What Is a Calorie Deficit, and How Big Should Yours Be?

A calorie deficit means you eat fewer calories than your body burns, so your body covers the gap from its stores, mostly body fat. The NHS puts it simply: to lose weight, you need to use more energy than you take in. To find your deficit, work out the calories you burn in a day and subtract the deficit you choose. For most adults a deficit of about 500 kcal a day is a good start: it sits at the low end of the US obesity guideline, and the NHS suggests losing 0.5 to 1 kg a week.

Key takeaways

• Deficit = the calories you burn minus the calories you eat.
• About 500 kcal a day is a sensible default. Lean people keep more muscle with smaller deficits.
• Eat enough protein and do strength training so the weight you lose is mostly fat.
• Judge your deficit by your weight trend over several weeks. One morning on the scale says little.

What a calorie deficit is

A calorie is a unit of energy in food and drink. Your body spends energy all day: on breathing, your heart and keeping warm, and on everything you do, from typing to training. When you eat less energy than you spend, your body takes the rest from what it has stored.

That gap is your calorie deficit. Eat 2,000 kcal on a day when you burn 2,500 kcal, and your deficit that day is 500 kcal. Over weeks, a steady deficit shows up as weight loss.

Food choice still matters for hunger, health and muscle, but it does not change this basic sum.

How to calculate your calorie deficit

You need two numbers: the calories you burn (your maintenance calories) and the size of the deficit you want.

Step 1: Find your maintenance calories. Maintenance calories, also called TDEE (total daily energy expenditure), are what you burn in a full day. They come from your resting energy times an activity factor. Our calorie calculator does this for you with the Mifflin-St Jeor equation, and the pillar guide on calories for weight loss shows the formula by hand. To see what makes up the number you start from, read what TDEE is and how to calculate it.

Step 2: Choose the size of your deficit. A 500 kcal deficit suits most adults, and the table below shows the other sizes.

Step 3: Subtract. Maintenance calories minus the deficit is your daily calorie target.

Worked example (a calculation with the formula, not a real person):

A man, 30, 178 cm, 85 kg, exercises 3 to 5 days a week.

A 500 kcal deficit for him is about 18% of his maintenance calories. In the first weeks it should take off roughly 0.45 kg a week, which is about 0.5% of his body weight.

How big should your deficit be?

The US guideline and the NHS land in a similar range. The 2013 US obesity guideline recommends a daily cut of 500 to 750 kcal for adults with overweight. The NHS suggests aiming to lose 0.5 to 1 kg a week.

The weekly numbers in the table use the old rule of thumb of 3,500 kcal per pound, about 7,700 kcal per kg. Kevin Hall and colleagues showed in The Lancet that this rule overstates weight loss over months, because a lighter body burns less. Treat the table as a guide to the first weeks.

Lean people: go slower

Lean people can keep more muscle by losing weight slowly. The International Society of Sports Nutrition states that slower rates of weight loss can better preserve lean mass in leaner people. In a study of 24 elite athletes, the group that lost 0.7% of body weight a week (an energy cut of 19%) gained 2.1% lean mass, while the group losing 1.4% a week (a cut of 30%) saw no gain (-0.2%). For lean, trained people, a review for bodybuilders recommends losing about 0.5% to 1% of body weight a week.

More weight to lose: speed matters less

For people with obesity, the evidence is more relaxed. A meta-analysis that compared gradual and rapid weight loss of the same total size found no significant difference in lean mass, and the gradual groups lost about 1 kg more fat. A larger deficit is an option here, as long as you can keep it up and eat enough protein.

How to keep muscle while you lose fat

You want the weight you lose to be fat, and two habits help you keep muscle.

Eat enough protein. Protein helps you keep muscle in a deficit. The ISSN position stand on protein gives 1.4 to 2.0 g per kg of body weight a day for active people, and says that higher intakes of 2.3 to 3.1 g/kg may help resistance-trained people keep lean mass in a deficit. The guide to protein per day turns those numbers into meals.

Lift something. Strength training tells your body to keep the muscle it has. In a meta-analysis of older adults with obesity, adding resistance training to a lower-calorie diet prevented about 93% of the lean mass loss seen with the diet alone. The NHS advises strengthening activities for all the major muscle groups on at least 2 days a week, with weights, bands or your own body weight. Ab exercises on their own do not burn the fat on your belly, as the guide to losing belly fat explains.

