What is a calorie deficit and how do you work out yours?

"Just eat in a calorie deficit" is advice people hear all the time, usually with no explanation of what it means or how to find one. It's simpler than it sounds, and you don't need an app full of graphs to get started.

What a calorie deficit actually is

Your body uses energy all day, even while you sleep. The NHS explains that to stay around the same weight, the calories you use should match the calories you eat and drink. Eat and drink fewer than you use and you're likely to lose weight, because your body draws on its stored fat for energy.

That gap is your calorie deficit. As a rough guide, the NHS says an average man needs about 2,500kcal a day and an average woman about 2,000kcal. Those are averages, though, and your own figure depends on your age, weight, height and how much you move.

Maintenance calories in plain English

Your maintenance calories are what you'd need to eat to stay the same weight. You'll also see it called TDEE, short for total daily energy expenditure. Nobody can measure it at home, so calculators estimate it in two steps.

1. Energy at rest. Most use the Mifflin-St Jeor equation, published in 1990, which estimates how much energy your body uses at rest from your weight, height, age and sex.
2. Activity. That number is multiplied by an activity factor, from about 1.2 if you mostly sit to 1.9 if you have a physical job and train as well.

Our free calorie deficit calculator does exactly this, and you can enter your height in feet and inches and your weight in stones and pounds if that's how you think. It also shows the sums, so you can work it out by hand.

The honest caveat: it's an estimate. Real people can burn noticeably more or less than an equation predicts, and labels and portions vary too. Treat your result as a starting point, not a verdict.

How big a deficit to aim for

For most adults I'd start 300 to 500kcal a day below your estimated maintenance. The NHS suggests cutting around 600 calories a day, which it calls safe and sustainable, so a gentler gap than that is a perfectly sensible place to begin, especially if you train.

Bigger isn't better. A new position stand from the International Society of Sports Nutrition, published on 13 October 2026, reviewed the research on diets for weight loss. It found that very low calorie diets (under 800kcal a day) and low calorie diets (800 to 1,200kcal) produce more weight loss in the short term but lack long-term advantages over less aggressive cuts. It also warned that they may increase muscle loss and can leave gaps in vitamins, minerals and fibre.

That's why our calculator won't suggest less than about 1,200kcal a day for women or 1,500kcal for men. If your sums take you below that, speak to your GP first.

The diet style matters less than the gap

The same review found no real difference in weight or body measurements between intermittent fasting, including time-restricted eating, and simply eating a bit less every day. It also said that how many meals you eat a day doesn't tend to make a crucial difference.

So pick the approach you can keep doing. If three meals suit you, eat three. If a later breakfast suits you, that's fine too.

What did matter in the review was holding on to muscle. It described resistance training and enough protein as essential for limiting muscle loss in a sustained deficit, whatever the diet, with total daily protein the main thing. Our guide on how much protein you need helps you set a figure. Two or three strength sessions a week is plenty, and the beginner's 3-day gym plan is built around simple moves like the goblet squat, lat pulldown and glute bridge.

Checking it's working

Weigh yourself a few times a week and compare weekly averages, not single mornings. Daily weight jumps around with water and salt. Give it two to four weeks before you judge it.

If the trend hasn't moved, check your logging before you cut further, then trim another 100 to 200kcal if you need to. If you're always starving or your training is suffering, ease off. Recalculate whenever your weight has changed by a few kilos, because a lighter body needs a little less. Our post on how many calories to cut to lose a stone covers why progress slows over time.

Please don't use a deficit if you're under 18, pregnant or breastfeeding, or have had an eating disorder, and speak to your GP first if you have a medical condition or take medicines for diabetes or blood pressure.

If you'd like a daily calorie target, a food diary and a training plan in one place, that's what a FitFitCity membership is for.