcal (women) or 1,500 kcal (men) without medical supervision.

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

Short term, yes: changes in body water can mask fat loss for a while. If nothing changes over 3 to 4 weeks, the deficit is probably smaller than you think, often because of untracked food or drinks.

Is it better to eat less or exercise more?

Both work, and combining them is best. Eating less usually creates the deficit more easily, while exercise, especially strength training, helps protect muscle and supports keeping the weight off.

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

No. Smaller portions, fewer sugary drinks and more vegetables can create a deficit without counting. Tracking simply makes the deficit visible, which helps many people, especially at the start.