What Is a Calorie Deficit? A Plain-English Guide

Why every diet comes down to a deficit

Diets argue about carbs and fat, but trials that compare them find the macronutrient split matters far less than the deficit itself.

• POUNDS Lost. In a 2009 trial in the New England Journal of Medicine, Sacks and colleagues gave 811 adults with overweight one of four reduced-calorie diets with different amounts of fat, protein and carbohydrate. At six months, people had lost about 6 kg (13 lb) on average, around 7% of their weight, whichever diet they followed. The authors concluded that reduced-calorie diets "result in clinically meaningful weight loss regardless of which macronutrients they emphasize."
• DIETFITS. In a 2018 trial in JAMA, Gardner and colleagues put 609 adults on a healthy low-fat or a healthy low-carbohydrate diet for a year. Average weight loss was 5.3 kg (11.7 lb) and 6.0 kg (13.2 lb), and the difference between them was not significant.

So choose the way of eating you can keep up. Low-carb works when it helps you eat less; so does low-fat, and so does eating in a shorter window. The deficit is what they have in common.

Food quality still matters. Protein helps you keep muscle and stay full, and fibre-rich, less processed food helps with hunger. These help you stick to a deficit; they do not replace it.

How big should a calorie deficit be?

Bigger is not better. For people who lift and want to keep their muscle, Helms and colleagues (2014, Journal of the International Society of Sports Nutrition) recommend losing about 0.5 to 1% of body weight a week.

Your weight	0.5% a week	1% a week
60 kg (132 lb)	0.3 kg (0.7 lb)	0.6 kg (1.3 lb)
80 kg (176 lb)	0.4 kg (0.9 lb)	0.8 kg (1.8 lb)
100 kg (220 lb)	0.5 kg (1.1 lb)	1 kg (2.2 lb)

To turn that into calories, many calculators, including the free calorie and macro calculator on this site, use the common estimate that a kilogram of body fat holds about 7,700 calories (about 3,500 per pound). On that estimate, a deficit of about 500 calories a day loses roughly 0.45 kg (1 lb) a week.

That is a fair place to start, but weight loss does not keep going in a straight line. In a 2013 commentary in the International Journal of Obesity, Thomas and colleagues compared the 3,500-calorie rule with the results of seven weight-loss studies and found it "grossly overestimates actual weight loss" over time. As you get lighter, you burn less, so the same food intake gives a smaller deficit. Kevin Hall and colleagues' 2011 model in The Lancet showed the same slowing. That is why plans need adjusting as you go.

Very large deficits bring more hunger, poorer training and more muscle loss. NICE, which sets health guidance in England, has advised against routinely using diets of 800 calories a day or less to manage obesity, and has said that anyone following one needs ongoing clinical support.

How to tell if you are in a calorie deficit

Your app's number is an estimate. Your weight trend is the answer.

1. Weigh yourself each morning, after the toilet and before eating or drinking.
2. Work out the average for each week.
3. Compare the weekly averages over two to three weeks.

If the average is falling, you are in a deficit. Expect the first week to drop faster than the rest, because some stored carbohydrate and the water held with it go first, so judge your rate from the second week. Daily jumps of a kilo or more are usually water, salt and food in your gut, not fat; how to read the scale explains why.

If the average has been flat for three weeks, you are probably at maintenance, whatever your app says. Tracking is hard for everyone: in a 1992 study in the New England Journal of Medicine, Lichtman and colleagues found that people who believed they could not lose weight on low calories were under-reporting what they ate by an average of 47% and over-reporting their activity by 51%.

Calorie deficit myths

"My body is in starvation mode, so I can't lose weight." Your energy use does fall as you diet, partly because you are smaller and partly because your body adapts and you tend to move less without noticing. A 2014 review by Trexler, Smith-Ryan and Norton in the Journal of the International Society of Sports Nutrition describes these changes. They slow fat loss, but they do not stop a real deficit working. A long stall usually means the deficit has shrunk or disappeared.

"Healthy food can't make you gain weight." It can, if you eat enough of it. Olive oil, nuts and granola are good foods with a lot of calories.

"I'll just burn it off in the gym." Training is vital for keeping muscle on a diet, but most sessions burn less than people expect, and it is easy to eat the calories back. Use food for the deficit and training to keep your muscle.

"A deficit means being starving all the time." Some hunger is normal, especially later in a diet. Constant, distracting hunger usually means the deficit is too big or your meals are not filling enough.

When a calorie deficit is not the right move

Speak to your doctor or GP before you start a diet if you are pregnant or breastfeeding, are under 18, have a health condition such as diabetes, take regular medicines, or have a history of disordered eating. If you are already lean, a deficit may not be what you need at all; building muscle at maintenance or in a small surplus might serve you better.

How to put it into practice

1. Estimate your maintenance with the free calculator.
2. Set a deficit that loses 0.5 to 1% of your body weight a week; the gentler end if you are already fairly lean.
3. Eat 1.6 to 2.2 g of protein per kg of body weight a day, aiming towards the top while you diet.
4. Keep lifting weights, and keep your daily steps up.
5. Judge progress by weekly averages, and adjust after two to three weeks, one small change at a time.

How to start a cut walks through each step in detail.

How coaching helps

Understanding a calorie deficit is the easy part. Staying in one when hunger rises, the scale stalls and life gets busy is harder. With one-to-one coaching from CoachAk, your calorie, macro and step targets are set in a private app alongside your training program. You log your weight daily, and each week you send a check-in that takes about three minutes, covering your weight, training and nutrition adherence, steps, sleep, energy, hunger and stress. Your coach replies with written feedback and adjusts your targets as your body responds, so your deficit stays the right size as you go.

If you want help setting yours up and sticking to it, apply for coaching.