Calorie Deficit Calculator

What is a calorie deficit?

A calorie deficit is simply using more energy than you take in from food and drink.

Your body burns calories continuously — breathing, keeping your heart beating, holding your temperature, digesting, thinking, and everything you do on top of that. When intake falls short of that total, the difference comes from stored energy, mostly body fat.

This is the mechanism behind every diet that works, whatever it is branded as. Low-carb, intermittent fasting, meal replacements, cutting out ultra-processed food — where they succeed, they succeed by producing a deficit, usually by making it easier to eat less without counting.

How this calculator works

1. Basal metabolic rate

What your body would burn lying still all day, from the Mifflin-St Jeor equation using your age, sex, height and weight. For most adults this is the majority of daily energy use.

2. Total daily energy expenditure

Your BMR multiplied by an activity factor, giving what you burn on a typical day. This is your maintenance figure — eat this and weight stays roughly stable.

3. Your deficit target

Maintenance minus 250, 500 or 750 calories. The result is a starting point, not a prescription. Everyone responds a little differently, so expect to adjust after two or three weeks of real data.

How big should your deficit be?

A smaller deficit is easier to live with; a larger one is faster but harder to hold. The right answer is usually the largest one you can sustain without it running your life.

Because these are subtracted from your maintenance, the same deficit produces different targets for different people. A 500-calorie deficit is 1,500 a day for someone maintaining at 2,000, and 2,500 a day for someone maintaining at 3,000. That is why a single "1,500 calorie diet" number suits almost nobody.

Why weight loss is never a straight line

People expect the same loss every week and are then unsettled when the scales stall or rise. Normal fluctuation is larger than a week's worth of fat loss, and it is driven by things that have nothing to do with fat:

• Water retention, which can swing a couple of kilos on its own
• Salt and carbohydrate intake, both of which hold water
• Hormonal cycles
• What is currently in your digestive system
• Muscle holding water while it repairs after hard training

The scales can stay flat for a fortnight while you are in a genuine deficit, then move several hundred grams overnight. Judge progress on the trend over three or four weeks, not on any single morning.

Your calorie needs change as you lose weight

A lighter body costs less to run. As you lose weight your maintenance figure falls, which means the deficit you started with gradually shrinks toward zero — this is why loss slows, and eventually stops, on a target that was correct three months ago.

Recalculate every few kilograms, or whenever your activity changes meaningfully. Adding the goal weight above will show how the timeline shifts.

Should you eat back exercise calories?

Usually not. If the activity level you chose reflects how you actually live, your normal training is already inside your maintenance figure — eating it back a second time counts it twice.

Genuinely exceptional days are different. A long ride, a race, or a full day walking in hills is not covered by "moderately active", and eating more on those days is sensible.

Be sceptical of the numbers, too. Watches and gym machines routinely overestimate calories burned, sometimes substantially, and a 600-calorie reward for a 300-calorie session closes a deficit quickly.

What happens if your deficit is too large

Eating far less looks like the fast route, and for a fortnight it is. The problems arrive after that:

• Hunger that stops being manageable
• Fatigue and poor concentration
• Training performance dropping, which costs you muscle
• More of the loss coming from lean tissue rather than fat
• Difficulty getting enough vitamins and minerals in the food that is left
• Most commonly of all, abandoning the whole thing

A deficit you keep for six months beats one you keep for three weeks, even though the second one looks better on day ten. This calculator holds every target at a 1,200-calorie floor for that reason, and says so when it has done it.

Making a deficit easier to hold

Being in a deficit does not have to mean being hungry all day. What you eat changes how the same calorie figure feels.

• Prioritise protein. It is the most filling macronutrient and it protects muscle while you lose fat. Work out your target with the protein calculator.
• Eat plenty of vegetables. Volume and fibre for very few calories — the cheapest fullness available.
• Choose fibre-rich carbohydrates. Wholegrains, beans and fruit keep you satisfied for longer than refined versions of the same calories.
• Drink water. Thirst is regularly mistaken for hunger.
• Strength train. The single most effective way to make sure what you lose is fat.
• Sleep. Short sleep raises appetite hormones and makes every other decision harder.
• Be consistent rather than perfect. Six good days and one ordinary one beats four perfect days and three abandoned ones.

All of that applied to a full week is the weight loss meal plan, and the wider guide is healthy weight loss.

Common mistakes

Guessing portions

Estimation error runs in one direction, and it is usually enough to erase a 500-calorie deficit entirely. Weighing food for a week or two recalibrates your eye, even if you never do it again.

Forgetting drinks and extras

Coffee syrups, juice, alcohol, oil in the pan, a handful of nuts. None of them feel like eating, and together they are frequently the whole deficit.

Picking an unrealistic target

The aggressive option produces the best first fortnight and the worst third month.

Watching only calories

A deficit made of biscuits is technically a deficit. It is also hungry, low in protein and short of nutrients — which is why it rarely lasts.

Common myths

"You have to exercise to lose weight"

No. Fat loss follows the deficit, and diet is the larger lever for most people. Exercise is worth doing for fitness, health and for keeping muscle — but you can lose weight without it, and you cannot outrun a diet that does not add up.

"Eating after 6pm makes you gain weight"

Total energy balance decides this, not the clock. If late eating causes a problem it is because of what and how much, not when.

"Carbs stop fat loss"

They do not. Low-carb approaches work when they reduce total intake, which they often do — but so does any other approach that reduces total intake.

"A bigger deficit is always better"

Only if you keep it. Most people do not, and the rebound usually costs more than the extra speed gained.

Important information

These estimates are for informational purposes only and do not constitute medical advice. Before starting a calorie deficit diet, consult a qualified healthcare professional, particularly if you have existing medical conditions, are pregnant, or are under 18.