How to tell if you are in a deficit

The scale tells you, but only over weeks. Day to day, water moves your weight up and down more than fat does.

Your muscles store carbohydrate as glycogen, and each gram of glycogen is stored with at least 3 g of water. When you start eating less, your body uses some of that glycogen, and the water goes with it. Part of a fast drop in the first week is this water. A carb-heavy dinner refills glycogen, and the water comes back with it. A study of 80 adults who weighed themselves daily found weight was highest on Sunday and Monday and fell again from Tuesday.

To read your trend:

• Weigh yourself at the same time each morning, after the toilet and before breakfast.
• Compare weekly averages, not single days.
• Give a new target a few weeks before you judge it.
• If your weekly average has not moved for several weeks, check your logging first, then lower your target a little. The guide to weight loss plateaus shows what to check, in order.

Cooking oil, drinks and bites while cooking can quietly eat into your deficit, and the pillar guide shows how big those gaps get. In Cal Clark you can snap the plate and add what the photo can't see, such as the oil or the sauce, so the meal is logged in one go.

Does your body fight back?

Partly, but less than "starvation mode" stories suggest. When you lose weight, your body burns fewer calories, mostly because there is less of you to move and keep warm. On top of that, it saves a little extra energy. Researchers call this adaptive thermogenesis, and a review in Obesity puts it at about 120 kcal a day on average, with large differences between people.

So your metabolism does not shut down in a deficit. Your deficit gets a little smaller as you lose weight, so work out your maintenance calories again after you lose a few kilograms, as Hall's model shows.

Plan your deficit by the week

A dinner out or a birthday does not cancel a week of eating slightly less, and a few lighter days can balance it. Cal Clark shows a weekly calorie budget next to the daily one, so one bigger evening fits into the week instead of feeling like a failed day. If you would rather have the week planned in advance, the calorie deficit diet plan lists seven days of meals with calories and protein.

Frequently asked questions

Can I build muscle in a calorie deficit?

It is possible, and a smaller deficit helps. In a 4-week trial, young men who trained hard six days a week on a large deficit gained 1.2 kg of lean mass when they ate 2.4 g of protein per kg, against 0.1 kg on 1.2 g/kg. Those men trained much harder than most people do. A meta-analysis on training in a deficit found that, on average, a deficit of about 500 kcal a day was enough to stop lean mass gains, so keep the deficit small if gaining muscle is your main goal.

Is a 1,000 calorie deficit too much?

For most people it is the upper limit. It lines up with the NHS top rate of about 1 kg a week, and it makes enough protein and nutrients hard to fit in. If your target would fall very low, talk to your doctor first.

How long does it take to see results in a deficit?

The scale often drops in the first week, partly from water. Fat loss shows in your weekly averages after a few weeks. At 0.5 kg a week, that is about 2 kg in a month.

Is 2,000 calories a deficit?

It depends on what you burn. A deficit is the gap between your maintenance calories and what you eat, so 2,000 kcal is a deficit only if you burn more than that. In the worked example above, the man burns about 2,820 kcal, so 2,000 kcal would be a deficit of about 820 kcal, which is above the 500 to 750 kcal the US guideline recommends. Someone who burns 2,000 kcal eats at maintenance on 2,000 kcal, and loses no weight. Find your own number with the calorie calculator.

How do I create a calorie deficit?

Eat a little less, move a little more, or both. Harvard Health suggests reaching the deficit through a mix of eating less and being more physically active. Swapping a sugary drink for water or trimming the oil in a pan cuts calories without a big change to your plate. A daily walk adds to the other side of the sum.

How do I know my deficit is too big?

Harvard Health lists persistent fatigue, irritability or an unusual preoccupation with food as signs the deficit is too aggressive. If that sounds like you, make the deficit smaller and check that you reach your protein target. Talk to your doctor if the signs stay.

Who should not start a calorie deficit on their own?

The NHS advises talking to a GP first if you have a medical condition, follow a special diet for your health, or have had an eating disorder. If you are pregnant, breastfeeding, under 18 or take medication for diabetes, ask your doctor before you cut calories.

Sources

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