How Big Should Your Calorie Deficit Be?

A calorie deficit is what makes you lose weight — nothing else does the job on its own. But "eat less than you burn" doesn't tell you how much less, and getting that number wrong is why so many diets either stall out or fall apart within a few weeks. Too small a deficit and nothing happens. Too large, and you're fighting hunger, fatigue, and muscle loss for a result you could have gotten more comfortably.

The maths behind "losing 1kg a week"

One kilogram of body fat stores roughly 7,700 calories of energy. To lose 1kg in a week, you need a total deficit of 7,700 calories across those seven days — which works out to about 1,100 calories a day below your maintenance level (your TDEE). Want to lose it over two weeks instead? That halves the daily number to around 550 calories. This is exactly the calculation the widget above runs: pick a weekly rate, and it converts that into the daily deficit and target intake needed to get there.

Why bigger isn't necessarily faster

A 1,100-calorie daily deficit sounds efficient on paper, but for most people it's difficult to sustain for more than a couple of weeks. Very large deficits increase the amount of muscle lost alongside fat, slow down your metabolism more than a moderate deficit would, and drive the kind of hunger that eventually leads to bingeing or simply quitting. There's also a floor: cutting calories too aggressively — especially below roughly 1,200–1,500 kcal a day for most adults — risks nutrient shortfalls regardless of how much weight you have to lose. A deficit you can't sustain for long enough to matter isn't actually faster; it just fails sooner.

Picking a rate that fits your body

A widely used guideline is to aim for roughly 0.5–1% of your bodyweight lost per week. For someone around 80kg, that's 0.4–0.8kg — comfortably inside the range in the calculator above. The more weight you have to lose, the more forgiving your body tends to be about a slightly larger deficit; the closer you get to a lean, healthy weight, the more you should slow down and lean towards the smaller end of that range to protect muscle and energy levels.

What to do when the scale stops moving

Weight loss is never a straight line — water retention, hormonal cycles, sodium intake, and sleep can all swing the number on the scale by a kilogram or more without any change in actual fat. Judge progress over 2–3 week averages, not single readings. If your trend genuinely flattens for three weeks or more at a stable calorie intake, it usually means your TDEE has dropped as you've gotten lighter — recalculate it and adjust your target down slightly rather than assuming the deficit has stopped working.

Diet breaks are a tool, not a failure

Taking a planned week or two at maintenance calories partway through a longer cut isn't giving up — it's a recognised strategy for making the whole process more sustainable. A diet break lets hormones that suppress appetite and metabolism during a sustained deficit recover somewhat, gives you a mental reset from constant tracking and restriction, and often results in less muscle loss overall than grinding through months of deficit without a pause. If you've got more than 8-10 weeks of cutting ahead of you, planning a break every 6-8 weeks tends to produce better long-term results than pushing straight through.

Why the 7,700 kcal rule drifts over time

The familiar arithmetic — about 7,700 kcal per kilogram of body fat, so a 500 kcal daily deficit should yield roughly half a kilo a week — is a reasonable starting approximation, and it's genuinely useful for setting an initial target. It's also known to overpredict loss the longer a diet continues, and it's worth understanding why rather than concluding the diet has failed.

Three things account for most of the drift. First, weight lost isn't purely fat: some is glycogen and its associated water, particularly in the opening fortnight, and some is lean tissue. Second, a lighter body simply costs less to maintain and move, so the same intake represents a progressively smaller deficit. Third, sustained restriction tends to reduce spontaneous daily movement — people sit more, fidget less, and move less briskly without deciding to.

The practical consequence is that a deficit set in week one has quietly shrunk by week twelve. That isn't a metabolism that has broken; it's a moving target. It's also precisely why recalculating periodically, rather than setting a number once and assuming it holds for six months, matters.

Protecting muscle while you lose fat

In a deficit the body will draw on whatever tissue is available, and without the right signals a meaningful share of what you lose can be muscle. Two interventions have strong evidence behind them, and neither is complicated:

• Enough protein. Roughly 1.6–2.2 g per kg of body weight daily, with the upper end more appropriate in steeper deficits. This is the single most effective dietary lever for preserving lean mass while losing weight.
• Resistance training. Continuing to lift gives the body an active reason to retain muscle. Maintaining intensity matters more than volume here — training loads generally shouldn't collapse just because calories have, even if total work drops somewhat.

The two work together, and neither substitutes for the other. Cardiovascular exercise has its own benefits and can contribute to the deficit, but it doesn't send the same retention signal that loaded resistance work does.

Rate versus adherence

Faster isn't better if it doesn't finish. Steeper deficits show consistently higher dropout rates, a greater share of weight lost as lean tissue, more pronounced hunger and preoccupation with food, and worse training quality. Steady approaches tend to win on total fat lost over any horizon longer than a couple of months, largely because they're still being followed.

A reasonable frame is that the best deficit is the largest one you can maintain without your sleep, training, mood, or social life visibly deteriorating. For most people that's 0.5–1% of body weight per week. If you're losing faster than that and feeling fine, there's no crisis — but check that training performance and sleep are genuinely holding rather than being tolerated.

Signals the deficit is too steep

• Training performance declining consistently over two or more weeks, not just on one bad day.
• Persistent hunger that dominates your attention, or increasingly frequent loss of control around food.
• Sleep worsening, especially waking hungry during the night.
• Noticeably low mood, irritability, or difficulty concentrating.
• In women, menstrual cycle changes or absence — a signal worth taking seriously and raising with a GP rather than pushing through.

This guide is for general education and planning purposes and isn't medical advice. Very low-calorie diets can cause health problems — if you're considering losing more than around 1kg a week, or you have a medical condition, speak to a doctor or registered dietitian before changing your intake significantly.