What a calorie deficit is (and how big yours should be)

Every diet that has ever worked, worked through the same door. Low carb, fasting windows, meal-replacement shakes, the cabbage soup your aunt still swears by. Each one is just a different costume on the same idea: eating less energy than your body spends. That gap has a name, the calorie deficit, and understanding it is worth more than any list of foods you are told not to touch.

Energy balance, minus the jargon

Your body burns energy around the clock, sleep included. Keeping your organs running takes most of it, digestion takes a cut, and movement handles the rest. Add up a normal day and you get your maintenance level, also called TDEE, and our maintenance calorie calculator will estimate it from your height, weight, age and activity.

Eat near maintenance and your weight holds. Eat above it and the surplus gets stored. Eat below it and your body makes up the difference from its own reserves, mostly fat. A deficit is that third state, held on purpose. If maintenance is 2,400 calories, eating 1,900 puts you 500 in the red, and it does not care whether you got there by smaller portions or a longer walk.

What a kilogram of fat costs

Body fat stores roughly 7,700 calories per kilogram. The older 3,500-calories-per-pound figure says the same thing in different units and traces back to a 1958 paper by Max Wishnofsky. Hold a 500-calorie daily deficit and you rack up 3,500 across a week, about half a kilo of fat on paper.

Treat that as a planning number, not a pinky promise. The first week or two usually drop faster on the scale, because lower carbs flush out stored glycogen and the water clinging to it. Later on, progress can trail the arithmetic. The 7,700 figure still earns its keep: it is why a brutal fortnight yields 1 kg and not 5, and why anyone advertising 5 kg of fat loss in a week is selling you something. Probably a tea.

Choosing the size of your deficit

Sensible rates run 0.25 to 1 kg a week, which maps to daily deficits of about 250 to 1,000 calories. A decent rule of thumb is 0.5 to 1 percent of body weight a week, so an 80 kg person aims for 0.4 to 0.8 kg.

• 250 a day buys about 0.25 kg a week. Slow and nearly invisible at the table. Suits people close to their goal or in no hurry.
• 500 a day buys about 0.5 kg a week. The standard prescription, big enough to see progress and small enough to live with.
• 1,000 a day buys about 1 kg a week. Defensible for larger bodies with a lot to lose, punishing for everyone else.

Our calorie deficit calculator turns your maintenance number into a daily target for whichever rate you pick.

Why bigger is rarely better

On paper, doubling the deficit halves the timeline. In practice the arithmetic runs into an actual human being. Hunger scales with the gap. 500 calories under maintenance is a manageable nag; 1,500 under is a daily brawl, and most people lose that brawl somewhere in week 3.

Steep deficits also cost you more muscle. The bigger the gap, the more of each lost kilogram comes off as lean tissue instead of fat, especially if you are not lifting or eating enough protein. A slower cut keeps more of the stuff you actually wanted to keep.

And then there is the boring truth nobody frames on their wall: a modest deficit you hold for 6 months beats an aggressive one you ditch in 3 weeks, every single time. The right size is the biggest deficit that does not make you miserable, and for most people that is nearer 500 than 1,000.

Plateaus are part of the deal

Expect the same menu to buy less over time. A lighter body burns fewer calories, at rest and per step, so the 500-calorie gap you opened at the start quietly shrinks as the weight comes off. People also move less in a deficit without clocking it. The fidgeting and pacing researchers file under NEAT drops off on its own. Add a little logging drift, an unweighed glug of oil here, a guessed weekend there, and a plateau can show up with nobody technically cheating.

Water muddies the reading too. Retention after a hard training week or a salty meal can hide fat loss for days, which is why the weekly average deserves your attention and the daily number does not. When the scale stalls for 2 or 3 weeks, re-run the TDEE calculator at your current weight, since the old target was set for a heavier you, and tighten your logging for one honest week before you change anything else. This is exactly where a habit streak helps: Joules lets you log meals from a photo and keeps the daily streak in view, which turns out to be a surprisingly good nudge to log the messy days instead of skipping them.

Where that leaves you

So: find your maintenance, subtract 300 to 500, and eat there for 3 weeks while the weekly average tells you whether the deficit is real. Adjust in small steps and re-run the numbers every 4 or 5 kg. And if you're carrying a medical condition or a history of disordered eating, set your targets with a professional rather than a calculator. This is general information, not medical advice, and I'd rather be boring about that than clever.