How to Calculate Your Calorie Deficit (and the Weight You'll Lose)

Picture planning a trip based purely on distance as the crow flies: no roads, no traffic, no tired legs — just a straight line on a map. A calorie-deficit projection is a bit like that. The arithmetic gives you a clean, motivating estimate of how something will shrink and when. Whether your real journey follows the line exactly is another question.

Here's the core idea in one sentence: eat fewer calories than you burn, and the shortfall is repaid from stored fat. That shortfall is your deficit — the engine of every weight-loss plan that works. Let's make the math explicit, then look at where it's a remarkably good guide and where real biology refuses to cooperate.

The formula, unpacked

Target intake  = TDEE − deficit
Expected loss  = (deficit × 7) ÷ 7,700  kg per week

Three ingredients:

• TDEE — Total Daily Energy Expenditure: everything you burn in a day, resting metabolism plus activity.
• Deficit — how far below TDEE you intend to eat.
• 7,700 — the rough energy content of 1 kg of body fat, in kcal. Strictly it's a rule-of-thumb figure (fat tissue isn't pure fat, and physiological efficiency isn't 100%), but it's the standard approximate used to translate shortfalls into expected weight change.

A worked example

Take a 30-year-old man, 175 cm, 75 kg, moderately active.

BMR  = 10 × 75 + 6.25 × 175 − 5 × 30 + 5 ≈ 1,699 kcal
TDEE = 1,699 × 1.55 ≈ 2,633 kcal

With a 500 kcal daily deficit:

Target intake = 2,633 − 500 = 2,133 kcal
Weekly loss   = (500 × 7) ÷ 7,700 ≈ 0.45 kg
Time to lose 5 kg ≈ 11 weeks

That's the tidy version: ~0.45 kg a week at the classic 500 kcal deficit. Raise or lower the number, and the projection scales accordingly:

Daily deficit	Expected weekly loss	Pace character
250 kcal	~0.2–0.3 kg	Gentle, flexible, very sustainable
500 kcal	~0.45 kg	The standard balanced target
750–1,000 kcal	~0.7–0.9 kg	Aggressive; harder to sustain and riskier to muscle

Choosing your own deficit — and a safety floor

Most people do best in the 250–500 kcal range. That's enough to see steady progress without leaving you ravenous, and it's small enough that your daily food still covers your real needs. Large deficits can drive muscle loss alongside fat, sap training performance, and quietly push overall intake dangerously low.

A useful general rule of thumb: avoid taking total intake below roughly 1,200 kcal/day for most women and 1,500 kcal/day for most men without professional supervision. Those are approximate floor figures, not precise prescriptions — and they're a warning that you've probably set the deficit too deep if you're hitting them. If you're older, very active, or planning a long cut, it's worth talking to a doctor or dietitian about the right starting point for you.

Why the scale won't follow the graph

The model assumes your body obeys the arithmetic precisely. In practice:

• Water weight announces itself. In the first week or two, glycogen and its associated water flush out, and the scale drops fast — that's not fat, and it'll slow. Conversely, salty meals, dehydration, and the menstrual cycle can add 1–2 kg of water that hides real fat loss for a week.
• Bodies adapt. As you get lighter, your TDEE drifts downward with you; your planned daily deficit shrinks slightly even if you eat the same target. Recalculate every few kilograms.
• Deficits are eaten, not declared. Food labels overestimate and undercount, portions drift, weekends leak calories, and a "500 kcal deficit" on paper is often 250–350 in reality. Tracking honestly for a few weeks reveals your true number.
• The last couple of kilograms resist. As you approach low body fat, the body defends its stores more stubbornly, and an aggressive deficit becomes counterproductive.

None of this means the arithmetic is useless — it means it's a map, not a guarantee. The map tells you the direction and a plausible arrival window; the actual journey needs occasional recalibration.

Making the deficit actually work

• Anchor to protein and fibre. They keep you full at the same calorie count, which is the real battle.
• Assume a fudge factor. Most people burn less than activity multipliers suggest. If after two weeks the scale isn't moving as predicted, trim 100–200 kcal rather than slashing everything.
• Keep training. Strength work under a deficit signals "keep the muscle"; that's the difference between losing weight and losing the weight you wanted to lose.
• Expect a wobble, not a retreat. Water-weight swings and the occasional disappointing week are noise. Judge by the fortnightly trend, not the daily number.

Key takeaways

• Deficit = TDEE − intake; expected loss ≈ (deficit × 7) ÷ 7,700 kg/week, an approximation.
• A 250–500 kcal daily deficit is the sustainable sweet spot for most people.
• Watch the safety floor — roughly 1,200 kcal for most women, 1,500 for most men — and go higher, not lower, if unsure.
• The formula is a roadmap, and your scale-declared weight follows it with lag and noise. Recalculate as you shrink and track trends, not single days.

FAQ

Why does everyone say 500 kcal = about 0.5 kg a week? Because 500 × 7 = 3,500 kcal per week, and 3,500 kcal is the historical shorthand for roughly 0.45–0.5 kg of stored fat. It's a useful approximation that holds reasonably well in overweight-to-moderate ranges, less so at extremes.

Can I create a deficit through exercise instead of eating less? Yes, and it can feel more forgiving — but exercise burns only a fraction of what appetite forgives, and most people overestimate burned calories by a wide margin. The most reliable setup combines a moderate intake cut with whatever activity you actually enjoy.

Is it better to diet for 12 weeks or take breaks? There's no single right schedule. Taking a short diet break at your new maintenance level can restore energy and morale, and it helps you practise the eating pattern you'll need after the cut ends — which is the part that decides whether the weight stays off.

Enter your numbers in the Calorie Deficit Calculator, starting from your TDEE or maintenance calories figure.